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Monographs of the Western North American Naturalist

Volume 11 2019

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Sandhill Crane use of riverine roost sites along the central Platte River in Nebraska, USA

DAVID M. BAASCH1,*, PATRICK D. FARRELL1, ANDREW J. CAVEN2, KELSEY C. KING2, JASON M. FARNSWORTH1, AND CHADWIN B. SMITH1

1Executive Director’s Office for the Platte River Recovery Implementation Program, 4111 4th Ave., Ste. 6, Kearney, NE 68845 2Platte River Whooping Crane Maintenance Trust, 6611 West Whooping Crane Drive, Wood River, NE 68883

ABSTRACT.—Wide channels with short bank vegetation, access to nearby foraging habitat, shallow water areas (<30 cm deep), and absence of disturbance features are factors commonly associated with suitable roost sites for Sandhill Cranes (Antigone canadensis). However, since channel width has typically been evaluated independently of channel depth and flow, it is possible that use of narrow channels is not limited so much by a requirement for wider channels but by deeper water that flows through these narrow channels. We used a discrete-choice modeling framework and 9 years of roost location data to evaluate the influence of channel-width measures and flow per linear unit of channel width on roost-site selection by Sandhill Cranes. Roost-site selection was influenced by maximum unvegetated channel width and flow per unit length of total unvegetated channel width of all channels. The relative selection ratio increased as maximum unvegetated channel width increased to 131 m for small groups (≤500 cranes) and 275 m for large groups (>5000 cranes), but the ratio was statistically similar across a wide range of maximum unobstructed channel widths. Medium-sized Sandhill Crane groups (501–5000 cranes) were less influenced by in-channel vegetated islands, and these groups selected channels based on wide total unvegetated channel widths. Our results also suggest that flows ≤39.05 m3/s (cms; 1379 cfs) in channels that are 275 m in unvegetated width maximize selection ratios for medium and large crane groups, so flows above this level may not improve Sandhill Crane roosting habitat conditions during the spring migration and staging season within the central Platte River. While Sandhill Cranes stage within the central Platte River valley for a longer time interval in the spring, Whooping Cranes (Grus americana) also use the Platte River as a stopover point. Both species share similar indices for roosting habitat, such as unobstructed channel width and shallow water depths. The results of our investigation could be used to identify a range of flows and channel width configurations expected to gen- erate the highest amount of suitable habitat for Sandhill Cranes roosting along the central Platte River.

RESUMEN.—Los canales anchos con riberas de escasa vegetación, accesibilidad a hábitats de forrajeo cercanos, áreas de aguas poco profundas (<30 cm) y ausencia de perturbación, son factores comúnmente asociados, a los sitios que son adecuados para la percha de las grullas canadienses (Antigone canadensis). Sin embargo, dado que la amplitud del canal ha sido típicamente evaluado, independientemente de su profundidad o de su flujo, es posible que el uso de canales estrechos no limite tanto al requisito de los canales más amplios, como aguas más profundas que fluyen a través de canales estrechos. Para evaluar la influencia que tienen la amplitud del canal y el flujo por unidad lineal, en la selección del sitio de percha de la grulla canadiense, usamos el marco del modelo de elección discreta y los datos obtenidos durante nueve años de la localización de los sitios de percha de la grulla canadiense. En todos los canales, la selección de sitio se relacionó, por la amplitud máxima del canal sin vegetación y con el flujo por unidad de longitud. La propor- ción relativa de selección de sitio, incrementó a medida que la amplitud máxima del canal sin vegetación aumentó, a 131 m en los grupos pequeños (≤500 grullas) y a 275 m en los grupos grandes (>5000 grullas), pero fue estadísticamente similar a lo largo de un amplio rango de amplitudes de canales sin obstrucciones. Los grupos medianos de grullas (501–5000 grullas) fueron menos influenciados por islas que poseen canales con vegetación. En este caso, los sitios seleccionados se basaron en la amplitud total de los canales sin vegetación. Nuestros resultados también sugieren que los flujos ≤39.05 cms (1379 cfs) en canales sin vegetación con 275 m de amplitud, maximizan la proporción de selección de los grupos medianos y grandes de grullas. Por lo tanto, aumentar el flujo por encima de este nivel podría no mejorar las condiciones del hábitat de percha de las grullas canadienses durante la migración de primavera y el inicio de la temporada, dentro del área central del Platte River. Mientras que las grullas canadienses permanecen dentro del área central del valle del Platte River durante un intervalo de tiempo largo en primavera, las grullas trompeteras (Grus americana) también usan el Platte River como punto de descanso. Ambas especies comparten índices similares en cuanto a su hábitat de percha, tales como la amplitud del canal sin obstrucciones y aguas poco profundas. Los resultados de nuestra investigación podrían usarse para identificar un rango de configuraciones de flujos y de amplitud de canales, que se espera generen hábitats más adecuados para que las grullas canadienses descansen a lo largo del área central del Platte River.

*Corresponding author: [email protected]

DMB  orcid.org/0000-0003-0747-1449 PDF  orcid.org/0000-0002-2861-8182 JMF  orcid.org/0000-0002-9146-0162

1 2 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 1–13

This is an open access article distributed under the terms of the Creative Commons Attribution License CC BY-NC 4.0, which permits unrestricted noncommercial use and redistribution provided that the original author and source are credited.

Each spring approximately 600,000 Sand- studies were conducted using sophisticated hill Cranes (Antigone canadensis) migrate physical/hydrodynamic modeling based on sur- through the central Great Plains from the win- veys of channel topography at roost locations tering grounds in Texas, New Mexico, south- (Prince 1990, Simons and Associates Inc. 1990, eastern Arizona, and Mexico en route to their Kinzel et al. 2005). These investigations have breeding grounds in Siberia, Alaska, and cen- primarily focused on assessing the channel tral and northern Canada (Kinzel et al. 2005, widths and river discharges necessary to opti- Krapu et al. 2011, Kruse and Dubovsky 2015, mize the area of shallow water habitat in reaches Dubovsky 2016). Nearly 80% of these cranes that are suitably wide for roosting. Various stage each spring along the Big Bend Reach of researchers have proposed suitable channel the Platte River in Nebraska, USA (Lewis 1976, width targets ranging from 50 m (164 feet) to Krapu et al. 1985, 2014, Pearse et al. 2017). ≥250 m (820 feet) and optimal river discharges While in this area, Sandhill Cranes forage ranging from 30 m3/s (cms; 1059 cfs) to 57 cms during the day for waste grain in cornfields (2013 cfs; Currier and Eisel 1984, Platte River and for native sedges and macroinvertebrates Management Joint Study Biology Workgroup in lowland meadows. At night they roost in 1990, Prince 1990, Kinzel et al. 2005). The shallow braided channels of the Platte River high degree of variability in proposed width (Sparling and Krapu 1994, Pearse et al. 2017). and discharge targets is likely due to the dy - This annual phenomenon is of critical conser- namic nature of the Platte River channel where vation concern because it is one of the largest bedforms and unvegetated channel width vary re maining mass wildlife migration events in widely across the reach and at a single location the world (Johnsgard 2002). through time. Furthermore, discharge can vary The central Platte River is a steep, sand-bed by up to 28 cms (1000 cfs) in a day during the braided river (Simons and Associates Inc. and spring migration season, substantially chang- URS Greiner Woodward Clyde 2000). As with ing the quantity and spatial distribution of most braided rivers, the central Platte is highly available roosting habitat (Kinzel et al. 2009). dynamic with a high width-to-depth ratio and Sandhill Cranes that use the Platte River a channel dissected by emergent bedforms at during the spring migration season utilize a low discharges (Simons and Associates Inc. and sys tem that is far from static. Spatial shifts in URS Greiner Woodward Clyde 2000, Murphy roost locations that appear to correspond to et al. 2004). Sandhill Cranes roost in shallow flow variability are commonly noted during water (<25 cm; 10 inches) that flows over sub- ob servational studies (J. Jenniges and M. Pey- merged bedforms, and occasionally cranes will ton personal communication) and during our roost on aerially exposed sandbars (Norling et systematic aerial monitoring surveys. For ex- al. 1992, Kinzel et al. 2005) in areas with views ample, a large number of Sandhill Cranes unobstructed by dense vegetation (Folk and have been observed roosting in fields and Tacha 1990). It is hypothesized that Sandhill wet mead ows under high discharge conditions Cranes roost in wide, shallow unvegetated chan - when shallow- water habitat is limited (Davis nels because they rely heavily on eyesight to 2001). These observations led us to hypothe- detect approaching predators (Armbruster and size that the distribution of Sandhill Crane Farmer 1982, Krapu et al. 1984, Folk and Tacha roost locations within and across years is a 1990, Davis 2001, 2003, Pearse et al. 2017). function of the interaction between available Given the importance of the central Platte channel widths and discharge as it affects River in the annual Sandhill Crane migration, the distribution and availability of suitably ecologists have conducted numerous studies shallow roosting habitat. The objective of this since the early 1980s to assess in-channel study was to test this hypothesis and to uti- habitat suitability for roosting and to identify lize the results to de scribe flexible flow targets threats to the quantity and quality of that habi- that acknowledge the range of spatial and tem- tat (USFWS 1981, Faanes and LeValley 1993, poral variability present in this system. As Davis 2003, Pfeiffer and Currier 2005, Krapu such, we evaluated the influence that channel- et al. 2014, Pearse et al. 2017). Several of these width measures and flow per linear unit of BAASCH ET AL. ♦ SANDHILL CRANE ROOST-SITE SELECTION 3

Custer Sherman Howard

Middle Loup R.

Merrick ^Chapman

S. Loup R. ^Grand Island Platte R.

Alda Hall ^ Buffalo Hamilton ^Wood River ^Shelton ^Gibbon Elm Creek Odessa ^ Kearney ^ ^ Platte R. Dawson g Blue R. W. Fork Bi

Adams Clay Nebraska Location Map Phelps Kearney

Legend ± ^ City 03.5710.5141.75 . Miles Little Blue R River 04.5913.5182.25 County Kilometers

Fig. 1. Platte River channels within the study area between Elm Creek and Chapman, Nebraska, USA, with the locations of cities included for reference. channel width (i.e., average channel depth) 135 km/h and at an elevation of 200 m above have on roost-site se lection by Sandhill Cranes. ground. The surveys were initiated 15–25 min The results of our analysis could be used to prior to sunrise and took approximately 1 h to help identify ideal flow conditions for all chan- complete. Survey flights were generally flown nel widths for Sand hill Cranes roosting on the from east to west; however, flights were flown central Platte River. from west to east later in the seasons when the highest concentration of birds shifted from METHODS east to west (Krapu et al. 2014). One observer was responsible for estimating the size of each Identifying Sandhill Crane Group Locations group of Sandhill Cranes by first counting a To determine distribution patterns of Sand- portion of each roost, then counting spatial hill Crane roosts, aerial surveys of Platte River replicates of that area within the roost as a channels between Chapman and Elm Creek, whole (Gregory et al. 2004, Bowman 2014). Nebraska, USA (Fig. 1), were conducted annu- A second researcher collec ted the latitude and ally by personnel of the Platte River Whoop- longitude over the center of each Sandhill ing Crane Maintenance Trust. Surveys were Crane roost using a handheld Global Posi- conducted via a fixed-wing single-engine air- tioning System (Garmin® eTrex, 72, or 72H plane (generally a Cessna 172) once every week handheld GPS, Olathe, KS) and re corded count from mid-February to mid-April, as ap prop riate esti mates obtained by the ob server. Individual weather conditions and aircraft availability roosts were defined as a group of cranes that permitted. This 120-km stretch of river was were separated from another group by more flown 6 to 9 times per year at approximately than 100 m (Iverson et al. 1987). 4 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 1–13

Fig. 2. Examples of how (a) total unvegetated channel width (TUCW) and (b) maximum unvegetated channel width (MUCW) were delineated.

Estimating Variables Included in Analyses along the channel throughout the study area, and encompassed all channels in split-flow We used aerial imagery (≤15-cm resolution) reaches. Photo-interpretation of unvegetated collected annually during periods of low flow width metrics was determined to provide (July/August or October/November) to photo- acceptable measurement accuracy based on interpret total unvegetated channel width previous comparisons of field-measured and (TUCW) of the channel that contained the use photo-interpreted unvegetated width mea- or available location and maximum unvege- surements along the central Platte River (Wer- tated channel width (MUCW) of the channel bylo et al. 2017). We divided the daily mean that contained the use or available location discharge, estimated from a U.S. Geological throughout our study area during the period Survey gaging station near Grand Island, of 2007–2017 (Fig. 2). Unvegetated width met- Nebraska (06770500), by the total unvegetated rics were delineated along 391 predefined width of all channels to develop a metric that transects using ESRI ArcMap Geographic In - equates to flow per unit width of channel formation System (GIS) software (Werbylo et (DISCH) in order to describe water condi- al. 2017). All transects were oriented perpen - tions encountered by cranes in all channels dicular to flow, were spaced at 300-m intervals throughout the entire reach. BAASCH ET AL. ♦ SANDHILL CRANE ROOST-SITE SELECTION 5

For each roost location, we developed indi- 2010, Carter et al. 2010, Unger et al. 2015, vidual choice sets by randomly generating Baasch et al. 2017). We used DCMs because 20 locations within 5.7 km of the used loca - changing habitat availability can be better cap - tion to characterize available roosts based on tured within this framework compared to other the average observed distance between subse- techniques (Cooper and Millspaugh 1999). quent night roost locations calculated from We evaluated our model set using a Cox telemetry data (Krapu et al. 2014). Thus a ran- proportional hazards regression function in R dom selection of locations within this distance statistical software (R Development Core Team represented a choice set of roosts that a crane 2015) with function gam in package mgcv, might have chosen on a particular night (Arthur which utilizes reweighting least-squares fit- et al. 1996, Compton et al. 2002, Baasch et al. ting of the penalized likelihood to determine 2010). All habitat metrics (MUCW, TUCW, the smoothness of the line and associated de- and DISCH) were calculated for the center of grees of freedom (Arthur et al. 1996, McCracken each Sandhill Crane group use location and et al. 1998, Cooper and Millspaugh 1999, for the 20 corresponding randomly selected McDonald et al. 2006, Wood 2006). Addition- in-channel available points within 5.7 km ally, generalized cross validation was used to upstream and downstream of the use location. determine the penalty for smoothing parame- We constructed an a priori set of 5 models ters of each iteration. Important habitat rela- with the following variables: maximum unvege- tionships were described for the top model in tated channel width (MUCW), total unveg - each group size–specific model selection pro - etated channel width (TUCW), and flow per cess in which models were ranked using infor- meter of unvegetated channel width (DISCH); mation theoretic methods with the Akaike models were tested on small (0–500 indi vid - information criterion (AIC), and top models uals), medium (501–5000 individuals), and were indicated as the most parsimonious large (>5000 individuals) crane groups sepa- model with an AIC value of ≤2.0 (Burnham rately. Although the same models were tested and Anderson 2002). with use-available data from each crane group Validating Variables Included in Analyses size, responses to channel width variables were expected to differ because large crane To validate results of the best model, we groups have been observed using wider chan- randomly partitioned the data sets of use loca- nels than medium groups and small groups tions and corresponding available locations for (Buckley 2011). Crane groups, regardless of small, medium, and large groups into training size, require a suitable water depth to utilize a data sets (2/3 of the choice sets) and test data roost location regardless of channel widths. sets (1/3 of the choice sets). We used training No variables were included in a model to- data to develop parameter estimates for best gether if substantial correlation (|r| ≥ 0.50) models and a comparison of available and use was present (Dormann et al. 2013). locations of the test data set to understand the We utilized general additive models (GAMs) reliability of a binary response (use/available) within a discrete choice model (DCM) frame- model. Predicted values of available locations work for group-specific sandhill crane roost within the test data set were scaled to the location selection from 2007 to 2017. A GAM number of use locations in the test data set. is a special case of a generalized linear model These were then binned into 20 percentile in which smoothing functions are applied to categories and compared to the number of covariates (Hastie and Tibshirani 1990, Wood test- data-set use locations in each bin. Pre- 2006). We employed GAMs with penalized dicted values were summed to calculate the regression splines, which estimate the degree number of expected use locations in each bin, of covariate smoothness with cross validation. which were then compared to the actual sum An assumption of DCMs is that individuals or of use locations in each bin with a linear re- groups of individuals make choices to maxi- gression model in order to identify the relia- mize their satisfaction, mirroring assumptions bility of the model based on the closeness of the of resource selection functions (Ben-Akiva and slope-relationship of 1. This method was re - Lerman 1985). DCMs have been applied to peated 1000 times to develop the average slope several studies of wildlife resource selection and 95% confidence intervals of model fit. (Cooper and Millspaugh 1999, Baasch et al. As defined by Howlin et al. (2004), a “good” 6 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 1–13

TABLE 1. Average and standard deviation (SD) of habitat variables evaluated in a use-availability discrete-choice analysis of Sandhill Crane (Antigone canadensis) roost locations on the central Platte River, 2007–2017. Variables include maximum unvegetated channel width of the channel where the use or available locations were (MUCW), total unvegetated channel width of the channel where the use or available locations were (TUCW), and discharge per linear meter of total unvegetated width of all channels (DISCH). MUCW TUCW DISCH Crane ______group size Use/available n Average SD Average SD Average SD Large Use 442 201 85 307 81 0.156 0.085 Large Available 8819 181 87 289 83 0.166 0.089 Medium Use 1116 160 78 258 72 0.159 0.091 Medium Available 22,265 151 80 252 78 0.167 0.102 Small Use 1084 135 67 239 74 0.163 0.108 Small Available 21,613 141 75 242 77 0.163 0.113

TABLE 2. Akaike information criterion (AIC) model selection results of our use-availability roost-site selection analysis by crane group size. Variables included maximum unvegetated channel width of the channel where the use or available locations were (MUCW), total unvegetated channel width of the channel where the use or available locations were (TUCW), and flow per meter of total unvegetated channel width of all channels (DISCH). Δ Group size Model df AIC AIC Weight Small MUCW 1087.26 23892.88 0.00 0.54 MUCW + DISCH 1087.79 23893.22 0.34 0.46 TUCW + DISCH 1085.02 23905.07 12.19 0.00 TUCW 1084.27 23908.67 15.79 0.00 Null 1083.00 23910.83 17.95 0.00 DISCH 1087.06 23911.24 18.36 0.00 Medium TUCW + DISCH 1119.63 24642.15 0.00 0.72 TUCW 1118.78 24645.14 2.99 0.16 MUCW + DISCH 1119.16 24645.97 3.83 0.11 DISCH 1116.02 24651.15 9.00 0.01 MUCW 1118.46 24657.48 15.34 0.00 Null 1115.00 24683.02 40.87 0.00 Large MUCW + DISCH 448.55 8908.24 0.00 0.93 TUCW 444.36 8914.66 6.42 0.04 TUCW + DISCH 445.71 8916.45 8.21 0.02 DISCH 445.25 8916.65 8.41 0.01 MUCW 444.70 8919.28 11.04 0.00 Null 441.00 8956.02 47.78 0.00 model had an average 95% confidence inter- at roost sites of medium and small crane groups val that incorporated 1 and not zero. An “ade- (Table 1). However, flow per meter of total un - quate” model had an average 95% confidence vegetated channel width was similar among interval that did not incorporate 1 or zero. If crane group sizes (Table 1). the average slope-relationship had a 95% con- Statistical modeling of habitat selection in - fidence interval spanning zero, the model was dicated that aspects of channel width were deemed “poor.” important for all crane group sizes. Flow metrics were also important for medium and large RESULTS crane groups, but not for small crane groups (≤500 individuals; Table 2). Maximum unvege - We documented 2642 Sandhill Crane roost tated channel width was an important predic- sites and 52,697 associated available locations tor of small and large crane group roost loca- within our study area between 2007 and 2017. tions. Selection ratios were maximized at 131 m Roost size averaged about 3500 individuals for small crane groups and 275 m for large with a median of 1000 cranes (range 1–84,330 crane groups but were statistically similar from cranes). The average maximum unvegetated 26 m to 190 m and ≥150 m, respectively (Fig. and total unvegetated channel widths were 3A, D). For medium-sized crane groups, the wider at roost sites of large crane groups than selection ratio for TUCW was maximized at BAASCH ET AL. ♦ SANDHILL CRANE ROOST-SITE SELECTION 7

267 m but was statistically similar from 69 m in the spatial distribution of Sandhill Cranes to 408 m (Fig. 3B). Flow per meter of total along the Platte River have been associated unvegetated channel width was an important with changes in river morphology (Krapu et predictor for medium and large crane groups. al. 1982, Faanes and LeValley 1993). Reduced Selection ratios were maximized at the lowest flows in the Platte River have allowed for the flows observed for medium and large crane encroachment of willows (Salix spp.), cotton- groups (0.0361 cms/m and 0.0382 cms/m of wood (Populus deltoides), eastern red cedar total unvegetated channel width of all chan- (Juniperus virginiana), and more recently phrag- nels, respectively). However, these results were mites (Phragmites australis) onto sandbars statistically similar from 0.0361 cms/m to used by Sandhill Cranes for roosting (Lewis 0.1420 cms/m and 0.0382 cms/m to 0.1006 1976, Williams 1978, Currier 1982, Norling et cms/m of total unvegetated channel width of al. 1992). As woody and exotic encroachment all channels for medium and large groups, re- have increased following the appropriation of spectively (Fig. 3C, E). Model validation results 70% or more of the Platte River’s flows, large indicated an adequate and good model fit for western portions of the Big Bend region have large and small groups, respectively, where the been abandoned by spring-staging Sandhill slope of the regression lines and 95% confi- Cranes as channels have become narrower and dence intervals averaged 0.323 (95% CI 0.081– deeper (Krapu et al. 1982, Faanes and LeValley 0.565) for large groups and 0.907 (95% CI 1993, Currier 1997). Faanes and LeValley 0.111–1.704) for small groups. However, model (1993) reported that the distribution of staging validation results for medium groups indi- Sandhill Cranes shifted eastward along the cated an adequate to poor model fit where the Platte River from 1957 to 1989, decreasing slope of the regression line and 95% confi- where woody vegetation encroachment and dence interval averaged 0.205 (95% CI −0.055 chan nel narrowing were the greatest and in- to 0.465), so results for medium-sized groups creasing along reaches that remained rela- should be interpreted with care. Arbitrary group tively wide and unvegetated. This abandon- size cutoffs are a potential reason for the re- ment has resulted in higher concentrations of duced certainty in medium group size model cranes in a smaller reach of river, increasing fit. Groups of 500 Sandhill Cranes need less the potential for an epidemic outbreak of dis- area of suitable depths to roost, whereas the eases (Vogel et al. 2013). Because of the larger groups of 5000 birds may select chan nels increased risk of disease, active channel width more like groups >5000 in size along the cen- has been artificially maintained and vegetation tral Platte River and thus require wider channels that provide more area of suitable depth. growth on sandbars has been mitigated over the last few decades by the efforts of environmental organizations such as the Platte Valley DISCUSSION Weed Management Area, the USFWS Part- The most important factor regulating the ners for Wildlife Program, the Platte River num bers of cranes using the central Platte Whooping Crane Maintenance Trust, and the River is roosting habitat availability (Krapu et Platte River Recovery Im plementation Pro- al. 1984, Tacha et al. 1992, Faanes and LeValley gram (Pfeiffer and Currier 2005). These groups 1993). Consequently, data on habitat availabil- currently manage in-chan nel vegetation in the ity and use are key components in understand - central Platte River nearly every fall through ing crane-habitat relationships and for de- use of heavy machinery and through airboat veloping sound conservation plans. Sandhill and aerial applications of herbicide. Crane distribution along the Big Bend of the In agreement with the literature, medium Platte River is determined by the availability and large Sandhill Crane groups selected areas of adequate roosting habitat more than any with wider channel widths (USFWS 1981, other factor (Krapu et al. 1984, Sparling and Krapu et al. 1984, Norling et al. 1990, Sidle et Krapu 1994). Substantial changes in river char - al. 1993), while small groups selected channels acteristics, including changes in river flow, with moderate widths. Nearly 100% of Sand- have been implicated in the cranes’ abandon- hill Cranes observed in many studies of the ment of large portions of the Platte River in central Platte River, including ours, roosted in west central Nebraska (USFWS 1981, Faanes river channels >50 m wide (Krapu et al. 1984, and LeValley 1993, Krapu et al. 2014). Changes Norling et al. 1990, Sidle et al. 1993). Large 8 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 1–13

Fig. 3. Influence of variables included in the top model on predicted relative selection ratios, with 90% confidence intervals, for (A) small, (B, C) medium, and (D, E) large crane groups. Variables included maximum unvegetated channel width of the channel where the use or available locations were (MUCW), total unvegetated channel width of the channel where the use or available locations were (TUCW), and flow per meter of total unvegetated channel width of all channels (DISCH).

Sandhill Crane groups used channels where 135 m and their selection was maximized at maximum unvegetated widths averaged 307 m 131 m TUCW. Medium-sized Sandhill Crane and selection for MUCW was maximized at groups used channels where maximum unvege- 275 m. Selection for small groups averaged tated widths averaged 258 m and selection for BAASCH ET AL. ♦ SANDHILL CRANE ROOST-SITE SELECTION 9

TUCW was maximized at 267 m. Sidle et al. roosting habitat, whereas narrower, more in - (1993) found that more than two-thirds of cised channels may provide adequate roosting cranes roosted in river channels wider than habitat during dryer periods when water in 150 m. Krapu et al. (1984), Davis (2001, 2003), those channels is appropriately shallow. This and Pearse et al. (2017) reported that most may be especially true when narrower chan- roosting Sandhill Cranes selected river channels are in close proximity to high-value forag- nels with widths >150–200 m and avoided ing locations such as wet meadows (Sparling river channels with widths ≤100–150 m. Davis and Krapu 1994). Flow per unit of channel (2001, 2003) and Krapu et al. (1984) also re- width is an indirect measure of average chan- ported that most roosting Sandhill Cranes nel depth and is an important metric to con- were observed in the widest river channels and sider, one that has never been evaluated di - avoided narrow channels. In contrast, Norling rectly. Sandhill Cranes respond to changes in et al. (1992) and Parrish et al. (2001) found that flow by adjusting where they roost within the Sandhill Cranes selected channels with mod- channel. Davis (2001) indicated that Sandhill erate widths and that large and small flocks Cranes use narrow channels—those that are were located in wide channels as well as nar- normally too deep for roosting—during peri- row channels. However, the discrepancy in ods of low flows and have been observed selection for various channel widths is likely roosting in shallow water areas adjacent to related to the way that “channel width” was islands, on top of cleared islands, and in grass- defined and estimated in each of these studies, lands adjacent to the river during periods of the timing of flights (middle of the night vs. high flows. In March 2017, flows exceeded after twilight) if crane groups in narrower 100 cms (3550 cfs) and the majority of the channels depart the river prior to being de- Sandhill Cranes roosted on land, forgoing the tected after twilight, or the flow during survey river during a period of high winds that cre- periods. Our findings indicate that the optimal ated white-capped waves in the main channel channel width for roosting Sandhill Cranes of the Platte River (Platt River Whooping largely depends on crane group size and water Crane Maintenance Trust, Brice Krohn, Vice depth as a function of discharge and channel President, personal communication). width (flow per unit width of channel). We found that suitable areas within the The availability of suitable roosting habitat river channel for Sandhill Crane roosts are for Sandhill Cranes can be maximized under identified not only by unobstructed visibility certain flow conditions because factors such as but partially by water depth as well, which is water depth and velocity that are important in similar to the results of a few other studies determining the suitability of an area for roost (Folk and Tacha 1990, Norling et al. 1992, Kin - sites are related to flow (FERC 1994, Kinzel zel et al. 2009). Although Pearse et al. (2017) et al. 2009). Norling et al. (1992) and Kinzel et were unable to establish a link between their al. (2005) found that Sandhill Cranes generally surrogate measure of water depth and Sand- roosted in shallow channels with velocities hill Crane roost-site selection, average maxi - <70 cm/s. Because channel width has always mum daily flows at Grand Island during the been evaluated independently of channel spring staging season were more than 50 cms depth, it is possible that narrow channel use (~1750 cfs) higher during our study period is not limited so much by a requirement for than they were during the time frame of the wider channels but by deeper water that flows Pearse et al. (2017) study (100 cms in 2007– through these channels (Latka and Yahnke 2017 vs. 45 cms in 2001–2005). Similarly, aver- 1986, Folk and Tacha 1990). Differences in age daily flows at Grand Island during the channel flow during study periods may ex - spring staging season were nearly 25 cms plain why Norling et al. (1992) and Parrish et (900 cfs) higher during our study period than al. (2001) found that Sandhill Cranes selected they were during the Pearse et al. (2017) study moderately wide channels (50–200 m) and period (51 cms [1791 cfs] and 26 cms [921 cfs], found no preference for the widest channels. respectively). The difference in how the flow- Theoretically, the widest channels should be to-depth relationship was estimated and in - the most preferred in times of higher dis- cluded within the model sets and the very dif- charge when dry sandbars within wide reaches ferent ranges of flow observed during our are temporarily submerged, creating shallow study and the Pearse et al. (2017) study likely 10 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 1–13 account for the differences in conclusions be- Our results further clarify the dynamics of tween the 2 studies about the importance of Sandhill Crane roosting habitat in relation flow-to-depth relationships to roost-site selec- to channel width and water depth as a factor tion by Sandhill Cranes along the central of discharge along the Big Bend of the Platte Platte River. Kinzel et al. (2009) predicted the River. Integrating our research with the exist- area of suitable depth for crane roosts as a non - ing literature can allow a specific target range linear relationship of river stage in which mod - of flows based on the management objective of erate flows corresponded to the greatest roost optimizing Sandhill Crane habitat during the area. Likewise, Farmer et al. (2005) re ported spring migration period. Relatively high sum- varying relationships between river dis charge mer flows that maintain the ecological integ- and available habitat for roosting Whooping rity of wet meadows, which are important for- Cranes (Grus americana), depending partially aging areas for Sandhill Cranes (Currier 1990, on channel width along the Platte River. Sparling and Krapu 1994, Currier and Hen- Our results suggest that flows between szey 1996), also maintain fish spawning habitat 0.0361 cms/m and 0.1420 cms/m of total un - during summer months (Poff et al. 1997) and vegetated channel width of all channels would reduce the need for mechanical maintenance maximize selection ratios for medium and of riverine systems by preventing woody and large Sandhill Crane groups. This equates to a exotic plant establishment within the high flow of approximately 13.65 cms (482 cfs; low- banks (Williams 1978, Currier 1997, Johnson est flows observed) to 39.05 cms (1379 cfs) for 1997). While our flow estimates are lower than channel widths of medium and large Sandhill most reported in the existing literature (Cur- Crane roosts (~275 m); flows above these lev- rier and Eisel 1984, Kinzel et al. 2005), our els reduce habitat availability. Channels of findings are best applied and understood in these widths typically occur in areas that have the context of the Platte River ecosystem as a been managed on a near-annual basis, such as whole. They should be interpreted in the con- the Rowe Sanctuary and properties of the text of extensive riverine management that has Platte River Whooping Crane Maintenance maintained river segments at historically more Trust and the Platte River Recovery Imple- similar but artificially wide MUCWs, given mentation Program. Similarly, Kinzel et al. cur rent hydrological conditions (Williams 1978, (2005) examined results at several stream flows Currier 1997, Pfeiffer and Currier 2005). It is and reported that the largest amount of avail- also important to note that our analyses of unable roosting habitat in channels that are gen- obstructed channel width measurements sug - erally 250 m wide occurs when the stream gest that regardless of flow, wide unvegetated flow is about 35–40 cms (~1200–1400 cfs). channel widths represent selected roosting Prince (1990) and Simons and Associates Inc. habitat for both medium and large Sandhill (1990), who used a Geographic Information Crane roosts, which make up the majority of System and hydraulic modeling approach, also Sandhill Cranes counted along the Big Bend estimated that optimum flows for Sandhill Reach of the Platte River. Sustained flows dur- Crane roosting habitat were between 30 cms ing the summer months help maintain these and 40 cms. Based on several models origi- wide-open channels. Reserving and retiming nally developed by the Platte River Manage- flows that are not necessary during Sandhill ment Joint Study Biology Workgroup (1990), Crane and Whooping Crane migrations to optimum flows for Sandhill Crane roosting meet the aforementioned objectives may pro- habitat on the Platte River near Grand Island, vide tangible ecological benefits to both Nebraska, were generally between 30 cms and species and save conservation resources by 50 cms. Currier and Eisel (1984), however, con- reducing the need for mechanical intervention cluded that marginal roosting habitat was avail- to maintain wide channels during and follow- able on the central Platte River at lower flows ing dry summers. (i.e., ≤30 cms), while adequate and ideal habitat was provided at the highest flows (i.e., ≥57 ACKNOWLEDGMENTS cms) within a 150-m-wide channel required by Sandhill Cranes, but these assessments We thank all members of the Platte River were based on qualitative judgements rather Re covery Implementation Program’s Techni- than on statistical analyses (Safina et al. 1989). cal Advisory Committee for their helpful and BAASCH ET AL. ♦ SANDHILL CRANE ROOST-SITE SELECTION 11 insightful comments. The Platte River Whoop - CURRIER, P.J., AND L.M. EISEL. 1984. The impact of flow ing Crane Maintenance Trust, the U.S. Fish level on Sandhill Crane and Whooping Crane roosting habitat on the Platte River, Nebraska. Whooping and Wildlife Service Rainwater Basin Joint Crane Habitat Maintenance Trust, Grand Island, NE. Venture, and the Platte River Recovery Imple- CURRIER, P.J., AND R.J. HENSZEY. 1996. 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UNGER, A.M., E.P. TANNER, C.A. HARPER, P.D. KEYSER, estimated unvegetated channel widths in a braided F. T. V AN MANEN, J.J. MORGAN, AND D.L. BAXLEY. river system: a Platte River case study. Geomorphol- 2015. Northern Bobwhite seasonal habitat selection ogy 278:163–170. on a reclaimed surface coal mine in Kentucky. Jour- WILLIAMS, G.P. 1978. The case of the shrinking channels: nal of the Southeastern Association of Fish and the North Platte and Platte Rivers in Nebraska. Volume Wildlife Agencies 2:235–246. 781. Department of the Interior, Geological Survey. [USFWS] UNITED STATES FISH AND WILDLIFE SERVICE. WOOD, S. 2006. Generalized additive models: an intro- 1981. The Platte River ecology study. Special re - duction with R. Chapman and Hall/CRC, New York, search report. Jamestown, ND. NY. https://doi.org/10.1201/9781420010404 VOGEL, J.R., D.W. GRIFFIN,, H.S. IP, N.J. ASHBOLT, M.T. MOSER, J. LU, AND J.W. SANTO DOMINGO. 2013. Received 19 June 2018 Impacts of migratory Sandhill Cranes (Grus Revised 23 August 2018 canadensis) on microbial water quality in the Central Accepted 5 September 2018 Platte River, Nebraska, USA. Water, Air, and Soil Published online 23 April 2019 Pollution 224:1576. WERBYLO, K.L., J.M. FARNSWORTH, D.M. BAASCH, AND P. D . F ARRELL. 2017. Investigating the accuracy of Monographs of the Western North American Naturalist 11, © 2019, pp. 14–23

Adult Whooping Crane (Grus americana) consumption of juvenile channel catfish (Ictalurus punctatus) during the avian spring migration in the Central Platte River Valley, Nebraska, USA

ANDREW J. CAVEN1,*, JENNA MALZAHN1, KEITH D. KOUPAL2, EMMA M. BRINLEY BUCKLEY3, JOSHUA D. WIESE1, RICK RASMUSSEN4, AND CAROL STEENSON5

1Platte River Whooping Crane Maintenance Trust, Wood River, NE 68883 2Nebraska Game and Parks Commission, Kearney, NE 68845 3Department of Communication, University of Nebraska at Kearney, Kearney, NE 68849 4Platte River Photography, Doniphan, NE 68832 5Retired. Department of Radiology, University of Minnesota, Minneapolis, MN 55455

ABSTRACT.—Stopover sites provide important forage resources and protection from predators to the Aransas-Wood Buffalo population of Whooping Cranes (Grus americana) as they migrate 4000 km across the Great Plains each spring and fall. Given the Whooping Crane’s expansive migration corridor, sensitivity to human disturbance, small population size, and protected status under the Endangered Species Act, it is challenging to gather detailed information regarding the particular forage resources that the cranes exploit at various stopover locations. On 22 March 2018 we observed and photo-documented an adult Whooping Crane consuming at least 5 individual juvenile channel catfish (Ictalurus punctatus) after it landed 100 m in front of our Sandhill Crane viewing blind on the south channel of the Platte River. Using the average exposed culmen length of an adult Whooping Crane for reference, we estimated that the length of the channel catfish ranged from 97 mm to 117 mm. Growth estimates developed from the Lower Platte River suggest that the depredated channel catfish were just over one year old. To the best of our knowledge, our observations represent the first definitive record of a Whooping Crane consuming fish in the Platte River, as well as the first record of a Whooping Crane depredating a channel catfish in the Great Plains. Given the relatively long distances at which Whooping Cranes are generally viewed (≥650 m), small-bodied fish may be a more common prey item during migration than indicated by current scientific literature. Our note demonstrates how wildlife photography and ecotourism can contribute to our understanding of species’ natural histories.

RESUMEN.—Las zonas de descanso ofrecen importantes recursos para forrajeo y protección contra depredadores a la población de Grullas Trompeteras (Grus americana), ya que estas migran 4000 km a través de las Grandes Llanuras cada primavera y otoño. Debido a la extensión de su ruta migratoria, la sensibilidad a la perturbación humana, la población reducida y el estado de protección bajo la Ley de Especies en Peligro de Extinción en la que se encuentra esta especie, resulta complejo la recopilación de información detallada acerca de los recursos que las grullas trompeteras explotan para forrajear, en las diferentes zonas de descanso. El 22 de marzo del 2018 detectamos y docu- mentamos fotográficamente una grulla trompetera adulta consumiendo por lo menos 5 ejemplares juveniles de peces gato (Ictalurus punctatus), después de aterrizó a 100 m de nuestra ventana con vistas a las Grullas Canadienses del canal Sur del río Platte. Tomando como referencia la longitud promedio del culmen de una grulla canadiense, esti- mamos que la longitud del pez gato varió de 97 mm a 117 mm. Las estimaciones de crecimiento obtenidas del río Lower Platte sugieren que el pez gato depredado tenía más de un año de edad. Hasta donde conocemos, nuestras observaciones representan el primer registro definitivo de una grulla canadiense en el río Platte, así como el primer registro de una grulla canadiense depredando a un pez gato en las Grandes Llanuras. Debido a que generalmente las Grullas Canadienses son detectadas a distancias relativamente largas (≥650 m), los peces de cuerpo pequeño pueden ser un objeto de presa más común durante los períodos migratorios de lo que indica la literatura científica actual. Nue- stro artículo demuestra cómo la fotografía de vida silvestre y el ecoturismo pueden contribuir a comprensión de la his- toria natural de las especies.

*Corresponding author: [email protected]

AJC  orcid.org/0000-0002-5482-8191 EMBB  orcid.org/0000-0003-4438-7094

14 CAVEN ET AL. ♦ WHOOPING CRANE CONSUMES CHANNEL CATFISH 15

Whooping Cranes (Grus americana) under- habitat throughout the migration corridor take a biennial north–south migration of approx- (Stah lecker 1997). imately 4000 km through the Great Plains of Forage patterns have been more thoroughly North America (Kuyt 1992, Pearse et al. researched on wintering and breeding grounds 2018). Their migration corridor averages about than they have been during migration. Whoop - 300 km in width and traverses the Central ing Crane diet on the wintering grounds in Platte River Valley (CPRV), Nebraska (Pearse coastal Texas is well studied, revealing that et al. 2018). The U.S. Fish and Wildlife Ser- the species primarily subsists on blue crabs vice designated the Platte River Bottoms be - (Callinectes sapidus), common fiddler crabs tween Lexington and Denman, Nebraska, as (Uca pugilator), razor clams (Tagelus plebeius), one of the 5 critical habitats in the Central snails (Littorina spp., Melampus coffeus, and Flyway for the Aransas-Wood Buffalo Popula- Cerithidea pliculosa), crayfish (Cambarus spp.), tion of Whooping Cranes (USFWS 1978). The acorns (Quercus spp.), and wolfberry (Lycium Platte River is a braided prairie river charac- carolinianum) fruits, while occasionally consum - terized by shifting exposed and submerged ing vertebrates such as snakes (Nerodia clarkii sandbars that provide night roosting and for- clarkii) (Allen 1952, 1954, Uhler and Locke aging habitat for both Sandhill Cranes (Antigone 1970, Blankinship 1976, Hunt and Slack 1989, canadensis) and Whooping Cranes (Smith Chávez-Ramírez 1996, Westwood and Chávez- 1971, Currier and Ziewitz 1987, Farmer et al. Ramírez 2005, Greer 2010, Geluso and Harner 2005, Kinzel et al. 2009). Historically, the 2013). Data from the breeding grounds at main channel of the Platte River was bor- Wood Buffalo National Park in Canada sug- dered by prairie habitats and exceeded 1.6 km gests that they forage on fish (Culaea incon- in width in many locations, but it has become stans, Pimephales promelas, and Cyprinidae), narrower and more wooded with the appropri- snails (Lymnaea stagnalis and Helisoma spp.), ation of river flows for human use (Williams dragonflies (Libellula spp. and Aeshna spp.), 1978, Currier 1982, Johnson 1994). and diving beetles (Rhantus binotatus, Acilius Today the CPRV landscape is a mosaic of semisulcatus, Graphoderus occidentalis, and irrigated agricultural fields, riparian wood- Dy tiscus alaskanus) (Novakowski 1966, Berge- lands, lowland tallgrass prairies, wet meadows, son et al. 2001, Sotiropoulos 2002, Classen and linear wetlands called “sloughs” (Currier 2008). Incidental accounts from reintroduced 1982, 1989, Kaul et al. 2006, Brei and Bishop populations have discovered Whooping Cranes 2008, Whiles et al. 2010, Krapu et al. 2014, consuming amphibians (Ranidae) and reptiles, Caven et al. 2017). The diversity of wetlands including turtles (Chelydra serpentina, Kinos- and agricultural fields in the CPRV provides ternon subrubrum) (Zimorski et al. 2013, Dinets quality stopover habitat for Whooping Cranes 2016). However, much less is known regarding (Lingle et al. 1991, Chávez-Ramírez and Weir the prey items selected by Whooping Cranes 2010, Jorgensen and Dinan 2016, Pearse et al. during migration (Allen 1952, Austin and 2017). Stopover sites are necessary for migrat- Richert 2001). During migration, Whooping ing birds as they provide forage resources and Cranes have been recorded eating agricultural protection from predators (Alerstam and Hög- waste grains such as sorghum (Sorghum vul- stedt 1982, Newton 2006). Whooping Cranes gare), wheat (Triticum aestivum), and corn (Zea prefer wide, unobstructed view widths away mays); nutsedge tubers (Cyperus spp.); inver- from forests, channel widths over 150 m, vege - tebrates including beetles, mollusks, and cray- tation under 1.5 m in height, and a lack of fish; as well as vertebrates including frogs human disturbances (Lingle et al. 1991, Faanes (Lithobates blairi), fish, snakes, salamanders, 1992, Faanes et al. 1992, Farmer et al. 2005, and small mammals (Allen 1952, USFWS 1978, Howlin and Nasman 2017, Pearse et al. 2017). 1981, Kauffeld 1981, Howe 1987, Kuyt 1987, One of the major risks facing Whooping Cranes Austin and Richert 2001, 2005, Geluso et al. is the loss of wetland habitats to develop - 2013). However, most of these data comes ment (Meine and Archibald 1996). Stahlecker from incidental observations made from long (1992) found that there were 4 or fewer suit- physical distances; therefore, very little species- able wetland roosting locations per 100 km2 of specific information exists regarding the vari- the migration corridor through Oklahoma. By ety of prey items consumed by Whooping contrast, Nebraska contains suitable stopover Cranes during migration. In this report, we 16 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 14–23

Fig. 1. Photo of an adult Whooping Crane (Grus americana), with a channel catfish (Ictalurus punctatus; estimated at just over one year old) in its bill, moving among Sandhill Cranes (Antigone canadensis) on 22 March 2018 at about 10:30 on the main channel of the Platte River, Hall County, Nebraska, USA. Photo by C. Steenson. add to the sparse information concerning imagery from 2016 (ESRI 2017, RWBJV 2017), Whooping Crane diet during migration by de- we estimated that the river channel was about scribing an observation novel to the scientific 450 m wide in this location. The Whooping literature regarding an adult feeding on multi- Crane walked along the edge of an exposed ple juvenile channel catfish (Ictalurus puncta- sandbar slowly bobbing its head, a physical tus) in the CPRV. behavior associated with foraging thought to On 22 March 2018, we observed an adult aid in gaze stabilization and prey detection Whooping Crane consuming juvenile channel (Cronin et al. 2005, 2007). At about 10:30 we catfish within the south channel of the Platte first detected the Whooping Crane catching a River (40°4545 N, 98°3045 W; 600 m eleva- small fish (Fig. 1). The Whooping Crane was tion; Figs. 1, 2). The viewing blind was located detected catching and consuming at least 4 on the south bank of the channel on the edge more individual fish over the next 1.75 h before of a 37.4-ha restored prairie. Ten photogra- it took flight and left our field of view (Fig. 2). phers entered the viewing blind about 1 h The Whooping Crane’s behaviors were photo- before sunrise (06:35) to photograph roosting documented using a 600 mm Nikon lens and a Sandhill Cranes (Antigone canadensis). At 100–400 mm Canon lens with a 1.4× extender. approxi mately 10:15 a Whooping Crane with The Whooping Crane appeared to be forag- adult plumage landed near a group of Sandhill ing in between 5 cm and 25 cm of water based Cranes about 100 m in front of the viewing on the level to which the tarsus was submerged blind. Weekly aerial Sandhill Crane surveys in photos (tarsus length: female mean = 27.7 cm estimated that there were about 58,000 Sand- [n = 7], male mean = 28.1 cm [n = 15]; hill Cranes within 1600 m of the viewing blind, Johnsgard 1983; Fig. 2). We estimated that the one of the highest densities on the river (Crane Whooping Crane spent between 70% and 75% Trust unpublished data). Using the “measure of its time foraging, with the remainder of its tool” in ArcGIS 10.5.1 and digitized aerial time dedicated to preening and interspecific CAVEN ET AL. ♦ WHOOPING CRANE CONSUMES CHANNEL CATFISH 17

Fig. 2. Whooping Crane (Grus americana), capturing a channel catfish (Ictalurus punctatus; estimated at just over one year old) in its bill, surrounded by onlooking Sandhill Cranes (Antigone canadensis) on 22 March 2018 at about 10:45 on the main channel of the Platte River, Hall County, Nebraska, USA. Photo by C. Steenson. agonistic behavior with Sandhill Cranes (see mation) had an average total length of 84 mm Ellis et al. 1998 for a description of agonistic (range 69–101 mm) and that 2-year-old chan- behaviors). We identified all 5 fish consumed as nel catfish ranged from 137 to 168 mm within juvenile channel catfish from a series of 19 pho- the Lower Platte River from 1988 to 1991. tos taken by 2 photographers noting the fol- These results suggest that the channel catfish lowing features: the presence of a forked cau- the Whooping Crane was consuming were just dal fin, the presence of an adipose fin, a lack of over 1 year old. scales, the body-depth-to-length ratio, and the North American Ictaluridae, including chan- placement of pectoral fins (Page and Burr 2011; nel catfish, are territorial and communicate Fig. 2). We approximated the length of the through chemical signals in low-light habitats channel catfish by using the average exposed (Bryant and Atema 1987, Jamzadeh 1992). culmen length (upper bill, 13.8 cm) of adult However, Brown et al. (1970) found that chan- Whooping Cranes (culmen length: female mean nel catfish ≤10 months old gathered in the = 13.7 cm [n = 7], male mean = 13.9 cm [n = bottom of pools during the day when struc- 15]; Johnsgard 1983). We estimated that the tural cover was absent; they also gathered dur- channel catfish ranged from 70% to 85% of the ing all hours of the day when temperatures crane’s exposed culmen length, resulting in were below 4 °C. Ambient temperatures were esti mated total body lengths ranging from as low as 8 °C and water temperatures were as 97 mm to 117 mm. Holland et al. (1992) found low as 5.7 °C during our observations (USGS that 1-year-old channel catfish (at annulus for- 2018, Weather Underground 2018), suggesting 18 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 14–23 that any aggregation behavior was more likely atromaculatus), shiner species (Notropis spp.), to have resulted from habitat structure or pre- the brassy minnow (Hybognathus hankinsoni), dation pressure. In the absence of shelter, it is and topminnow/killifish species (Fundulus spp.). possible that these juvenile channel catfish Stable isotope analysis of Whooping Crane were aggregated in the bottom of a shallow feathers suggests that fish are an important pool within the wide and open braided river prey item for the Aransas-Wood Buffalo popu- channel. Power (1984) found that the spatial lation (Duxbury and Holroyd 1996). Bergeson distribution of armored catfish (Loricariidae) et al. (2001) directly observed Whooping Cranes appears to be determined more by avian preda - consuming brook sticklebacks (Culaea incon- tor avoidance than by available food resources. stans) on their breeding grounds in Wood Buf- Though fish are not common forage resources falo National Park. Brook sticklebacks are also for Sandhill Cranes, cranes have occasionally present in the Platte River Basin (Goldowitz been detected depredating fish in the Platte 1996). Research from the breeding grounds River (Lewis 1979, Mergler Niemeier and also suggests that pond habitats containing Niemeier 1982). Given that Sandhill Crane small-bodied fishes such as brook stickleback, abundance can exceed 600,000 individuals dur- fathead minnow (Pimephales promelas), and ing the spring migration staging period in dace species (Phoxinus spp.) received higher mid to late March (Dubovsky 2018), it is pos- Whooping Crane use than ponds dominated sible that channel catfish exhibit predator by invertebrate species (Bergeson 1998, Classen avoidance aggregation behavior as a response 2008). On their wintering grounds, Whooping to perceived Sandhill Crane predation risk. It Cranes have also been observed consuming is also feasible that there was simply an abun- fish, including topminnow/killifish, sheepshead dance of juvenile channel catfish in this micro - minnow (Cyprinodon variegatus), flathead grey habitat. As Phelps et al. (2011) notes, juvenile mullet (Mugil cephalus), and speckled worm-eel channel catfish in the Mississippi River pre- (Myrophis punctatus) (Allen 1952, Greer 2010). ferred shallow islands and off-channel habitats Channel catfish are important prey items with sandy substrate and slow-flowing water. for a number of wading bird species, such as Finally, it is conceivable that Whooping Cranes Great Blue Herons (Ardea herodias) and Great selected for juvenile channel catfish among Egrets (Ardea alba) (Stickley 1995, Glahn et the available small-bodied fishes because the al. 1999, 2000, Werner et al. 2001, Dorr and catfish represented preferred forage. Research Taylor 2003). According to Stickley (1995) and regarding Double-crested Cormorants (Pha- Glahn et al. (1999), Great Blue Herons showed lacrocorax auritus) suggests that channel cat- a preference for smaller-sized channel catfish fish are easily digestible and provide greater ranging from 110 to 160 mm. Glahn et al. energy content than gizzard shad (Dorosoma (1999) and Werner et al. (2001) similarly found cepedianum) or bluegill (Lepomis macrochirus) that Great Egrets preferred catfish ranging (Brugger 1993). from 75 to 103 mm. Our observations suggest To the best of our knowledge, our observa- that Whooping Cranes select channel catfish tions represent the first scientific record of a sizes similar to those selected by Great Blue Whooping Crane depredating a channel catfish Herons and Great Egrets (97–117 mm). Willard in the Platte River and in the Central Flyway. (1977) found that wading bird species demon- Despite the Platte River supporting 56 species strate similar depth preferences for hunting of fish (Goldowitz 1996, Chadwick et al. 1997), fish; Great Blue Herons and Great Egrets pre- there have been no detailed reports of Whoop - ferred hunting depths of 15 to 25 cm, while ing Cranes depredating fish there. However, Snowy Egrets and Little Blue Herons gener- Whooping Cranes have been re ported con- ally foraged in shallower water of 5 to 15 cm. suming fish (unidentified) in the Rainwater Our observations again demonstrated signifi- Basin, just south of the CPRV (USFWS 1981). cant overlap in fish foraging depths between Research suggests that fish are a potentially Whooping Cranes (5 to 25 cm) and wading important food source for Whooping Cranes birds in the family Ardeidae. Water depth data during migration (Armbruster 1990), but only collected from Whooping Crane stopover sites Allen (1952) discusses potential prey species suggest that on average Whooping Cranes including the white sucker (Catostomus com- roost and forage in 14 to 20 cm of water (Howe mersonii sucklii), the creek chub (Semotilus 1987, 1989, Pearse et al. 2017). Both Great CAVEN ET AL. ♦ WHOOPING CRANE CONSUMES CHANNEL CATFISH 19

Blue Herons and Great Egrets were predomi- classification of forage (Lingle et al. 1991, Jor- nantly observed depredating channel catfish gensen and Dinan 2016). Therefore, it is pos- in the spring and fall when catfish diseases, sible that small-bodied fish species and the such as enteric septicemia, were more prevalent young of larger fish species may be relatively (Stickley 1995, Glahn et al. 1999, 2000). Dis- common prey items for Whooping Cranes dur - ease could reduce the ability of channel catfish ing migration but may also be underrepre- to escape and defend themselves from de pre - sented in scientific records due to low detec- dation attempts (Glahn et al. 2000). Whooping tion probability. Crane migration may overlap with periods of Given the challenges of studying the be hav - channel catfish disease prevalence in the CPRV, ior of this cryptic endangered species during making the catfish a more easily attainable migration, incidental observations play an im- food source. portant role in furthering our understanding Stopover sites throughout the Central Fly- of Whooping Crane natural history, and such way provide Whooping Cranes with necessary observations provide an opportunity for citi- energy reserves and safe refuge during migra- zen scientists to make significant contributions tion (Alerstam and Högstedt 1982, Stahlecker (Geluso and Harner 2013, Geluso et al. 2013). 1992, 1997, Meine and Archibald 1996, Austin During the early spring, thousands of people and Richert 2001, Newton 2006, Pearse et al. flock to the CPRV to observe the Sandhill 2017). This observation underscores the impor- Crane migration (Dority et al. 2017); the num- tance of managing the CPRV holistically to ber of tourists watching the river is vastly benefit a broad swath of native species, such greater than the sum of researchers on the as channel catfish, brook sticklebacks, plains ground. Publically reported Whooping Crane leopard frogs (Lithobates blairi) and others sightings and associated photographs provide that fill niches in the ecosystem’s food web a rich information repository, which can be and provide important forage resources to used to better understand habitat-use patterns endangered species such as the Least Tern and track changes in the migration corridor (Sternula antillarum) and the Whooping Crane over time (Austin and Richert 2001, Niemuth (Wilson et al. 1993, Bergeson et al. 2001, et al. 2018, Pearse et al. 2018). Photographs Sherfy et al. 2012, Geluso et al. 2013). also provide information regarding Whooping Despite the importance of gathering infor- Crane behavior and ecology, such as the docu- mation on Whooping Crane forage resources mentation of prey items and potential preda- during migration, systematically studying crane tors (Geluso et al. 2013, Geluso and Harner behavior at stopover locations presents several 2013, Caven et al. 2018). Publicly sourced pho - challenges. First, Whooping Cranes can be tographs provide a form of visual evidence that challenging to detect even in relatively limited can be validated and further investigated, and ranges such as their wintering grounds (Stro- photographs often provide additional details bel and Butler 2014). Whooping Cranes, num- depending on factors such as resolution, qual- bering about 500 individuals (Butler and Har- ity, and distance (Pimm et al. 2015). Publicly rel 2018), migrate through a corridor spanning sourced images have been a useful conserva- over 100 million ha of the Great Plains (Pearse tion research tool for studying various wildlife et al. 2018), and they avoid human distur- species including sea turtles (Long and Azmi bances (Pearse et al. 2017); they are therefore 2017), whale sharks (Davies et al. 2012), and challenging to locate while in migration. cheetahs (Weise et al. 2017), as well as for Moreover, Whooping Cranes are sensitive to monitoring protected habitat areas (Walden- human disturbances, including people on foot Schreiner et al. 2018). In this case, images of or approaching vehicles (Lewis and Slack an adult Whooping Crane eating juvenile chan- 2008); therefore, government wildlife manage- nel catfish provided documentation of a novel ment authorities require a distance of approxi- forage event and allowed us to visually examine mately 650 m for public observation and most characteristics and approximate measurements research investigations (USFWS and NGPC of the prey. Our research further dem onstrates 2015). Consequently, most behavioral studies the value of publically sourced imagery for natu - have been conducted at distances that only ral history research and provides valuable in- allow for a broad interpretation of foraging be - sight into the variety of diet items ex ploited re - havior in particular habitats but not the visual gionally by Whooping Cranes during migration. 20 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 14–23

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First description of a Bald Eagle (Haliaeetus leucocephalus) attempting depredation on an adult Whooping Crane (Grus americana) of the Aransas-Wood Buffalo population

MATTHEW R. RABBE1,*, ANDREW J. CAVEN2, AND JOSHUA D. WIESE2

1United States Fish and Wildlife Service, Nebraska Ecological Services Office, Wood River, NE 68883 2Platte River Whooping Crane Maintenance Trust, Wood River, NE 68883

ABSTRACT.—Twice annually, the last remaining wild and self-sustaining migratory population of Whooping Cranes (Grus americana) migrates through central Nebraska on its approximately 3900-km journey between Aransas National Wildlife Refuge on the Gulf Coast of Texas and Wood Buffalo National Park in Alberta, Canada. On 27 March 2018, a juvenile Bald Eagle (Haliaeetus leucocephalus) was observed attacking a Whooping Crane on the Loup River near Rockville, Nebraska. The encounter, documented by a private landowner, was forwarded to the U.S. Fish and Wildlife Service as part of the decades-long citizen-science effort undertaken to track and record public sightings of Whooping Cranes during migration. Crane species have few avian predators, and observations of depredations upon these crane species are rare. The Whooping Crane fended off the Bald Eagle, utilizing a “jump-rake” defense; neither species appeared harmed by the clearly aggressive interaction. The episode was reflective of recent observations of Bald Eagles depredating Sandhill Cranes on the Platte River during the spring migration. To our knowledge, this is the first description in scientific literature of a Bald Eagle attacking a Whooping Crane from the Aransas-Wood Buffalo population.

RESUMEN.—Dos veces al año, la población migratoria remanente de grullas trompeteras (Grus americana) salvajes y autosuficientes migra por el centro de Nebraska, en un viaje de aproximadamente 3900 km entre el Refugio Nacional de Vida Silvestre Aransas (en la costa del golfo de Texas) y el Parque Nacional Wood Buffalo (Alberta, Canadá). El 27 de marzo de 2018, se observó a un águila calva joven (Haliaeetus leucocephalus) atacando a una grulla trompetera en el río Loup cerca de Rockville, Nebraska. El encuentro, documentado por un vecino de la zona, fue enviado al Servicio de Pesca y Vida Silvestre de los EE.UU. como parte de una labor científica realizada durante décadas por ciudadanos comunes cuyo objetivo es el de rastrear y registrar avistamientos públicos de grullas trompeteras durante sus períodos migratorios. Estas especies de grullas tienen pocos depredadores aviares, por lo que es raro observar actos de depredación sobre ellas. La grulla trompetera se defendió del águila calva mediante una defensa de tipo “rastrillo”; sin embargo, ninguna de las especies pareció dañarse ante esta interacción claramente agresiva. El episodio fue el reflejo de recientes observaciones de águilas calvas depredando a grullas canadienses en el río Platte durante su migración en primavera. Hasta donde sabemos, esta es la primera descripción en la literatura científica de un águila calva atacando a una grulla trompetera de la población de Aransas-Wood Buffalo.

The Whooping Crane (Grus americana; species has sustained considerable long-term WHCR), federally listed as endangered growth with numbers now reach ing about under the U.S. Endangered Species Act 500 individuals (Butler and Harrell 2018). (ESA; 93rd U.S. Congress 1973, as amended), The species’ biannual migration through the is one of North America’s rarest and most Great Plains includes large portions of cen- iconic avian species (Gray and Harrell tral Nebraska, where a remaining network 2017). The Aransas-Wood Buffalo popula- of intact wetlands and rivers is used for tion, the only wild and self-sustaining migra- principal stopover habitat on the approxi- tory population, numbered as few as 16 mately 3900-km journey the species makes birds in 1941. Recent estimates suggest the every spring and fall (Lingle et al. 1991,

*Corresponding author: [email protected]

MRR  orcid.org/0000-0002-9322-4374 AJC  orcid.org/0000-0002-5482-8191

24 RABBE ET AL. ♦ BALD EAGLE ATTACKING A WHOOPING CRANE 25

Stahlecker 1997, Farmer et al. 2005, Pearse also used by Bald Eagles (Haliaeetus leuco- et al. 2017). Stopover sites provide neces- cephalus; BAEA) throughout the year as sary caloric resources and secure roosting breeding, migratory, and wintering habitat. places for WHCRs throughout their migra- In Nebraska, BAEAs are most abundant dur- tion corridor (Alerstam and Högstedt 1982, ing their spring migration from late February Newton 2006). to early March (Sharpe et al. 2001). Mature Nebraska is known for having more miles eastern cottonwoods (Populus deltoides) adja- of river than any other state, and is the only cent to the Loup River offer perching trees state within the migration corridor where and nesting habitat in close proximity to prey WHCR use of riverine stopover sites may sources preferred by BAEAs whose use of the outnumber palustrine or lacustrine wetland area has expanded over time (Steenhof et al. stopover sites (Austin and Richert 2005). 1980, Anthony and Isaacs 1989, Buehler et al. Some of the earliest and most frequent migra- 1992, Sharpe et al. 2001, Bishop et al. 2011, tory records of WHCRs were in Nebraska Jorgensen and Dinan 2018). BAEAs were for- (Allen 1952). As conservation efforts and aware- merly on the ESA, but range expansion and ness increased and the ESA was enacted into an increasing population led the U.S. Fish and law, the United States Fish and Wildlife Ser- Wildlife Service (USFWS) to remove them in vice (USFWS) cataloged historic records of 2009, though they remain protected under WHCR sightings. Opportunistic sightings of the Bald and Golden Eagle Protection Act WHCRs were collected and recorded using a (USFWS 2018a). citizen-science approach, wherein a network On 27 March 2018, a juvenile BAEA was of state and federal contacts gathered, investi- observed and photographed attacking a gated, and recorded Whooping Crane sight- WHCR on the Loup River near Rockville, ings during migration (Niemuth et al. 2018). Nebraska, midway through the WHCR’s 29-d Over time, these efforts increased and the stopover (John Conkin, Canadian Wildlife scope of collected information expanded to Service, personal communication). The attack include descriptions of the behaviors observed, provides novel information regarding the site characteristics, circumstances of observa- behavioral strategies of both species. The tion (e.g., binoculars at 600 m), and photo- report was forwarded by a local landowner graphic or video documentation (if available). on the Loup River who had observed the The effort collectively became referred to as group many times previously. At approximately the Cooperative Whooping Crane Tracking 17:30, the landowner concealed himself near Project (WCTP), and the records are updated a previously known roost location on the with each spring and fall migration. Con- bank of the Loup River. At this point, the firmed sightings include those documented WHCRs were about 400 m away. A short by photographic or video records, by a pro- time later, they flew up and over the ripar- fessional biologist or qualified individual, or ian forest to an adjacent cornfield. At 19:00, from observer interviews and circumstances after feeding for an hour, the WHCRs got up surrounding the sighting. and landed on the river directly in front of The Loup River system in Nebraska the landowner at a distance of 75 m (approxi- (North, South, Middle, and main-stem Loup mate coordinates: 41.141598°, −98.876501°). River collectively) is a tributary of the Platte There, he began closely observing them. The River and part of a vast network of rivers WHCRs were engaged in both social displays within the state that provide habitat for and bathing in the water, and did not appear WHCRs (Sharpe et al. 2001). The Loup River alert, vigilant, or disturbed in any way. At is primarily a spring-fed, sandy-bottom braided 19:15, a single juvenile BAEA flew over river, with wide channels containing shallow them, causing a sudden change in behavior. sandbars that are used as night-roosting and The family group instantly became alert and diurnal-foraging habitat by migrating WHCRs alarmed, adopting the characteristic alert (Stahlecker 1997, Austin and Richert 2005). posture described by Ellis et al. (1998), with The neighboring landscape consists of agri- their heads extended upward and somewhat cultural fields, lowland grasslands and wet forward, watching and following as the BAEA meadows, and riparian forest (Kaul et al. flew overhead (Fig. 1). Moments later, a sec- 2006, Bishop et al. 2011). The Loup River is ond juvenile BAEA was observed in the 26 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 24–32

Fig. 1. A family group of Whooping Cranes (Grus americana) displaying alert behavior as a 2-year-old Bald Eagle (Haliaeetus leucocephalus) hovers overhead on 27 March 2018 within the main channel of the Middle Loup River, Sher- man County, Nebraska, USA. Photo by L. Burman. vicinity. At approximately 19:30, another fly- Water depth varied from 0 cm deep (exposed over by one of the 2 BAEAs prompted the sandbar) to approximately 10 cm at the landowner to try to take a picture of the location of the attack. Data from the WCTP BAEA. As the landowner clicked the shutter, and radio-tracking telemetry data indicate the BAEA dove straight down and attacked repeated high use of the Loup River by one of the adult WHCRs. The encounter was WHCRs (Austin and Richert 2005, Pearse et brief and the attempted depredation was al. 2015, USFWS 2018b). The Loupe River is unsuccessful as the attacked adult WHCR likely used annually, though efforts to docu- engaged with the BAEA, utilizing an agonis- ment its use in WCTP are less intensive than tic “attack and mob” behavior termed a in other areas of Nebraska such as the Platte “jump-rake” by Ellis et al. (1998) and Heatley River (Hefley et al. 2015). (2002). The WHCR leaped into the air with We observed 2 BAEAs during the attack its wings spread wide and met the BAEA, and aged them using the plumage molt pro- slashing at its opponent with its talons (Fig. gression portrayed by McCollough (1989). The 2). The BAEA is believed to have made brief BAEA involved in the attack was a 2-year-old physical contact with the crane’s wing, though juvenile and the second BAEA was a 1-year- neither bird exhibited signs of injury. old juvenile. The WHCRs did not immediately The events were photographed by using a attempt to fly away and remained in the vicin- Canon Eos 60D camera equipped with a ity following the attack. Upon review of the 150–600-mm lens (images captured using a photographs, the juvenile WHCR was noted 600-mm lens). Following the report, we vis- as having GPS tracking equipment on its left ited the site of the observation to visually leg with the identification 5A (ID: 2017-5A). assess habitat conditions. The river channel Given that the individual was <1 year old, we at this location was approximately 215 m determined that it had been recently affixed wide (measure tool, Google Earth Pro 2018). with radio-tracking equipment and was likely RABBE ET AL. ♦ BALD EAGLE ATTACKING A WHOOPING CRANE 27

Fig. 2. A second-year Bald Eagle (Haliaeetus leucocephalus) engaged in attack on a Whooping Crane (Grus americana) as it defends itself using the “jump rake” agonistic behavior on 27 March 2018 in the main channel of the Middle Loup River, Sherman County, Nebraska, USA. Photo by L. Burman. transmitting at the time of its stay in Nebraska. 5.85, n = 3245, 1942–2018). However, in Whooping Crane 2017-5A was banded in Nebraska, an average length of stay for the 2017 as a chick on its breeding grounds and population was 4.04 d (SD = 6.85, n = 801, likely traveled with its family throughout the 1942–2018). The individuals’ encounter with spring migration north (John Conkin, Cana- the BAEA described here occurred near the dian Wildlife Service, personal communica- midpoint of the stopover, 13 d after they first tion). Radio-tracking data indicated that the arrived in the area. During the remaining por- crane group departed their previous known tion of their stay, they were observed numer- stopover location near Coats, Kansas, at approxi- ous times by the public with no discernable mately 09:30 on 14 March 2018, and arrived injuries noted (WCTP 2018). It remains un - on the Middle Loup River at approximately known whether the attack had any lasting 17:50 that same day. On 27 March, shortly impact on the cranes’ length of stay or the after the attack occurred, radio-tracking data remainder of their migration. indicated that the family group permanently BAEA populations have continued to moved roost locations approximately 2.4 km increase in size throughout Nebraska, includ- upstream from their former roosting area and ing on the Loup River system (Sharpe et al. stayed an additional 16 evenings before de - 2001, Jorgensen and Dinan 2018). BAEAs are parting on 12 April 2018. The 29-d stopover also common year-round in the general vicin- represents one of the longer stopovers docu- ity of the encounter, and they were observed mented during migration in the United States on a follow-up site visit to the area with the (USFWS 2018b). Records from the WCTP landowner on 28 September 2018 (Lawrence (2018) indicate that the average length of stay Burman personal communication). An eagle at stopovers for WHCRs in the Aransas-Wood nest was also observed approximately 900 m Buffalo population during migration through- downstream (41.1352°, −98.86801°) of the out the United States is <3 d (x– = 2.87, SD = attack location. The landowner provided 28 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 24–32 records of nesting on the property since vulnerable (e.g., sick, injured, etc.) (Knight 2014 from memory and photo documenta- and Knight 1983, Stalmaster and Plettner tion, suggesting that the area contains suit- 1992, Lefebvre and Bouchard 2003, Caven et able BAEA habitat (Granger 1992, Bower- al. 2018). However, WHCRs have a diverse man et al. 2002). defensive and agonistic repertoire. They have Juvenile BAEAs are more likely than adults been recorded effectively fending off depreda- to exhibit migratory or nomadic behavior, and tion attempts from a variety of predators using they follow temporospatial prey availability “bill-stab,” “jump-rake,” and “wing-spread” until nesting territories are established in displays (Nesbitt and Archibald 1981, Ellis et adulthood (Hodges et al. 1987). With GPS al. 1996, 1998, Heatley 2002). It is notable that equipment, Wheat et al. (2017) tracked 28 cranes are behaviorally oriented to defend BAEAs of different age classes and social sta- themselves and exhibit no known antipredator tuses and found that movement strategies escape behaviors (Lima 1993). BAEAs have varied among adults (nesting, localized, migra- been filmed “prey testing” WHCRs in Wiscon - tory, or nomadic), while immature BAEAs in sin (part of the Experimental Eastern Migra- the study were all nomadic or migratory. The tory Population; Fuad Azmat / YARNutopia by juveniles involved in this attack may have Nadia 2014). The 2 WHCRs in the film dis- migrated to the area, following prey availabil- played agonistic defensive and territorial be - ity. Juvenile BAEAs of multiple age classes haviors described by Ellis et al. (1998) and were recently associated with the depredation Heatley (2002) in the presence of 5 BAEAs and consumption of Sandhill Cranes (SACRs) (adult and juvenile) as the WHCRs competed on the Central Platte River (Caven et al. for access to an appropriate roosting location 2018). BAEAs’ diets are often variable; in in shallow water near a submerged tree, sug- times when resources are scarce, they modify gesting that the adult cranes viewed the BAEAs their diets to exploit readily available prey as a potential depreda tion threat (Fuad Azmat / (Stalmaster and Plettner 1992). Juvenile YARNutopia by Nadia 2014). BAEAs learn foraging techniques from older Golden Eagle (Aquila chrysaetos) depreda- individuals (Knight and Knight 1983, Stal- tion of cranes may be more common; depreda- master and Plettner 1992). Research suggests tion has been documented on multiple SACRs that juvenile and subadult BAEAs are less and one WHCR (Windingstad et al. 1981, Ellis efficient predators and less discerning in prey et al. 1999, Heatley 2002, Stehn and Haralson- selection than adults. They may seek prey Strobel 2014). Ellis et al. (1999) described items associated with a higher risk of injury numerous successful attempts on SACRs and and experience caloric deficiencies at higher one unsuccessful attempt on a WHCR during rates than adults do, and therefore experience aircraft-led migration flights associated with higher rates of mortality (Fischer 1982, Harper the Eastern Migratory Population of WHCRs 1983, Stalmaster and Gessaman 1984, Stal- (experimental reintroduced population); the master and Plettner 1992). Additionally, forag- WHCR was struck by a Golden Eagle but sur- ing locations occupied by other BAEAs are vived and recovered. Windingstad (1981) doc- often selected over unattended sites (Knight umented a successful attack on a single juve- and Knight 1983, Stalmaster and Gessaman nile WHCR within the Grays Lake National 1984). In this way, multiple individuals may Wildlife Refuge Population (formerly reintro- congregate where abundant resources are duced experimental population now extir- being exploited. pated) wherein the individual, migrating with Large prey such as WHCRs have high its foster SACR parents, was killed by a edible and caloric contents, and similar large Golden Eagle on the individual’s first south- prey contribute to most of the energetic ward migration. Stehn and Haralson-Strobel needs of wintering BAEAs (Stalmaster and (2014) detected talon marks suggestive of Plettner 1992). Given the large concentra- Great Horned Owl (Bubo virginianus) depre- tions of SACRs and other waterfowl that are dation during the necropsy of one WHCR potential prey resources for BAEA during found on the wintering grounds in Texas, spring migration along wooded riparian areas which is the only probable mortality resulting in Nebraska, it is possible that BAEAs could from avian predation recorded for the Aransas exploit WHCRs if they were available or Wood-Buffalo Population of WHCRs. RABBE ET AL. ♦ BALD EAGLE ATTACKING A WHOOPING CRANE 29

During late winter and spring, BAEA num- WHCRs in the reintroduced eastern flock, bers peak in Nebraska as migrating BAEAs particularly due to bobcats (Lynx rufus). concentrate on the river, where abundant food WHCRs prefer wide unobstructed views, supplies include fish and migratory water birds shallow water, and gentle bank slopes for (Lingle and Krapu 1986, Jorde and Lingle roosting (Austin and Richert 2005, Farmer et 1988, Stalmaster and Plettner 1992, Caven et al. 2005, Pearse et al. 2017, Farnsworth et al. al. 2018, Jorgensen and Dinan 2018). Inter- 2018). Models suggest that distance to nearest actions between WHCRs and BAEAs are forest is positively related to WHCR roosting- likely promoted by the increase of cottonwood habitat use on the Platte River (Farnsworth et gallery forests along the banks of rivers in the al. 2018, Baasch et al. 2019). While the loca- Platte River Basin since the late 1800s (Loup, tion where the attack occurred had suitably Platte etc.; Williams 1978, Johnson 1994, Cur- wide channels, the distance to the nearest rier 1997), as well as the robust recovery of trees and other tall vegetation along the BAEA populations within riverine ecosystems banks, where the presence of predators is in Nebraska since the 1970s (Sharpe et al. increased, was approximately 100 m less than 2001, Jorgensen and Dinan 2018). the distance to the nearest forest (181 m) pre- As suggested by Caven et al. (2018) regard- scribed in Baasch et al. (2019). Trees and ing SACRs, it is possible that BAEAs prey other tall vegetation can provide cover and on or attempt to prey on WHCRs more com- habitat for both terrestrial and aerial preda- monly than previously understood. Until tors of waterbirds (Anthony and Isaacs 1989, recently, it was widely believed that even most Buehler et al. 1992, Ruiz-Olmo et al. 2003, SACRs preyed upon by BAEAs were likely Whittingham and Evans 2004, Cole et al. sick, injured, or already dead, and that healthy 2009), which may pose a risk to WHCRs. For- adult cranes were likely too large and aggres- est cover has increased in the Platte River sive to be considered prey by BAEAs (Wood Basin and in other lowland ecosystems in et al. 1993). Given the orders-of-magnitude Nebraska (Currier 1997, Briggs et al. 2002, difference in population sizes of North Amer- Kaul et al. 2006, Bishop et al. 2011). WHCRs ica’s 2 Gruidae species (660,000+ SACRs— face numerous threats during migration Dubovsky 2018; ∼500 WHCRs—Butler and (Lewis et al. 1992, Stehn and Haralson-Stro- Harrell 2018), the implications of recent obser- bel 2014), and recent research suggests that vations (Caven et al. 2018) are of highest con- predation may be a leading cause of mortality cern regarding WHCRs in central Nebraska. among Whooping Cranes (Pearse et al. 2018). WHCRs and SACRs often spatially and tem- Ongoing efforts to remove woody cover to porally overlap during migration (Lingle et al. improve contiguous areas of quality riverine 1991); predators of SACRs can threaten WHCRs roosting habitat (Farnsworth et al. 2018) and as well. Radio-tracking data indicate that the wet-meadow and slough foraging habitat WHCRs observed in this work abandoned (Chávez-Ramírez and Weir 2010) for the ben- their roost following the attack and relocated efit of WHCRs may also reduce predation 2.4 km upstream, suggesting that they no and the occurrence of WHCR–BAEA interac- longer deemed the habitat suitable or safe tions, while benefiting other wet-meadow and after the attack. Though it is possible that braided-river species of concern (Kirsch 1996, WHCRs migrating through wooded rivers in Rosenberg et al. 2016, Caven et al. 2017). A the Great Plains are preyed upon infrequently management approach may be preferred, one by BAEAs, it may be more likely that BAEA that considers the regional habitat needs of recovery, range expansion, and increased pres- both species while also minimizing their ence on the landscape could result in a interaction. For instance, a plan could include reduction of the availability of suitable WHCR clearing trees and reducing the presence of habitat secure from predators. predator perches in some sections of the river Frequent harassment by BAEAs may in- with wide channels, while allowing other sec- crease WHCR movement during stopovers or tions of the river not targeted for WHCR could alter stay length in wooded riverine habitat to maintain aging cottonwood galleries habitats. Cole et al. (2009) linked inappropri- to encourage continued BAEA use at those ate roosting habitat to several predations of locations. 30 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 24–32

ACKNOWLEDGMENTS toring data along the Big Bend of the Platte River, NE. Journal of Insect Conservation 21:183–205. We extend a special thank you to Lawrence CAVEN, A.J., J.D. WIESE, W.R. WALLAUER, AND K.J. MOSHER. 2018. First description of a Bald Eagle Burman who kindly provided information (Haliaeetus leucocephalus) actively depredating an and photographs surrounding this record and adult Sandhill Crane (Antigone canadensis). West- access to his property to conduct follow up ern North American Naturalist 78:216–220. investigations. We also thank the USFWS CHÁVEZ-RAMÍREZ, F., AND E. WEIR. 2010. Wet meadow and the Crane Trust for funding the writing of literature and information review. Technical Report, this natural history note. A special thanks to Platte River Recovery Implementation Program, Kearney, NE. 42 pp. John Conkin, Aaron Pearse, and Dave Brandt COLE, G.A., N.J. THOMAS, M. SPALDING, R. STROUD, R.P. for providing WHCR satellite tracking data. URBANEK, AND B.K. HARTUP. 2009. 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Temporospatial shifts in Sandhill Crane staging in the Central Platte River Valley in response to climatic variation and habitat change

ANDREW J. CAVEN1,*, EMMA M. BRINLEY BUCKLEY2, KELSEY C. KING3, JOSHUA D. WIESE1, DAVID M. BAASCH4, GREG D. WRIGHT5, MARY J. HARNER2, AARON T. PEARSE6, MATT RABBE7, DANA M. VARNER8, BRICE KROHN1, NICOLE ARCILLA1, KIRK D. SCHROEDER7, AND KENNETH F. DINAN7

1Platte River Whooping Crane Maintenance Trust, Wood River, NE 68883 2University of Nebraska at Kearney, Kearney, NE 68849 3Washington State University–Vancouver, Vancouver, WA 98686 4Platte River Recovery Implementation Program, Kearney, NE 68845 5U.S. Forest Service, Nebraska National Forest, Halsey, NE 69142 6U.S. Geological Survey, Northern Prairie Wildlife Research Center, Jamestown, ND 58401 7U.S. Fish and Wildlife Service, Wood River, NE 68883 8Rainwater Basin Joint Venture, Grand Island, NE 68803

ABSTRACT.—Over 80% of the Mid-Continent Sandhill Crane (Antigone canadensis) Population (MCP), estimated at over 660,000 individuals, stops in the Central Platte River Valley (CPRV) during spring migration from mid-February through mid-April. Research suggests that the MCP may be shifting its distribution spatially and temporally within the CPRV. From 2002 to 2017, we conducted weekly aerial surveys of Sandhill Cranes staging in the CPRV to examine temporal and spatial trends in their abundance and distribution. Then, we used winter temperature and drought severity measures from key wintering and early migratory stopover locations to assess the impacts of weather patterns on annual migration chronology in the CPRV. We also evaluated channel width and land cover characteristics using aerial imagery from 1938, 1998, and 2016 to assess the relationship between habitat change and the spatial distribution of the MCP in the CPRV. We used generalized linear models, cumulative link models, and Akaike’s information criterion corrected for small sample sizes (AICc) to compare temporal and spatial models. Temperatures and drought conditions at wintering and migration locations that are heavily used by Greater Sandhill Cranes (A. c. tabida) best predicted migration chronology of the MCP to the CPRV. The spatial distribution of roosting Sandhill Cranes from 2015 to 2017 was best predicted by the proportion of width reduction in the main channel since 1938 (rather than its width in 2016) and the proportion of land cover as prairie-meadow habitat within 800 m of the Platte River. Our data suggest that Sandhill Cranes advanced their migration by an average of just over 1 day per year from 2002 to 2017, and that they continued to shift eastward, concentrating at eastern reaches of the CPRV. Climate change, land use change, and habitat loss have all likely contributed to Sandhill Cranes coming earlier and staying longer in fewer reaches of the CPRV, increasing their site use intensity. These historically unprecedented densities may present a disease risk to Sandhill Cranes and other waterbirds, including Whooping Cranes (Grus americana). Our models suggest that conservation actions may be maintaining Sandhill Crane densities in areas that would otherwise be declining in use. We suggest that management actions intended to mitigate trends in the distribution of Sandhill Cranes, including wet meadow restoration, may similarly benefit prairie- and braided river–endemic species of concern.

RESUMEN.—Más del 80% de la población de grullas canadienses (Antigone canadensis), de la zona central del conti- nente (MCP por sus siglas en inglés), estimada en más de 660,000, descansa en el valle central del Río Platte (CPRV por sus siglas en inglés) durante su migración de primavera, desde mediados de febrero hasta mediados de abril. Diversos estudios indican que su distribución espacial y temporal podría estar cambiando dentro del CPRV. Desde el año 2002 hasta el 2017 realizamos sondeos aéreos semanales de grullas canadienses en el CPRV para estudiar las tendencias

*Corresponding author: [email protected]

AJC  orcid.org/0000-0002-5482-8191 EMBB  orcid.org/0000-0003-4438-7094 KCK  orcid.org/0000-0001-6340-3649 DMB  orcid.org/0000-0003-0747-1449 GDW  orcid.org/0000-0003-4932-9555 ATP  orcid.org/0000-0002-6137-1556

33 34 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 temporales y espaciales relacionadas a su abundancia y distribución. Usamos mediciones de temperatura durante el invierno y de la severidad de la sequía de lugares claves de invernada y de sitios de descanso durante su migración temprana para evaluar el impacto de los patrones climáticos en la cronología migratoria anual del CPRV. También analizamos la amplitud del canal y las características de la cubierta terrestre usando imágenes aéreas de 1938, 1998 y 2016 con el fin de evaluar la relación entre el cambio de hábitat y la distribución espacial de la MCP en el CPRV. Utilizamos modelos lineales generalizados, modelos de enlace acumulativo y el criterio de información de Akaike adecuados a muestras pequeñas (AICc), para comparar modelos temporales y espaciales. Las condiciones climáticas y de sequía en los sitios de invernada y migración más usados por la grulla canadiense mayor (A. c. tabida) predijeron mejor la cronología migratoria de la MCP en el CPRV. La reducción de la amplitud del canal principal desde 1938, junto con el porcentaje de cubierta terrestre como hábitat de pradera dentro de los 800 m del río Platte, fue el mejor predictor de la distribución espacial de la grulla canadiense desde el año 2015 hasta el 2017. Nuestros estudios indican que las grullas canadienses adelantaron su migración en un promedio poco más de un día por año entre el 2002 y el 2017 y que continuaron desplazándose hacia el este, concentrán- dose en los extremos orientales del CPRV. El cambio climático, el cambio de uso del suelo y la pérdida del hábitat proba- blemente contribuyeron a la migración temprana de esta especie y a su permanencia más prolongada en algunos sectores del CPRV, aumentando la intensidad del uso del sitio. Estas densidades sin precedentes podrían presentar un riesgo de enfermedad para la grulla canadiense y otras aves acuáticas, incluidas las grullas trompeteras (Grus americana). Nuestros modelos indican que las medidas actuales de conservación podrían ser la causa de preservación de la densidad poblacional de la grulla canadiense en áreas en las que, de otra forma, su presencia estaría disminuyendo. Sugerimos que las medidas de control destinadas a mitigar la tendencia de distribución de la grulla canadiense, incluyendo la restauración de los prados húmedos, pueden beneficiar de igual manera a las especies endémicas, praderas y ríos trenzados de nuestro interés.

A diversity of avifauna has substantially the MCP funnels through a relatively nar- shifted ranges or migratory paths in response row stretch of the Central Platte River Valley to broad landscape-level changes, such as (CPRV), spanning about 132 km of central increased agricultural production (Svedarsky Nebraska (Kinzel et al. 2006, Krapu et al. et al. 2000, Coppedge et al. 2001b), modifica- 2014). Individual cranes stage in the CPRV for tions in hydrological regimes (Monda and an average of 3–4 weeks, where they build up Reichel 1989, Brand et al. 2011), or altera - fat reserves before continuing their migration tions in forest cover (Dolman and Sutherland north to the breeding grounds (Krapu et al. 1995, Shaw et al. 2013). Concurrently, long- 1985, 2014, Davis 2003). In recent years, Sand- term research demonstrates that, in response hill Cranes have been observed overwintering to global climate change, several species of avi- in the CPRV, a location well north of their fauna have altered migratory and breeding historical wintering range (Tacha et al. 1984, chronologies and shifted distributions (Bradley Krapu et al. 2011, Harner et al. 2015). Fur- et al. 1999, Swanson and Palmer 2009, Visser et thermore, first arrival dates of Sandhill Cranes al. 2009). Given the landscape-level and cli- to Nebraska during northward migration have matic changes being observed in the Anthro- occurred earlier in recent decades, based on pocene (Meybeck 2003, Travis 2003), avifauna first reported sightings by the public (Harner clearly shift migratory patterns in response to et al. 2015). Research indicates that crane multiple independent factors. However, these species have adjusted their migration chronolo- factors can interact in their influence on indi- gies in response to climate change, but there vidual species, with habitat loss limiting the has been little investigation linking fluctua- ability of wildlife to adapt to global climate tions in temperature and drought to variation change (Travis 2003, Opdam and Wascher 2004). in migration timing (Alonso et al. 2008, Min- The Mid-Continent Sandhill Crane (Antigone gozzi et al. 2013, Harner et al. 2015, Jorgensen canadensis) Population (MCP) breeds from the and Brown 2017). eastern edge of Hudson Bay, west to Siberia, Since the 1950s, Sandhill Cranes have aban- and south into the northern Great Plains (Tacha doned large portions of the western CPRV et al. 1984, Krapu et al. 2011). Wintering range from Overton west to North Platte, Nebraska, of the MCP is also expansive, extending from following declines in appropriate roosting habi- the coastal plain of eastern Texas, west to tat. Recent research indicates that their abun- southeastern Arizona, south into Chihuahua, dance in the western half of the CPRV may Mexico, and north through the Texas Panhan- still be declining (Walkinshaw 1956, USFWS dle into central Oklahoma (Tacha et al. 1984, 1981, Faanes and Le Valley 1993, Buckley 2011). Krapu et al. 2011). However, from late Febru- Large efforts have been undertaken since the ary through the middle of April, over 80% of early 1980s to maintain and improve riverine CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 35 habitat throughout the CPRV, through the (Krapu et al. 1984, Reinecke and Krapu 1986, clearing of riparian woodlands, restoration of Sparling and Krapu 1994). This disproportion native meadow habitat, controlled burning, is likely a result of their need to accumulate a chemical treatment of invasive species, and basic level of protein and other nutrients in disking of the river channel during low flows their diets that are insufficiently present in (Currier 1984, 1991, Pfeiffer and Currier 2005, corn (Krapu et al. 1982, Reinecke and Krapu Kinzel 2009, Riggins et al. 2009, Smith 2011, 1986). Sidle et al. (1993) documented a nega- Rapp et al. 2012, Krapu et al. 2014). How- tive relationship between the number of Sand- ever, the impact of these efforts on the roost- hill Crane roosts within a 1.6-km segment of ing distribution of Sandhill Cranes remains river and the distance to wet meadow habitat. largely unexamined. Concurrently, Sparling and Krapu (1994) found Several studies investigated Sandhill Crane that minimum daily flight distances were high- roosting habitat in the CPRV and generally est to riverine roosting sites, followed by native found that Sandhill Cranes prefer channels meadows, suggesting that these habitats were wider than 150 m, bank vegetation <1.5 m tall, the most limited resources within the CPRV and shallow water depths (<20 cm); they also for Sandhill Cranes. Faanes and Le Valley (1993) prefer a lack of human disturbance, including also showed that Sandhill Crane roosting den- roads, bridges, and structures (Krapu et al. sities were increasing in areas with relatively 1984, Iverson et al. 1987, Folk and Tacha 1990, abundant remaining wet meadow habitat. Norling et al. 1992a, Davis 2001, 2003, Pearse Pearse et al. (2017) demonstrated that the et al. 2017). In addition to being an important probability of Sandhill Cranes roosting in a predictor of Sandhill Crane roosting habitat portion of the river is related to the amount of use, channel width is indicative of broader cornfield nearby only when channels are rela- riverine habitat features in the CPRV, including tively narrow; however, their study did not channel morphology, hydrology, and sinuosity, evaluate the impact of lowland grasslands on that can also impact Sandhill Crane habitat roosting distributions. Anteau et al. (2011) found suitability (Schumm 1963, Williams 1978, John- that the use of cornfields by cranes increased son 1994, Kinzel et al. 2009, Horn et al. 2012). with the quantity of wet grassland habitat The U.S. Fish and Wildlife Service (USFWS within 4.8 km. Sparling and Krapu (1994) sug- 1981) documented that in the late 1970s, 60% gest that native grasslands serve as diurnal of the CPRV was more than 150 m wide. Over activity centers from which cranes base forag- 2 decades later, Davis (2003) found that only ing expeditions. Wet grasslands within 800 m 25% of the CPRV was wider than 150 m and of riverine roosting sites are important for that these areas contained 90% of the roosting pre-roost aggregations, where Sandhill Cranes Sandhill Cranes. Pearse et al. (2017) recently gather to continue to forage into the evening found that about 60% of the available roosting and engage in important pair-bonding behav- habitat was less than 100 m in width. Channel iors (Johnsgard 1983, Tacha 1988). Though width may serve as a broad indicator of habi- corn is an important food source, it is wide- tat quality, yet Sandhill Crane declines have spread (east to west) throughout the CPRV been noted in the western portion of the and is not likely restricting the amount of CPRV even where quality roosting habitat still available Sandhill Crane habitat (Sparling and exists (Buckley 2011). Krapu 1994, Krapu et al. 2014). However, In addition to channel characteristics, land research indicates that waste corn availability cover features also influence Sandhill Crane may be declining as a result of harvest effi- roosting distributions (Sidle et al. 1993, Spar- ciency and increased populations of arctic- ling and Krapu 1994, Pearse et al. 2017). Sand- nesting geese; this decline could potentially hill Cranes require 3 general habitats while in lead to longer Sandhill Crane flight distances the CPRV: wide channels for roosting, crop- (north and south) to agricultural foraging areas, lands (predominantly cornfields) to meet ener- particularly late in the spring staging period getic needs, and wet meadows for protein and (Pearse et al. 2010, Krapu et al. 2014). Wide other nutrients (Krapu et al. 1982). Sandhill channels and wet meadows are the most lim- Cranes spend a disproportionate amount of time ited habitat resources in the CPRV and influ- foraging in areas broadly defined as grasslands ence the distribution of Sandhill Crane roosts compared to their availability in the CPRV (Krapu et al. 1984, Currier and Ziewitz 1987, 36 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Iverson et al. 1987, Sidle et al. 1989, 1993, fields in addition to other crops (Dappen et Faanes and Le Valley 1993, Davis 2003). How- al. 2008, Krapu et al. 2014). The study area ever, there has not been a comprehensive habi- also contains significant expanses of riparian tat assessment estimating the availability of woodland and lowland prairie habitats (Cur- these resources in the CPRV following large- rier 1982, Kaul et al. 2006, Krapu et al. 2014). scale restoration efforts over recent decades. The study area was divided into 11 survey To provide a clear and updated description “segments,” delineated by bridges often used of the timing and habitat-use patterns of for reference in both research and conserva- roosting Sandhill Cranes during spring migration efforts in the CPRV (Currier 1991, Buck- tion in the CPRV, we used aerial survey data ley 2011, Krapu et al. 2014; Fig. 1). For addi- spanning 16 years to examine spatial and tem- tional analyses, the 11 segments were grouped poral trends, as well as variation in Sandhill into 3 larger “reaches,” which include seg- Crane roosting behavior. We then conducted ments 1–4 as the East Reach, segments 5–7 as an a priori investigation into the correlates of the Central Reach, and segments 8–11 as the temporal and spatial shifts and variation in West Reach (Fig. 1). Sandhill Crane roosting within the CPRV. We The Platte River is characterized by a used weather data from major wintering and series of submerged and emergent bed-forms early spring migration stopover locations to that migrate and change depending on river investigate the impacts of temperature and flows, a pattern characteristic of braided river drought on the arrival dates of significant per- systems (Smith 1971). These bed-forms, com- centiles of the Sandhill Crane population to monly referred to as sandbars, can become the CPRV (Wilson 2013). Finally, we investi- stabilized by early successional woody vegeta- gated the spatial distribution of Sandhill Cranes tion during low-flow conditions that have in relation to land cover and channel charac- become increasingly common with the heavy teristics within segments of the CPRV to assess appropriation of river flows for agricultural the effectiveness of recent restoration and use and power generation (Williams 1978, management efforts, and to determine which Currier 1982, 1997, Johnson 1994, Horn et al. habitat resources may be proximate factors in 2012). The impounding and appropriation of spatial shifts. Investigating the influences of river flows over the last century have pro- both climatic variation and landscape-level moted forest development within former chan- change together can help elucidate how large- nels throughout the CPRV, leading to wide- scale independent factors may interact to spread declines in channel width (Williams influence localized patterns of species’ tem- 1978, Currier 1982, Johnson 1994). Dominant porospatial occurrence. tree species in the CPRV include early successional species such as the eastern cottonwood METHODS (Populus deltoides) and the peachleaf willow (Salix amygdaloides), which readily establish Study Area seedlings on moist exposed soil (Currier 1982, Our study area focused on the main chan- 1997). These species were the first to colonize nel of the Platte River and adjacent land cover within the former high banks of the Platte within a 132-km reach of the CPRV from Chap - River in the late 1800s. Eastern cottonwood man (N 40.984692°, W −98.144053°, WGS 84; development peaked between 1935 and 1960, 539 m elevation) to Overton (N 40.681662°, with Russian olive (Elaeagnus angustifolia) W −99.540353°, WGS 84; 702 m elevation), and eastern red cedar ( Juniperus virginiana) Nebraska, where the majority of the MCP stage establishment in the understory beginning 20 during their spring migration (Fig. 1; Kinzel years later, on average (Currier 1982, O’Brien et al. 2006, Krapu et al. 2014). The Platte and Currier 1987). Active channel area in River provides shallow, secure roosting habi- portions of the CPRV has been reduced by tat for migrating Sandhill Cranes and Whoop- over 90% in the last century, with losses being ing Cranes (Grus americana) (Krapu et al. 1982, most pronounced in the western reaches Farmer et al. 2005, Kinzel et al. 2006). The (Williams 1978, Sidle et al. 1989). CPRV is a highly productive agricultural region, The Platte River extends the range of tall- and the landscape largely comprises irrigated grass prairie habitat west into the central corn (Zea mays) and soybean (Glycine max) mixed-grass ecoregion of the Great Plains by CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 37

Fig. 1. Study area map showing 11 survey bridge segments and 3 reaches (East, Central, and West) delineating the study area of the Central Platte River Valley, Nebraska, USA. providing moisture via subirrigation to a rela- wetter portions of subirrigated herbaceous habi- tively deep-rooted herbaceous perennial plant tat within the CPRV, where differentiated in the community in the CPRV (Currier 1989, Kaul text. Based on current research, it is unclear et al. 2006). Slight changes in elevation within whether Sandhill Cranes exhibit a strong prefer- this subirrigated ecosystem provide for a vari- ence for wet meadows over lowland tallgrass ety of wetland habitats (Currier 1982, 1989, prairie habitats in the CPRV (Sidle et al. 1993, Henszey et al. 2004). Subirrigated prairie sys- Sparling and Krapu et al. 1994, Davis 2003, tems in the CPRV have been categorized in Krapu et al. 2014). However, VerCauteren multiple ways. Brei and Bishop (2008) catego- (1998) suggested that cranes used grasslands rized the drier portions of these subirrigated with shallower ground water to a greater extent. herbaceous plant communities dominated by Lowland tallgrass prairie and wet meadow big bluestem (Andropogon gerardii) and switch- plant communities are not spatially distinct, but grass (Panicum virgatum) as “xeric wet mead- rather are integrated with one another, expand- ows,” whereas Rolfsmeier and Steinauer (2010) ing and contracting depending on the moisture considered these systems “Sandhills mesic tall- regime across several growing seasons (Currier grass prairies.” Rolfsmeier and Steinauer (2010) 1989, Henszey et al. 2004). Analyses and dis- categorized wetter portions of these systems cussion referring to lowland tallgrass prairie dominated by prairie cordgrass (Spartina pecti- and wet meadow jointly will be referred to as nata) and sedges, such as Emory’s sedge (Carex “meadow-prairie” habitat. emoryi) and woolly sedge (Carex pellita), as Aerial Surveys “northern chordgrass wet prairies.” By contrast, Currier (1982) considered these systems “wet From 2002 to 2017, we conducted weekly meadows,” Brei and Bishop (2008) categorized aerial surveys of Sandhill Crane roosts from them as “mesic wet meadows,” and Henszey et mid-February (12–18 February) to mid-April al. (2004) labeled them as “sedge meadows.” For (16–22 April). We made every effort to keep this study we use the term “lowland tallgrass the surveys as close to 1 week apart as possi- prairie” (Kaul et al. 2006) to refer to slightly ble, following the methods described by higher and drier areas, and the term “wet Buckley (2011). We conducted surveys along meadow” (Currier 1982) to refer to lower and a 132-km section of the Central Platte River 38 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 from Chapman to Overton, Nebraska (Fig. 1). number of cranes detected at riverine roost- We conducted between 6 and 9 surveys per ing locations. However, in a few of the years, year (x– = 7.8, SD = 1.0), depending on funding, survey directions were rotated weekly. In all, recent ambient weather conditions (extended approximately 85% of surveys were conducted periods of weather that preclude surveys), by moving from east to west and 15% were and pilot availability. The surveys were done completed by moving from west to east. This to gain a better understanding of the spatial practice potentially undercounted cranes in and temporal variation in densities and pro- western segments for a considerable portion portions of Sandhill Cranes roosting across of the migration and in eastern segments near different river segments of the CPRV. We began the end of spring staging. Because Sandhill the surveys at civil twilight, approximately 15– Cranes increase flight distance to foraging 25 min before sunrise, contingent upon envi- sites throughout the spring staging period ronmental conditions, when it was light enough (Sparling and Krapu 1994, Buckley 2011), to clearly distinguish roosting Sandhill Cranes. survey segments that were assessed latest in Aerial surveys were conducted from a single- the morning and near the end of migration engine, fixed-wing aircraft (predominantly a likely sustained the most significant under- Cessna 172) at an altitude between 200 m counts during individual surveys (i.e., late- and 215 m and a ground speed of 115 to 135 season eastern segment surveys). km/h. We completed the majority of surveys Two observers and a pilot conducted aerial in 55–75 min. We generally followed recom- surveys. One observer counted the number of mendations of Ferguson et al. (1979) for con- Sandhill Cranes in river roosts and feeding ducting aerial Sandhill Crane surveys on the aggregations in adjacent fields, photographed Platte River; surveys were not flown during Sandhill Crane roosts for bias-estimation reduced visibility conditions (low clouds, fog, (beginning in 2016), and directed the course precipitation) or during high-wind events of the pilot (when to circle, etc.). If necessary, (>35 kpm) that could significantly lower large groups were circled and recounted for detection probabilities. accuracy. The second observer recorded count The route was flown along the south chan- data, saved GPS waypoints, spotted forth- nel of the Platte River, which is generally the coming Sandhill Crane roosts, and assisted largest or “main channel,” where the vast with photographing roosts. We recorded GPS majority of Sandhill Cranes roost (Krapu et waypoints near the center of Sandhill Crane al. 2014). Crane groups were considered to roosts unless the roosting aggregation was be different roosts when they were separated large (n > 20,000 Sandhill Cranes), in which by more than 100 m (Iverson et al. 1987). case we marked both the beginning and the Because a significant proportion of Sandhill end of the roost with 2 separate waypoints. Cranes can leave the roost before sunrise Following methods of Gregory et al. (2004) (Lewis 1974, 1978, Norling et al. 1992b), cranes and Bowman (2014), we estimated Sandhill were detected to the limit that they could be Crane roost sizes by counting a group of positively identified with binoculars in off- between 50 and 100 cranes, and then creating channel habitats along the survey path, such a mental polygon around those groups. The as wet meadows and cornfields. These detec- polygon was then multiplied in space to pro- tions were likely reflective of use in adjacent vide counts for small roosts (>2000 cranes), or bridge segments (Sparling and Krapu 1994). further grouped into larger units (500–1000 Research indicates that Sandhill Crane den- cranes) and multiplied in space to estimate sities peak earlier in eastern survey segments abundance for roost sizes larger than ∼2000 and are generally higher in eastern and cen- cranes (Gregory et al. 2004, Bowman 2014). tral survey segments than in western ones, Bias Estimation particularly until late in migration (Krapu et al. 1982, Faanes and Le Valley 1993, Buckley The sensitivity of aerial count data to changes 2011, Krapu et al. 2014, B. Taddicken per- in when cranes arrive or depart, and in their sonal communication). Therefore, surveys were distribution within the CPRV, has been signifi- flown from east to west for the first 7–8 sur- cantly improved by the addition of bias esti- vey weeks, and from west to east during the mation procedures, which use photographs of last 2–3 survey weeks, to maximize the total a subset of counted flocks (Ferguson et al. CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 39

1979, Johnson et al. 2010). We added a bias as a control accompanying the uncorrected estimation component to our aerial survey count data, and also to account for some of the protocol in 2016, which compared aerial sur- variation in accuracy across observers (Greg - vey counts to photo-interpreted counts of a ory et al. 2004). However, we retained basic subset of observed roosts (Ferguson et al. 1979, analyses and summaries of uncorrected abun- Gregory et al. 2004). Bias estimates were not dance indices, when appropriate (i.e., not biased conducted until 2016, due to limited staffing by potential population growth), in order to resources. Immediately following the aerial enhance the interpretability of results and to count of a designated roost, observers photo- provide a descriptive account of temporospa- graphed it by circling the area a single time tial distributions of Sandhill Cranes during and taking multiple photographs of the entire the spring staging period in the CPRV. roost for later bias estimation. The number of Wintering and Migration Weather Parameters photo subplots was proportional to the number of roosts counted per survey, up to a maxi- To examine factors that may influence the mum of 10 photo subplots because of time phenology of Sandhill Crane migration, tem- constraints. We selected a variety of roost sizes perature and Palmer Drought Severity Index between 500 cranes and 10,000 cranes, as (NOAA–NCDC 2017) data were selected from roosts larger than 10,000 cranes were difficult areas spatially centered within key Sandhill to photograph with high resolution in a single Crane wintering and early spring stopover frame. We averaged percent bias across all regions (Fig. 2; Tacha et al. 1984, Iverson et subplots, regardless of directionality or roost al. 1985, Krapu et al. 2011, 2014). We first size, to create a measure of estimated absolute selected southern New Mexico (NM division 8; percent bias of cranes observed per aerial sur- NOAA–NCDC 2017) because it is centered vey (e.g., +–15%). We felt this approach was within the western wintering distribution of appropriate because our bias estimates were the MCP, north of wintering concentrations in not significantly correlated with roost size Chihuahua, Mexico, east of those in south- estimates (r = 0.158). We also calculated a eastern Arizona, and southwest of those in measure of relative percent bias that consid- east-central and northeastern New Mexico. A ered the directionality of error estimates and representative aggregation exists within this that could be used to correct estimates up or range that roosts in the backwaters of Caballo down (e.g., −11%; Ferguson et al. 1979, Greg - Lake, NM (N 32.898267°, W −107.295119°; ory et al. 2004). Mitchusson 2003). Much of the Western Alaska– Bias estimates only accounted for the dif- Siberia (WA–S) breeding affiliation winters ference between the number of Sandhill in this western range, and it includes mostly Cranes detected in photo subplots and the Lesser Sandhill Cranes (A. c. canadensis; Jones number detected in aerial roost counts, and et al. 2005, Krapu et al. 2014). Secondly, we did not account for flocks not spotted within selected Coastal Texas (TX division 8; NOAA– the flight path (i.e., all estimates represent NCDC 2017) because it represents the south- minimums present). As bias estimates were not eastern-most major wintering region for Sand- obtained for all years, the 2016 and 2017 aer- hill Cranes. This region is a major wintering ial Sandhill Crane counts that are used in ground for both the Eastern Canada–Min- most of the analyses in this paper were not nesota (EC–M) and Western Canada–Alaska error-corrected for standardization. However, (WC–A) breeding affiliations, both of which we used bias estimates collected during peak contain significant numbers of Greater Sand- count dates to approximate the number of Sand- hill Cranes (A. c. tabida; Jones et al. 2005, hill Cranes counted via our survey methods Krapu et al. 2014). A representative aggrega- between Chapman, Nebraska, and Overton, tion exists within this range that winters in Nebraska, during 2016 and 2017. We then and around Aransas National Wildlife Refuge, compared our estimates to those produced Texas (N 28.113560°, W −96.888864°; Hunt and by the USFWS (Dubovsky 2016, 2017) to Slack 1989). The third wintering location cho- determine the degree to which our methods sen to model the influences of winter and may under- or overrepresent Sandhill Crane early spring weather on migratory timing of densities. We used both proportional metrics Sandhill Cranes in the CPRV was the Texas and aerial count indices in analyses to serve Panhandle (TX division 1; NOAA–NCDC 2017). 40 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Fig. 2. United States map showing select wintering and stopover locations of the Mid-Continent Sandhill Crane Popu- lation, wherein breeding affiliation (BA) is presented in color (adapted from Krapu et al. 2011). The east-central Canada–Minnesota BA (EC–M) is depicted in orange; the west-central Alaska BA (WC–A) is in green; the northern Canada–Nunavut BA (NC–N) is in yellow; and the western Alaska–Siberia BA (WA–S) is in blue. Gray outlines broadly surrounding key wintering populations in the U.S. map depict the National Oceanic and Atmospheric Administration’s regional climate divisions used in this study (from south to north): coastal Texas (TX division 8), southern New Mexico (NM division 8), Texas Panhandle (TX division 1), southwest Oklahoma (OK division 7), central Kansas (KS division 5), and central Nebraska (NE division 5).

This location is the largest wintering concen- Washita National Wildlife Refuge (N 35.394310°, tration, and the area serves both as wintering W −99.335130°; Lewis 1975). This area serves grounds and as an early spring migration as a wintering location for the EC–M and WC– stopover location for birds wintering farther A breeding affiliations, as well as a stopover to the south (Krapu et al. 2011, 2014). This location both for Sandhill Cranes wintering area contains the largest wintering concentra- along the Texas Coast and for NC–N birds tions of Northern Canada–Nunavut (NC–N) wintering in the Texas Panhandle (Krapu et and WA–S breeding affiliation Sandhill Cranes, al. 2014). including mostly Lesser Sandhill Cranes, as For analyses, we included mean daily average well as a large number of WC–A (larger pro- temperatures and the mean Palmer Drought portion A. c. tabida) cranes (Jones et al. 2005, Severity Index (PDSI) values at all wintering Krapu et al. 2014). A representative aggrega- sites for the months of January and February. tion exists within this range that winters in Mean daily average temperatures across sea- and around Muleshoe National Wildlife Refuge, sonal periods have been used to assess the Texas (N 33.835346°, W −102.755188°; Iverson effect of climatic variation and climate change et al. 1985). Our fourth and final wintering on bird migration distances (Visser et al. 2009), location was sited in southwestern Oklahoma timing of arrival on breeding grounds (McKin- and north-central Texas (OK division 7; NOAA– ney et al. 2012), and a host of other phenologi- NCDC 2017). This climate division borders cal and chronological questions (Thackeray et the Red River of north-central Texas to the al. 2016, Pancerasa et al. 2018). Seasonal PDSI south, a known wintering location for Sandhill values have been linked to the timing of nest Cranes, but is also close to wetland wintering initiation (Brown and Brown 2014) as well as sites in southwestern Oklahoma, such as at habitat use (Igl and Johnson 1999). CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 41

We also included in our model, conditions channel width in this system. Williams (1978) at a key spring migration stopover site south of defined channel width as the distance from the Platte River and north of the wintering bank to bank, after subtracting stabilized islands grounds. Krapu et al. (2011, 2014) indicated with perennial vegetation. Krapu et al. (1984) that Quivira National Wildlife Refuge in Kansas similarly defined unobstructed channel width (KS division 5; N 38.092525°, W −98.488117°; as the breadth of river unbroken by stands of NOAA–NCDC 2017) is one of the most widely woody vegetation. Pearse et al. (2017) esti- used spring migration stopover locations for mated unobstructed channel width from the central and eastern wintering concentrations. perspective of cranes, determining it as the We included the mean temperature and mean distance across the channel, including bare soil PDSI for February and March in our model and vegetation under 1.5 m in height. Werbylo for Quivira National Wildlife Refuge and sur- et al. (2017) considered unvegetated channel rounding wetlands. Finally, we included mean width as including sandbars with <25% vege- temperature and mean PDSI data averaged tative cover. For the purposes of this study, we across February through April for the CPRV considered the “unobstructed channel width” (NE division 5; N 40.708077°, W −98.788742°) as the distance from bank to opposite bank, to examine the effects of local temperature and including areas of active flow and unstabilized drought variables on the phenology of Sandhill sandbars. We defined unstabilized sandbars Crane staging in the CPRV (Krapu et al 2014). as either those which are bare of vegetation or those which include only early successional Habitat Assessment vegetation under ∼1.5 m in height and signifi- To investigate the changes in landscape cant exposed bare ground (>50%). These sand- features that could affect Sandhill Crane habi- bars are generally lower elevation and are tat use within the CPRV, we measured unob- scoured on a regular basis (Currier 1982). structed channel widths and classified key Islands showing signs of initial stabilization land cover types. We used georeferenced and (including significant perennial shrub cover orthorectified aerial imagery collected in June [Salix exigua, etc.] and perennial exotic plant or early July of 1938, 1998, 2015, and 2016 in cover [Phragmites australis, etc.]) or islands order to measure a systematic random sample generally exceeding 1.5 m in height, with lim- of unobstructed channel widths across all seg- ited exposed bare ground, were not included ments (RWBJV 2017; imagery provided by the as part of the unobstructed active channel Platte River Recovery Implementation Program width (Fig. 3). and the Rainwater Basin Joint Venture’s image We manually measured “total unobstructed library). We used aerial imagery from 1998 and channel width” (UOCW) in ArcGIS 10.5.1 by 2016 to classify key land cover types within an broadly following the techniques of Werbylo 800-m buffer to the north and south of the et al. (2017; Fig. 3). We placed point features main channel of the Platte River (1.6 km wide). every 800 m in the center of the main channel When determining our spatial sampling area, of the Platte River, beginning at a random we considered the impacts of human develop- starting point and using the oldest imagery ment (disturbances such as buildings, bridges, available to us for each section of river (gener- and roads within ∼700 m are associated with ally 1938). We then measured the width of the decreased use of roost sites within narrow chan- river channel (except for stabilized islands), nels; see Pearse et. al. 2017), eve ning pre-roost making every effort to situate the measure- and morning post-roost aggregation behavior ment lines as close to perpendicular to each (called “peripheral” or “secondary” roosting, and bank as possible. For purposes of comparison generally occurring in meadows within 800 m between years, we saved channel width mea- of the river; see Wheeler and Lewis 1972, surements for each year of imagery as a unique Johnsgard 1983, Tacha 1988), and the esti- feature class in ArcGIS 10.5.1. Points ran- mated distribution of Sandhill Crane roosts in domly landing near bridges were moved 100 m the CPRV (91% of crane roost locations occur past or before a bridge. We also calculated in the main channel; see Krapu et al. 2014) “maximum unobstructed channel width” (maxi- CHANNEL WIDTH.—Though a clear trend mum width unbroken by woody encroach- exists regarding channel losses in the CPRV, ment and stabilized islands; MUCW). Islands many researchers have differentially defined exceeding 1600 m in length or 400 m in width 42 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Fig. 3. Caption on page 43.

were considered to separate the Platte River LAND COVER.—To assess land cover, we into multiple channels, while vegetated islands classified areas of tree cover and meadow- smaller than those dimensions were consid- prairie cover from aerial imagery acquired in ered obstructions within individual channels. the summers of 1998 and 2016 in ArcGIS At each point feature, we also recorded the 10.5.1 (RWBJV 2017). A true-color orthophoto number of active channels of the Platte River with a 15-cm pixel resolution was used for in each year of imagery. 2016, and a MrSid (.sid) false-color composite CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 43

Fig. 3. Total unobstructed channel width measurements. A, 1938, black and white imagery. B, 1998, color infrared imagery. C, 2016, color infrared imagery. Channel width was measured every 800 m near the Dippel Island restoration site, bridge segment 6, southwest of Gibbon, Nebraska, USA (ArcGIS 10.2.1). All images were collected via aerial photography. Unobstructed channel width measurements are presented in blue. of Landsat data with a pixel resolution of 30 cm To assess tree cover, imagery was resampled was used for 1998. For the purposes of this to a 3.28-ft (1-m) pixel resolution, and classifi- study, lowland tallgrass prairie (Kaul et al. cation methods were broadly based off of work 2006) and wet meadow (Currier 1982) habi- quantifying cedar encroachment (Coppedge tats were not differentiated, and an analysis et al. 2001a, Ghimire et al. 2014) and delineat- was performed to classify an aggregated ing range habitats (Cingolani et al. 2004). meadow-prairie land cover type. We manu- Using image segmentation in the Spatial Ana- ally delineated areas of contiguous herba- lyst toolbox in ArcGIS 10.5.1, adjacent pixels ceous vegetation within 800 m of the Platte with similar spectral properties were grouped River that were dominated by nonwoody species together. As Cushnie (1987) noted, land cover (including grasses [Poaceae], sedges [Cyper- classification can be improved via the coars- aceae], rushes [Juncaceae], and herbaceous ening of image resolution, because creating forbs [Asteraceae, Lamiaceae, etc.; Currier internally homogenous units can lessen “noise” 1982, Kaul et al. 2006]) using a “heads-up” in the image. We performed pixel-based super- classification approach on imagery resam- vised classification using the interactive maxi- pled to a 1-ft (30.5-cm) pixel resolution for mum likelihood classifier tool in ArcGIS on consistency between years (Cushnie 1987, the segmented images, where pixels were Grossman et al. 1994, Ghimire et al. 2014). assigned to one of 5 classes: tree cover, water, Areas with scattered trees exceeding 30% herbaceous (grassland and agricultural pro- vegetative cover were considered woodland duction), sand/nonvegetated, or developed. habitats and were therefore not included in Tree cover training samples (n = 83) were areas of meadow-prairie vegetation (Currier selected from areas that had been surveyed 1982, Grossman et al. 1998). Two individual during habitat-monitoring efforts from 2015 observers reviewed all “heads-up” classifica- to 2017, which included both deciduous and tions of meadow-prairie habitat to ensure coniferous species. We used the majority fil- accuracy. ter, the boundary clean tool, and manual 44 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 inspection to correct the raw classification avian populations is more biologically mean- (see Ghimire et al. 2014). Final raster classi- ingful and sensitive to weather parameters fications were converted to polygons, visu- than first detection dates. Therefore, we used ally inspected, and corrected for accuracy 7 Sandhill Crane count metrics to assess arrival: (see Ghimire et al. 2014). Tree cover included (i–iii) the Julian date that more than 5%, 15%, scattered individuals or groups of trees within and 25% of a year’s peak count was first woodlands, as well as larger polygons of con- detected via aerial surveys; (iv–v) the percent tiguous forest habitat; we labeled this land of the year’s peak count observed in survey cover category as “woodland-forest” for analy- weeks 4 (05–11 March) and 5 (12–18 March), ses. We then calculated the proportion of land which generally predate peak Sandhill Crane cover within 800 m of the main channel of the abundance in the CPRV but occasionally sup- Platte River that was classified as woodland- port large abundances (Krapu et al. 2014, forest and as meadow-prairie in 2016, as well Pearse et al. 2015); (vi) the Julian date of peak as the change in proportion of meadow-prairie Sandhill Crane count; and (vii) the Julian dates from 1998 to 2016 within each of the 11 that Sandhill Crane counts were greater than bridge-delineated segments (Fig. 1). 125,000 by survey year in our trend analyses. Although this final metric (vii) uses raw count Data Analyses data, it is likely not biased by potential popu- We examined the chronology of the spring lation growth in its ability to assess an advanc- MCP migration in the CPRV and investigated ing migration. Counts exceeding 125,000 should temporal trends within our data. To summa- become more frequent throughout the survey rize seasonal variation, we calculated sample period in the face of population growth in the means, standard deviations, and maximum absence of temporal shifts. However, if counts values for Sandhill Crane counts by survey exceeding 125,000 are becoming more com- week. We then calculated sample means and mon only early in the migration across years, standard deviations for peak count and survey this metric will be a useful indicator of a week of peak count for each segment. Addi- shifting migration. We also analyzed the pro- tionally, we created boxplots of Sandhill Crane portion of the peak count detected in survey counts per survey week for selected individ- week 8 (02–08 April) by year to assess whether ual segments, in order to illustrate the varia- or not cranes were leaving progressively ear- tion in peak timing from east to west across lier from 2002 to 2017. Survey week 8 (02–08 our survey area (ggplot2 package, R Core Team April) generally postdates peak Sandhill Crane 2015, Wickham 2009). We calculated sample abundance in the CPRV, but it occasionally means, standard deviations, and maximum and supports large numbers of cranes (Krapu et minimum values for the Julian date (1–365 or al. 2014, Pearse et al. 2015). Therefore, it may 1–366 for leap year) of peak Sandhill Crane be a good indicator of shifting departure dates. migration across all years, as well as Julian To summarize the spatial distribution of date of counts exceeding 125,000, to best Sandhill Cranes within the CPRV, we calcu- approximate the time period when large num- lated sample means and standard deviations bers of Sandhill Cranes are present in the of abundance metrics for each of the 11 seg- CPRV via our survey data. We used 125,000 ments in our study area from 2002 to 2017, Sandhill Cranes, because that number repre- including the count per survey, the maximum sents approximately 25% of a conservative esti- value of count per survey during a single mate of the number of cranes in the CPRV study year, the density per kilometer, and the (Kinzel et al. 2006, Dubovsky 2017); 25% is a proportion of cranes counted in each segment biologically significant proportion (Wilson 2013). per survey year. For comparison, we calcu- To investigate potential trends in the tim- lated the mean density per kilometer and pro- ing of Sandhill Crane staging in the CPRV, portion of Sandhill Cranes per segment from we used bivariate ordinary least squares lin- 2015 to 2017. We used data from 2015 to 2017 ear regression to model the yearly change in because it best represented the distribution of arrival dates of key proportions of the MCP Sandhill Cranes following recent restoration from 2002 to 2017 (stats package, R Core efforts, and therefore was likely most reflec- Team 2015). Wilson (2013) noted that the tive of their responses. To further investigate arrival of “significant percentiles” of migratory Sandhill Crane distributions and the trends CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 45 therein, we calculated the aforementioned sum- MuMIn package, Barton 2016; stats package, mary statistics for larger reaches of the CPRV R Core Team 2015). We modeled Sandhill as well (East: 1–4; Central: 5–7; West: 8–11; Crane arrival metrics (>30K, >100K, WK4, Fig. 1). We then used a one-way analysis of WK5, >15%) using mean annual winter tem- variance (ANOVA) test with a Tukey’s Honest perature and PDSI values for major winter- Significant Differences (HSD) post hoc test to ing locations, and mean annual late-winter examine differences in Sandhill Crane roost- and early spring temperature and PDSI val- ing densities across these larger river reaches ues for major spring migration stopover loca- and segments (stats package, R Core Team tions. We included the year as a control vari- 2015). To determine long-term trends in Sand- able to account for any long-term trends in hill Crane use per segment and in larger river migration not driven by annual weather (i.e., reaches, we used bivariate ordinary least squares population growth, changes in observer, etc.). linear regression models to examine the rela- No mean winter or spring temperatures were tionship between the proportion of the total substantially correlated (r ≥ 0.50) with corre- crane count per segment and survey year. sponding PDSI values or year for key winter- To investigate the association between win- ing and migration locations, suggesting that ter moisture conditions (PDSI) and tempera- multicollinearity was not an issue in our mul- tures on Sandhill Crane migration phenology, tivariate models (Dormann et al. 2013). We we calculated Pearson’s product-moment cor- tested for correlations across sites within relation coefficients for winter and early spring years to determine whether any wintering/ weather data in relation to a number of Sand- migration regions were sufficiently similar to hill Crane metrics meant to serve as indica- warrant combination for analyses. We deter- tors of early migration (Hmisc package, Har- mined that the climate regions were distinct rell 2017; R Core Team 2015, NOAA–NCDC and should be included individually in the 2017). Sandhill Crane arrival metrics included analysis. Most sites were significantly corre- the Julian date that the Sandhill Crane count lated with at least one other site regarding reached 30,000 via our survey methods (here- either mean temperatures or PDSI values. after, >30K), the Julian date that the count However, most sites were not significantly reached 100,000 (hereafter, >100K), the Julian correlated on both metrics. For instance, date that the count reached 15% or more of mean temperatures in Oklahoma and New the peak count for that year (hereafter, >15%), Mexico were not significantly correlated (r and the total Sandhill Crane counts during = 0.37), but PDSI values were correlated (r = survey week 4 (hereafter, WK4) and week 5 0.81). Similarly, temperatures in Kansas and (hereafter, WK5). We hypothesized that the Oklahoma were significantly correlated (r = PDSI and mean temperatures across winter- 0.67), but PDSI values were not correlated (r ing and migration locations would be negatively = 0.16). Even those sites that were signifi- correlated with the arrival dates of >100K, cantly correlated regarding both metrics did >30K, and >15% Sandhill Cranes, suggesting not approach statistical singularity. For in- that warmer, drier winters are related to ear- stance, temperatures (r = 0.86) and PDSI lier arrival dates of larger numbers of cranes values (r = 0.76) on the Texas Coast and in (Harner et al. 2015). Following the same logic, the Texas Panhandle were significantly corre- we surmised that these same metrics would be lated but appeared sufficiently distinct to positively related to the numbers of cranes in warrant individual inclusion in the analysis. survey weeks 4 and 5. We also included a null model regressing the To determine which wintering locations are dependent variable by 1. We ran a total of most associated with trends in early arrivals 35 models—7 models (southwest Oklahoma of large numbers of Sandhill Cranes in the drought [PDSI] and temperature [Temp], Texas CPRV, we used Akaike’s Information Criterion coast drought and temperature, Texas Pan- corrected for small sample sizes (AICc) to handle drought and temperature, southern compare generalized linear models (GLMs) New Mexico drought and temperature, cen- of weather and drought indices at wintering tral Kansas drought and temperature, central and migration locations in relation to Sandhill Nebraska drought and temperature, and a Crane arrival metrics (Nelder and Baker 1972, null model) for each of the 5 Sandhill Crane Hurvich and Tsai 1989, Burnham et al. 2011; arrival metrics (>30K, >100K, WK4, WK5, 46 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 and >15%). We report only the top-perform- the habitat metrics that best explained the ing models in the results, achieving an trend in the proportional use of segments Akaike weight of ≥0.10 (Wagenmakers and from 2002 to 2017 (use metric iii, 13 models Farrell 2004). total, including null model; McCullagh 1980, To examine the influence of land cover Christensen 2015, R Core Team 2015). Trends characteristics and channel width measure- in the proportional use of each segment were ments on Sandhill Crane habitat use within coded as ordered factors (−1 = negative, 0 = segments of the CPRV, we summarized the no trend, 1 = positive) and models were com- following habitat metrics: (i) percent woodland- pared using AICc (Wagenmakers and Farrell forest cover in 2016, (ii) percent meadow- 2004; MuMIn package, Barton 2016; stats prairie cover in 2016, (iii) change in percent package, R Core Team 2015). meadow-prairie cover from 1998 to 2016, (iv) percent conservation management in 2016 (% RESULTS of land within 800 m of the main channel owned or managed by conservation organiza- From 2002 to 2017, we conducted 108 aer- tions in 2016 including state [i.e., USFWS] ial survey counts between the dates of 11 Feb- and nonstate [i.e., The Nature Conservancy] ruary and 16 April, surveying a total of 1162 actors), (v) median longitude per segment, (vi) segments (11 segments multiple times) and change in UOCW from 1938 to 2016, (vii) counting a total of 12,831,526 cranes across change in MUCW from 1938 to 2016, (viii) 14 survey years; our surveys were not con- change in the number of active channels from ducted in 2011 and 2012 due to lack of finan- 1938 to 2016, (ix) change in UOCW from cial resources and staffing. Our mean estimate 1998 to 2016, (x) change in MUCW from 1998 per segment was 11,043 +– 21,336 Sandhill to 2016, (xi) UOCW in 2016, and (xii) MUCW Cranes across all years. Comparisons of aerial in 2016. Sandhill Crane habitat use metrics ocular estimates with bias-corrected estimates included the following: (i) the statistical trend derived from photo subplots in 2016 and 2017 in the proportion of Sandhill Cranes detected revealed a mean absolute error of +–9.44% (SD per segment from 2002 to 2017, (ii) the mean = 3.42%, range +–3.20% to +–14.0%) across sur- proportion of Sandhill Cranes per segment veys (n = 14), with a relative error of −3.42%, from 2015 to 2017, and (iii) the mean density considering the directionality of error esti- of Sandhill Cranes per segment from 2015 to mates (SD = 8.4%, range −14.0% to 13.5%). 2017. We then used Pearson’s product-moment Our bias-corrected peak count of Sandhill correlation analyses between habitat metrics Cranes roosting between Chapman and Over- and Sandhill Crane use metrics per segment ton was 418,759 +– 46,901 in 2016 and 429,916 (Hmisc package, Harrell 2017; R Core Team +– 27,386 in 2017. 2015). After that, we constructed a series of Temporal Trends bivariate generalized linear models with Gauss- ian distributions, using each habitat metric Mean Sandhill Crane counts per survey week as an explanatory variable and Sandhill Crane from 2002 to 2017 for the survey region from use metrics ii and iii (mean proportion and Chapman to Overton, Nebraska, demonstrated mean density per segment 2015 to 2017, that survey week 7 (26 March–01 April) had the respectively) as dependent variables (26 mod- highest mean Sandhill Crane count, followed els total, including null models; Hurvich and by weeks 8 (02–08 April) and 6 (19–25 March); Tsai 1989, Burnham et al. 2011; stats package, all exceeded an average of 200,000 Sandhill R Core Team 2015). Because we were limited Cranes (Table 1). High counts of over 400,000 to 11 observations (one per segment for all Sandhill Cranes were observed in all weeks spatial analyses variables), we used bivariate from week 4 (05–11 March) to week 8 (02–08 models (Vittinghoff and McCulloch 2007) and April) (Table 1). Mean date of peak Sandhill selected the best-fit model among candidate Crane count per year was 25 March (x– = day models by using AICc (Wagenmakers and 84.1), with a standard deviation of approxi- Farrell 2004; MuMIn package, Barton 2016; mately 9 days (SD = 9.2 days). However, we stats package, R Core Team 2015). Lastly, we observed peak numbers from as early as employed ordered logistic regression with a 8 March (day 67) to as late as 8 April (day 98). cumulative “probit” link function to determine Aerial counts of Sandhill Cranes exceeding CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 47

TABLE 1. Mean (with standard deviation) and maximum Sandhill Crane counts by survey week (1–10) on the main channel of the Platte River from Chapman to Overton, Nebraska, from spring 2002 through spring 2017. N = 108 aerial surveys with 103 calendar weeks surveyed (when multiple surveys were conducted in a calendar week, the highest count was used). The total number of survey years was 14, and surveys were not conducted in 2011 and 2012. Not all segments were flown every week, primarily because of in-flight changes in weather. – Survey week n Dates x SD Maximum 1 2 12 Feb to 18 Feb 8073 103 8146 2 6 19 Feb to 25 Feb 15,891 24,948 66,017 3 10 26 Feb to 04 Mar 41,298 60,212 194,825 4 12 05 Mar to 11 Mar 73,560 119,180 405,857ˆˆ 5 13 12 Mar to 18 Mar 112,672 112,971 404,170ˆˆ 6 13 19 Mar to 25 Mar 206,241ˆ 105,679 410,066ˆˆ 7 11 26 Mar to 01 Apr 254,468ˆ 110,734 541,100ˆˆ 8 14 02 Apr to 08 Apr 212,017ˆ 125,183 567,525ˆˆ 9 11 09 Apr to 15 Apr 94,891 75,866 270,015 10 11 16 Apr to 22 Apr 19,346 36,667 109,025 ˆIndicates a mean aerial count of >200,000 Sandhill Cranes within a survey week. ˆˆIndicates a maximum aerial count of >400,000 Sandhill Cranes within a survey week.

TABLE 2. Bivariate ordinary least squares linear regression analyses of Julian date of peak Sandhill Crane count (Peak), Julian dates of all Sandhill Crane counts over 125,000 (>125K), Julian date when Sandhill Crane counts first reached 5%, 15%, and 25% of peak yearly count (>5%, >15%, >25%), and the proportion of peak Sandhill Crane count observed during survey week 4 (05 Mar–11 Mar; % WK 4), 5 (12 Mar–18 Mar; % WK 5), and 8 (02 Apr–08 Apr; % WK 8) by survey year (coefficient) from 2002–2017. n = 45 for the SACR > 125,000 analysis and n = 14 for all other analyses. “DV” = dependent variable. 2 Metric DV B SE t P R df Peak Julian date −1.324 0.364 −3.63 0.0034** 0.524 12 >125K Julian date −1.134 0.277 −4.10 0.0002*** 0.281 43 >5% Julian date −1.413 0.428 −3.30 0.0063** 0.476 12 >15% Julian date −1.155 0.309 −3.73 0.0029** 0.537 12 >25% Julian date −1.434 0.334 −4.29 0.0010** 0.605 12 %WK 4 Proportion 0.034 0.013 2.59 0.0268* 0.402 10 %WK 5 Proportion 0.035 0.012 2.89 0.0151* 0.430 11 %WK 8 Proportion −0.039 0.016 −2.38 0.0343* 0.322 12 *P < 0.05 **P < 0.01 ***P < 0.001

125,000 were observed from 27 February to 1.16 days per year (P < 0.01; Table 2, Fig. 4). 11 April, with a mean date of 24 March (x– = The proportion of the peak Sandhill Crane day 82.6, SD = 9.9 days). Median Sandhill count observed during survey week 5 (12– Crane counts were highest in week 7, exceed- 18 March) increased by 3.5% per year from ing the 75th percentile of both weeks 6 and 8. 2002 to 2017 (P < 0.05; Table 2, Fig. 4). The However, the 1.5 times interquartile range of proportion of the peak Sandhill Crane count Sandhill Crane counts for week 6, denoting observed during survey week 8 (02–08 April) extreme but not outlying values, exceeded decreased 3.9% per year (P < 0.05; Table 2). weeks 7 and 8 (see Wickham 2009). Cranes in the survey area did not demon- Bivariate ordinary least squares linear regres- strate a uniform temporal peak among seg- sion analyses demonstrated that counts of more ments; instead, there was a distinct difference than 125,000 Sandhill Cranes advanced an aver- in eastern and western portions. From 2002 to age of approximately 1.13 days per year across 2017, easternmost segments 1–3, spanning the 16-year survey period (P < 0.01; Table 2, from Chapman to Alda, Nebraska, had the Fig. 4). Similarly, peak Sandhill Crane count highest mean counts during survey week 6 advanced on average 1.32 days per year (P < (19–25 March); segment 4 had the highest 0.001), and the Julian date on which more mean counts during week 7 (26 March– than 15% of the year’s peak Sandhill Crane 01 April); and central and western segments count was first observed advanced on average 5–11, from Wood River to Overton, Nebraska, 48 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Fig. 4. Sandhill Crane abundance and arrival metrics fit with ordinary least squares bivariate regression lines by survey year, 2002–2017. A, Julian date (DOY) of peak Sandhill Crane count by year (YR). B, Julian dates (DOY) of Sandhill Crane counts exceeding 125,000 by year (YR). C, Julian date (DOY) when Sandhill Crane counts exceeded 15% of peak by year (YR). D, Proportion of the peak Sandhill Crane count observed in week 5 (12–18 March; PROP) by year (YR). CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 49

A 200000

150000

SACR 100000

50000

0 1 2 3 4 5 6 7 8 9 10 WKSV

150000 B

100000 SACR 50000

0 1 2 3 4 5 6 7 8 9 10

WKSV

Fig. 5. Weekly Sandhill Crane counts. A, Bridge segment 3 (HWY 281 to Alda Rd.). B, Bridge segment 7 (Gibbon to HWY 10). In bridge segment 3, median counts are statistically similar for weeks 6 and 7, but the upper interquartile range and whisker are highest for week 6. In bridge segment 7, median and upper quartile counts are highest during week 8, though the whisker in week 7 slightly exceeds that in week 8. SACR = Sandhill Crane, WKSV = survey week. peaked during survey week 8 (02–08 April; channel (Table 4, Figs. 6, 7) and, along with Table 3). High counts in eastern segments segments 2 and 5, each averaged over 10% of (1–4) began to decline just as central and west- the recorded Sandhill Cranes per survey sea- ern segments (5–11) reached peak numbers son (Table 4). Sandhill Crane densities per (Table 3; Fig. 5). Segments 3 and 7 have histor- kilometer from 2002 to 2017 varied signifi- ically held some of the highest densities of cantly across survey segments (F = 22.13, P < cranes, and they were separated from east to 0.001; Appendix 1). Tukey HSD post hoc west by areas of relatively lower density (Krapu tests revealed that segment 3 (HWY 281 to et al. 2014). The median weekly count was Alda) had a significantly higher density than highest during week 6 in segment 3, but far- all other segments, excluding segment 4 (Alda ther west in segment 7, the median weekly to Wood River). Segment 4 had a higher den- count was highest during week 8 (Fig. 5). sity of cranes than all segments with the exceptions of segments 7 and 3 (Gibbon to Spatial Trends HWY 10). However, a comparison of data from From 2002 to 2017, mean counts of Sand- the most recent surveys (2015 to 2017) to hill Cranes per segment were highest for seg- those of earlier years suggests that Sandhill ments 3, 4, and 7, respectively (Table 4). These Crane densities are increasing over time in segments also had the highest observed Sand- the east (Table 4, Fig. 6). The 3 highest Sand- hill Crane densities per kilometer of river hill Crane densities for 2015 to 2017 were 50 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

TABLE 3. Mean yearly peak Sandhill Crane count and mean survey week of yearly peak across aerial surveys (N = 108) and years by survey bridge segment and length, 2002–2017. When multiple average Sandhill Crane counts by survey week were within one standard deviation of each other, we used the median count to determine peak survey week. See Fig. 1 for map of segments. SACR = Sandhill Crane. – – Segment/location km x peak SACR x peak survey week 1 Chapman–HWY 34 17.1 18,423 6 (19 Mar to 25 Mar) 2 HWY 34–HWY 281 11.6 36,114 6 (19 Mar to 25 Mar) 3 HWY 281–Alda 10.7 68,913 6 (19 Mar to 25 Mar) 4 Alda–Wood River 8.5 39,561 7 (26 Mar to 01 Apr) 5 Wood River–Shelton 13.9 38,180 8 (02 Apr to 08 Apr) 6 Shelton–Gibbon 10.1 25,046 8 (02 Apr to 08 Apr) 7 Gibbon–Hwy 10 9.1 42,457 8 (02 Apr to 08 Apr) 8 HWY 10–Kearney 11.4 10,107 8 (02 Apr to 08 Apr) 9 Kearney–Odessa 14.7 4065 8 (02 Apr to 08 Apr) 10 Odessa–Elm Creek 11.0 3971 8 (02 Apr to 08 Apr) 11 Elm Creek–Overton 13.6 851 8 (02 Apr to 08 Apr)

TABLE 4. Mean weekly Sandhill Crane count (x–), maximum mean weekly count for a single survey year (max), mean count per kilometer of river channel (x–/km), and mean proportion of the yearly Sandhill Crane counts (x– Prop.) during 108 aerial surveys in 2002–2017; mean count per kilometer of river channel (x–/km 15–17) and mean proportion of the yearly Sandhill Crane counts (x– Prop. 15–17) during 2015–2017. Metrics are shown by survey bridge segment and reach of river from Chapman to Overton, Nebraskaa. “Reach” refers to a larger section of the study area and includes multiple bridge segments. The East Reach includes segments 1–4, the Central Reach includes segments 5–6, and the West Reach includes segments 8–11. See Table 3 for a description of bridge segments and Fig. 1 for a map of segments and reaches. Reach/ x–/km x– Prop. – – – segment x SD max x/km x Prop. 15–17 15–17 East 75,085 35,350 140,269 1568 59.6% 2107 75.8% 1 5786 3838 13,335 338 5.2% 523 7.4% 2 14,168 6501 29,952 1221 11.7% 1918 3 17.6% 3 33,203 1 20,188 81,583 1 3103 1 25.4% 4023 1 32.2% 4 21,928 2 10,316 43,507 3 2580 2 17.3% 3137 2 18.6% Central 42,640 21,975 96,515 1288 33.8% 812 20.3% 5 15,672 7698 26,923 1128 13.2% 546 5.8% 6 9623 6238 23,769 953 7.8% 382 3.2% 7 17,345 3 11,510 47,233 2 1906 3 12.8% 1697 11.3% West 7173 3538 13,331 141 5.7% 100 4.0% 8 4002 2168 9119 351 3.1% 311 2.8% 9 1539 1081 4204 105 1.3% 47 0.6% 10 1419 1534 4707 129 1.1% 45 0.4% 11 213 410 1316 16 0.2% 25 0.2% aSuperscripts 1, 2, and 3 are used to rank Sandhill Crane abundance per bridge segment via the various metrics. segments 3, 4, and 2, respectively, with seg- and 3 (32.2% +– 2.9% vs. 21.6% +– 8.2%), while ment 7 being the only segment in the west- decreases were most prominent in segments 5 ern two-thirds of the survey area to support (16.3% +– 6.6% vs. 7.6% +– 2.7%) and 6 (10.2% over 10% of the Sandhill Cranes (Table 4, +– 4.3% vs. 3.6% +– 1.3%; Fig. 8). Fig. 6). The same pattern is clear when com- When 11 survey segments were grouped paring discrete survey periods; the mean pro- into 3 larger reaches of the CPRV (East [1–4], portion of Sandhill Cranes counted per sur- Central [5–7], and West [8–11]), survey data vey year from 2013 to 2017 was higher than from 2002 to 2017 showed that the East Reach from 2002 to 2010 in each of the eastern seg- accounted for 59.6% of the Sandhill Cranes ments (1–4) and lower in each of the central counted (Table 4). When the most recent sur- and western segments (5–11; Fig. 8). Com- veys from 2015 to 2017 were examined, the paring 2013 to 2017 and 2002 to 2010, in - eastern segments accounted for 75.8% of the creases were most pronounced in segments 1 Sandhill Cranes counted (Table 4). Concur- (mean +– SD; 8.5% +– 4.9% vs. 3.3% +– 2.5%) rently, when the discrete survey periods were CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 51

100 km

N B

100 km

Fig. 6. Mean Sandhill Crane density per kilometer by bridge segment. A, 2002 to 2017. B, 2015 to 2017. compared, 73.4% of Sandhill Cranes were demonstrated a significant decline in the pro- detected in eastern segments from 2013 to portion of cranes using it annually (−1.2% 2017, compared to 51.8% from 2002 to 2010 annually, P < 0.001; Table 5). Negative trends (Fig. 8). Sandhill Crane densities varied widely were also noted in segments 6 (Shelton to Gib- across East, Central, and West reaches of the bon; −0.7% annually, P < 0.01) and 9 (Kearney CPRV (F = 27.7, P < 0.001; Appendix 1). Den- to Odessa; −0.1% annually, P < 0.01; Table 5). sities in the East and Central reaches were Analysis of larger reaches showed that the significantly higher than in the West reach, proportion of Sandhill Crane use in the east- but not significantly different from each other ern segments increased 2.3% annually (P < across all data from 2002 to 2017 (Table 4, 0.001). By contrast, there was a significant Fig. 7, Appendix 1). decline in the proportion of Sandhill Cranes Bivariate ordinary least squares linear using the central segments (−2.0% annually, regression models showed statistically signifi- P < 0.001) and a marginal decline in the pro- cant trends in the proportion of Sandhill Cranes portion of cranes using the western segments using particular segments from 2002 to 2017 (−0.2% annually, P < 0.10; Table 5). (Table 5). Segment 1 (Chapman to HWY 34) Temporal Factors demonstrated a positive trend in the proportion of cranes using it on a yearly basis (+0.5% Daily average winter (January–February) and annually, P < 0.05; Table 5). The proportion early spring (February–March/April) tempera- of cranes using segment 3 (HWY 281 to Alda tures from major Sandhill Crane wintering Road) increased 1.4% per year (P < 0.001; and migratory stopover regions were nega- Table 5). Segment 5 (Wood River to Shelton) tively correlated with the arrival date of >30K 52 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Fig. 7. Sandhill Crane density (mean count per kilometer; x–/km) from 2002 to 2017. A, Density by bridge segment (1–11). B, Density by river reach (Central, East, and West). Black horizontal lines denote median values, while the tops and bottoms of boxes denote the upper and lower interquartile ranges (75th and 25th percentiles), respectively. Extend- ing “whiskers” denote values of 1.5 times the interquartile range; areas outside of this range constitute outliers and are marked with points. For a description of bridge segments and reaches, see Fig. 1.

Sandhill Cranes in all locations but southern correlations, suggesting that as temperatures New Mexico (Table 6). Average temperatures increased in these locations, Sandhill Cranes from southwestern Oklahoma (r = −0.84, P arrived earlier in the CPRV (Table 6). Simi- < 0.001), the Texas Panhandle (r = −0.81, P larly, the arrival date of >15% and the Sand- < 0.001), and the Texas Coastal Plain (r = hill Crane counts during survey week 4 were −0.80, P < 0.001) demonstrated the strongest significantly correlated with temperatures at CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 53 Prop. SACR – x

Survey Bridge Segment

Fig. 8. The mean proportion of Sandhill Cranes (x– Prop. SACR) counted in each survey bridge segment per year from 2002 to 2010 (patterned bars) and from 2013 to 2017 (gray bars). Error bars represent one standard deviation from the mean.

TABLE 5. Bivariate ordinary least squares linear regression models for trends in the proportion of Sandhill Cranes per bridge segment (1–11) and river reach (including East [1–4], Central [5–7], and West [8–11]) by survey year, 2002–2017. See Table 3 for a description of bridge segments and Fig. 1 for a map of segments and reaches.

2 Reach/segment DV B SE t P R df East Prop. 0.0233 0.0028 8.25 0.001*** 0.850 12 1 Prop. 0.0049 0.0020 2.51 0.027* 0.344 12 2 Prop. 0.0023 0.0024 0.99 0.343 0.075 12 3 Prop. 0.0135 0.0029 4.63 0.001*** 0.641 12 4 Prop. 0.0025 0.0018 1.39 0.190 0.138 12 Central Prop. −0.0200 0.0035 −5.76 0.001*** 0.735 12

5 Prop. −0.0116 0.0021 −5.56 0.001*** 0.720 12 6 Prop. −0.0072 0.0017 −4.15 0.001** 0.589 12

7 Prop. −0.0011 0.0021 −0.54 0.601 0.023 12 West Prop. −0.0021 0.0010 −1.98 0.071 0.246 12 8 Prop. −0.0007 0.0006 −1.19 0.256 0.101 12 9 Prop. −0.0013 0.0004 −3.12 0.009** 0.449 12 10 Prop. −0.0001 0.0005 −0.20 0.846 0.003 12 11 Prop. 0.00004 0.0002 0.28 0.784 0.006 12 *P < 0.05 **P < 0.01 ***P < 0.001 all locations except southern New Mexico, and 0.10; Table 6). New Mexico weather data was most strongly correlated with temperatures in not related to arrival dates in the CPRV for the Texas Coastal Plain (r = −0.64, P < 0.05, any metric. Average temperatures in central and r = 0.73, P < 0.01, respectively; Table 6). Kansas were related to all crane arrival met- The average February and March tempera- rics (Table 6). tures in central Kansas had the strongest cor- Sandhill Cranes >30K was best predicted relation with >100K Sandhill Cranes (r = by weather and drought conditions in the −0.68, P < 0.01) and Sandhill Crane counts Texas Panhandle (wt = 0.59), closely followed in survey week 5 (r = 0.75, P < 0.001; Table by weather conditions in southwestern Okla- 6). The Palmer Drought Severity Index in homa (wt = 0.40; Table 7). Both of these mod- Oklahoma averaged for January and Febru- els demonstrated a highly significant negative ary had a marginal negative correlation with relationship between average temperature >100K, as well as >15% Sandhill Cranes (P < and arrival date on the Platte River, but they 54 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

TABLE 6. Pearson’s product-moment correlation coefficients relating average temperature (Temp) and Palmer’s Drought Severity Index (PDSI) measures for key wintering areas (January–February) and migration locations (Febru- ary–March/April) to the Julian date (JD) when Sandhill Crane counts (SACR) reached 30,000 (30K), 100,000 (100K), and 15% of a respective year’s peak count, as well as Sandhill Crane counts for survey week 4 (WK4; 5 March–11 March) and week 5 (WK5; 12 March–18 March), 2002–2017. J–F = January–February, F–M = February–March, and F–A = February–April. Wintering/ Coefficient JD SACR JD SACR SACR SACR JD SACR stopover area (months) >30K >100K WK4 WK5 >15% SW Oklahoma Temp (J–F) −0.84*** −0.35 0.65* 0.60* −0.59* PDSI (J–F) −0.37 −0.53 0.46 0.42 −0.46 Texas Panhandle Temp (J–F) −0.81*** −0.36 0.58* 0.46 −0.54* PDSI (J–F) −0.37 −0.44 0.31 0.33 −0.40 Texas Coastal Plain Temp (J–F) −0.80*** −0.50 0.73** 0.70** −0.64* PDSI (J–F) −0.12 0.01 0.12 0.12 −0.08 S. New Mexico Temp (J–F) −0.40 −0.23 0.28 0.10 −0.31 PDSI (J–F) −0.24 −0.34 0.18 0.34 −0.24 Central Kansas Temp (F–M) −0.78** −0.68** 0.70* 0.75** −0.62* PDSI (F–M) −0.12 −0.33 0.02 0.29 0.03 Central Nebraska Temp (F–A) −0.61* −0.51 0.62* 0.50 −0.53* PDSI (F–A) −0.21 −0.18 0.06 0.34 0.03 *P < 0.05 **P < 0.01 ***P < 0.001

TABLE 7. All generalized linear models with a delta weight ≥0.10 used to predict the relationship between temperature (Temp) and Palmer Drought Severity Index (PDSI) at key Sandhill Crane wintering areas (Location) and the Julian date (JD) when Sandhill Crane counts (SACR) reached 30,000 (30K), 100,000 (100K), and 15% of a respective year’s peak count (top), as well as Sandhill Crane counts (bottom) for survey week 4 (WK4; 05–11 March) and week 5 (WK5; 12–18 March) at the staging grounds in the Central Platte River Valley, Nebraska, 2002–2017. In the models, survey year functions as a control variable along with average temperature (Temp) and drought (PDSI) from particular locations functioning as covariates. “logLik” refers to log likelihood. Dependent variable Location Temp. B PDSI B logLik AICc delta weight JD SACR >30K TX Panhandle −2.6848*** −0.4317 −36.72 90.9 0.00 0.59 JD SACR >30K Southwest OK −2.5137*** −0.4121 −37.11 91.7 0.78 0.40 JD SACR >100K Central KS −1.2798** −1.3338ˆ −37.32 92.1 0.00 0.90 JD SACR >15% PK TX Coast −1.2768*** −1.1903* −33.68 84.9 0.00 0.71 JD SACR >15% PK Southwest OK −1.2665** −1.0991* −34.90 87.3 2.43 0.21 SACR WK4 (05–11 Mar) TX Coast 21,624** 16,910* −145.80 311.6 0.00 0.84 SACR WK4 (05–11 Mar) Southwest OK 21,035* 14,572 −147.90 315.8 4.19 0.10 SACR WK5 (12–18 Mar) TX Coast 21,059** 15,189ˆ −159.46 337.5 0.00 0.55 SACR WK5 (12–18 Mar) Central KS 18,694** 10,487 −159.93 338.4 0.94 0.35 ˆP < 0.10 *P < 0.05 **P < 0.01 ***P < 0.001 also included PDSI as a nonsignificant vari- arrival dates of 100,000 Sandhill Cranes to able (Table 7). For every unit increase in the CPRV. average temperature across January and Feb- Average temperatures (P < 0.001) and ruary in the Texas Panhandle, the model pre- PDSI (P < 0.05) in Coastal Texas (wt = 0.71), dicted a 2.7-day earlier arrival of at least followed by average temperatures (P < 0.01) 30,000 Sandhill Cranes (Table 7). Sandhill and PDSI (P < 0.05) in southwestern Okla- Cranes >100K was best predicted by climate homa (wt = 0.21), best predicted the >15% conditions in central Kansas (wt = 0.90), and Sandhill Crane arrival dates. For every unit included both average temperature (P < increase in the average winter temperature 0.01) and, marginally, PDSI (P < 0.10) as in Coastal Texas, the model predicted that significant variables, suggesting that increased >15% of Sandhill Cranes would arrive 1.3 days temperatures and drought in Kansas (to earlier to the CPRV, and that for every unit some degree) were associated with advanced increase in the PDSI, cranes would arrive CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 55

TABLE 8. Summary of the percent of land owned and managed by conservation organizations within 800 m of the main channel of the Platte River in 2016, the percent land cover classified as woodland-forest and meadow-prairie in 2016, and the percent change in land cover classified as meadow-prairie from 1998 to 2016 per survey bridge segment. See Fig. 1 for map of segments and study area. Segment/ Woodland Meadow-prairie Δ Meadow-prairie Conservation mgmt. location 2016 (%) 2016 (%) 1998–2016 (%) 2016 (%) 1 Chapman–HWY 34 21.5 20.7 −0.42 0.0 2 HWY 34–HWY 281 13.3 12.7 −0.78 0.7 3 HWY 281–Alda 3.4 49.6 1.32 67.4 4 Alda–Wood River 9.8 29.4 6.89 43.0 5 Wood River–Shelton 21.3 6.9 −0.60 16.7 6 Shelton–Gibbon 11.9 16.2 6.26 14.1 7 Gibbon–HWY 10 4.3 25.9 3.38 60.7 8 HWY 10–Kearney 18.8 13.3 12.11 38.2 9 Kearney–Odessa 30.7 2.0 −0.93 13.1 10 Odessa–Elm Creek 29.1 10.3 1.97 34.2 11 Elm Creek–Overton 30.6 7.0 1.48 40.5

1.2 days earlier (Table 7). In addition, week 4 CPRV were evident in the percent change of Sandhill Crane counts were best predicted by meadow-prairie from 1998 to 2016; segment environmental conditions in Coastal Texas (wt 8 increased in meadow-prairie cover by 12.1%, = 0.84), including both temperature (P < while segments 4 and 6 saw between a 6% and 0.01) and PDSI (P < 0.05) as significant inde- 7% increase. The proportion of area within pendent variables in the model. Our model 800 m of the main channel of the Platte River suggests an increase of 21,624 Sandhill Cranes managed by conservation organizations ranged in survey week 4 (05–11 March) with every from 0% in segment 1 to over 60% in seg- degree increase in average winter tempera- ments 3 and 7 (Table 8). tures in Coastal Texas, and a 16,910 increase On average, from 1938 to 2016, UOCW of in Sandhill Cranes for a one-unit increase in the main channel of the Platte River decreased PDSI (Table 7). Temperature in southwestern 59%, MUCW decreased 57%, and the num- Oklahoma also demonstrated a notable impact ber of total active channels of the Platte River on week 4 Sandhill Crane counts (wt = 0.10). increased 12% (Appendix 2, Fig. 10). It is impor- Week 5 Sandhill Crane counts were best pre- tant to note that our channel width analysis dicted by environmental conditions in Coastal did not consider peripheral channels, which Texas (wt = 0.55), including both average tem- have arguably demonstrated greater losses in perature (P < 0.01) and, marginally, drought portions of the CPRV (Williams 1978, Currier conditions (P < 0.10), followed by conditions 1991, Johnson 1994). The greatest decrease in in central Kansas (wt = 0.35; Table 7). channel width of the main channel was evident in segment 9 (Kearney to Odessa), which Spatial Factors decreased 82% in UOCW and 79% in MUCW Our land cover analysis revealed that in (Appendix 2). Concurrently, segment 9 increased 2016, the proportion of woodland-forest cover 140% in the number of active channels, because and the proportion of wet meadow–tallgrass the once singular and wide channel (x– = 1110 m, prairie (meadow-prairie) cover within 800 m SD = 121 m, in 1938; Appendix 2, Fig. 10) of the main channel of the Platte River varied became fragmented by several stabilized and widely across the CPRV (Table 8, Fig. 9). Seg- relatively large wooded islands (Figs. 3, 9; ments 9, 10, and 11 each had approximately Appendix 2). By contrast, segment 4 decreased 30% woodland-forest cover, whereas segments the least, with a loss of 15% of UOCW, a loss 3 and 7 both had <5%, and segment 4, <10% of 11% of MUCW, and a decrease in the woodland-forest cover (Table 8). There was number of active channels. This pattern was over 25% meadow-prairie cover in segments also observed for segment 3, which decreased 3, 4, and 7, whereas segments 5, 9, and 11 21% in the number of active channels and each contained <10% (Table 8, Fig. 9). Efforts largely maintained its channel width (Figs. 3, by conservation organizations to restore wet 9; Appendix 2). Historically, segments 3 and meadow and tallgrass prairie throughout the 4 were narrower than the western segments 56 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

Fig. 9. Proportion of meadow-prairie and woodland-forest land cover within 800 m of the main channel of the Platte River per bridge segment within the Central Platte River Valley in 2016.

(8–11) because the river was divided into mul- Appendix 2). The western segments (8–11) tiple channels in those areas (x– = 304 m, SD observed the greatest declines in channel = 40 m, and x– = 377 m, SD = 106 m, respec- width and the highest increases in the num- tively, in 1938; Appendix 2), and the complete ber of active channels (Fig. 10, Appendix 2). loss of some northern side channels to woody All segments improved when both MUCW accretion between 1938 and 2016 absorbed and UOCW from 1998 to 2016 were consid- much of the declines in flow (Figs. 3, 9; ered (Appendix 2). CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 57

Fig. 10. Proportional change in total (UOCW) and maximum (MUCW) unobstructed channel width within the main channel of the Central Platte River, as well as the proportion change in the total number of active channels of the Platte River (CHAN), 1938–2016.

TABLE 9. Pearson’s product-moment correlation coefficients for the relationships between habitat covariates and Sandhill Crane abundance metrics per bridge segment, including channel width measurements, percent meadow-prairie, percent woodland-forest, and percent of conservation managed area in 2016, channel width changes observed since 1938 and 1998, respectively, and change in meadow-prairie from 1998 to 2016. Sandhill Crane abundance variables include the trend in proportional use from 2002 to 2017 (Trend) as well as the mean proportion (x– Prop.) and density of Sandhill Cranes (x–/km) observed per bridge from 2015 to 2017. Segments (1–11) include all bridge segments between Chapman and Overton, Nebraska; Longitude (E–W) = median longitude; D UOCW 1938–2016 = percent change in unobstructed channel width from 1938 to 2016; D MUCW 1938–2016 = percent change in maximum unobstructed channel width from 1938 to 2016; D No. Active Channels = percent change in the number of active channels of the Platte River; D UOCW 1998–2016 = percent change in unobstructed channel width from 1998 to 2016; D MUCW 1998–2016 = percent change in maximum unobstructed channel width from 1998 to 2016; x– UOCW 2016 = mean unobstructed channel width in 2016; x– MUCW 2016 = mean maximum unobstructed channel width in 2016; % Woodland 2016 = percent woodland within 800 m of the main channel of the Platte River in 2016; % Meadow 2016 = percent meadow-prairie within 800 m of the main channel of the Platte River; D Meadow 1998–2016 = percent change in meadow-prairie from 1998 to 2016; % Conservation 2016 = percentage of land owned or managed by conservation organizations within 800 m of the main channel of the Platte River. – – Covariate Trend x Prop. x/km Segments (1–11) −0.43 −0.67* −0.58 Longitude (E–W) 0.41 0.68* 0.59 D UOCW 1938–2016 0.44 0.88*** 0.92*** D MUCW 1938–2016 0.51 0.82** 0.88*** D No. Active Channels −0.48 −0.60* −0.60* D UOCW 1998–2016 −0.46 −0.37 −0.34 D MUCW 1998–2016 −0.42 −0.53 −0.44 x– UOCW 2016 0.22 0.17 0.20 x– MUCW 2016 0.31 0.29 0.37 % Woodland 2016 −0.31 −0.78** −0.80** % Meadow 2016 0.67* 0.87*** 0.88*** D Meadow 1998–2016 −0.04 −0.09 0.03 % Conservation 2016 0.33 0.42 0.52 *P < 0.05 **P < 0.01 ***P < 0.001 58 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

The proportion of meadow-prairie per seg- 2003, Pearse et al. 2010, Buckley 2011, Krapu ment in 2016 was the only land cover metric et al. 2014). Annual weather influenced chronol- significantly correlated with the trend in the ogy of migration to the CPRV, and we found annual proportion of Sandhill Cranes per seg- indications of advancing migration arrival, ment from 2002 to 2017 (Fig. 9, Table 9). The with the most recent years showing significant 2 habitat metrics having the strongest correla- numbers of Sandhill Cranes arriving in late tions with the mean proportional use of bridge February. By contrast, variation in the spatial segments from 2015 to 2017 were the change distribution of Sandhill Cranes roosting along in UOCW from 1938 to 2016 and the propor- the Platte River was relatively patterned. tion of meadow-prairie in 2016 (Table 9). The Sandhill Crane distributions were related to change in MUCW from 1938 to 2016 and the the availability of quality river roost sites and proportion of woodland-forested area in 2016 meadow-prairie habitats that are important also demonstrated strong relationships with for foraging and social behavior. Long-term proportional use from 2015 to 2017 (Table 9). changes in hydrology and land cover were The density of Sandhill Cranes from 2015 to related to roosting Sandhill Cranes shifting 2017 was highly correlated with change in eastward, creating higher densities in eastern UOCW (r = 0.92, P < 0.001) and MUCW (r segments. Our results demonstrated how tem- = 0.88, P < 0.001) from 1938 to 2016, and porospatial changes in a species’ regional with the proportion of meadow-prairie (r = occurrence may be simultaneously associated 0.88, P < 0.001) and woodland-forest in 2016 with multiple independent processes that can (r = −0.80, P < 0.01) (Table 9). These vari- interact in their influence; in this case, these ables also demonstrated strong correlations processes are landscape-level habitat changes among themselves (Appendix 3): change in within the MCP’s main spring staging area UOCW from 1938 to 2016 was highly corre- and wamer and drier winters associated with lated with both meadow-prairie cover (r = climate change on Sandhill Crane wintering 0.82, P < 0.001) and woodland-forest cover (r grounds (see Runkle et al. [2017] and Fitzpat- = −0.86, P < 0.001), while meadow-prairie rick and Dunn [2019] for climate data and pro- cover and woodland-forest cover exhibited a jections). Our results indicated that Sandhill strong negative correlation (r = −0.81, P < Cranes are arriving at the CPRV earlier, stay- 0.001; Appendix 3). ing longer, and concentrating in limited reaches The top bivariate model for predicting Sand- with higher-quality riverine and meadow- hill Crane density from 2015 to 2017 was the prairie habitats in increasing densities. In this change in UOCW (wt = 0.81) followed by the way, climatic variation and landscape-level change in MUCW (wt = 0.10) from 1938 to habitat change are related to increasing site 2016 (Table 10). Change in UOCW from 1938 use intensity in portions of the CPRV. to 2016 (wt = 0.48) and the proportion of Temporal Dynamics meadow-prairie in 2016 (wt = 0.41) best determined the proportion of Sandhill Cranes Our population indices were generally lower using each segment from 2015 to 2017 (Table than those produced by the USFWS-coordi- 10). The proportion of meadow-prairie in nated spring survey of the MCP for the CPRV 2016 (wt = 0.42) was the best predictor of the (Kinzel et al. 2006, Dubovsky 2016, 2017), statistical trend in the proportional use of seg- suggesting that density estimates derived from ments by Sandhill Cranes from 2002 to 2017 our study may have negative bias. The USFWS (Table 10, Fig. 11). predawn aerial surveys of the river were ceased in the early 1980s in favor of a daytime survey DISCUSSION composed of transects running perpendicular to the river because the daytime surveys were Our investigation of 14 years of Sandhill deemed to provide more reliable population Crane aerial survey data suggested that the abundance indices (Ferguson et al. 1979, Ben- migration chronology in the CPRV has high ning and Johnson 1987). However, repeated annual variation. We documented peak counts predawn aerial surveys of the Central Platte over a wider range of dates than the majority River can produce useful depictions of river of published records; this range is likely a roosting distributions and relative densities result of our long-term data set (Davis 2001, over time (Davis 2003, Buckley 2011), and can CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 59

TABLE 10. Statistical models, with a delta weight ≥0.10 as measured by AICc, used to predict average density of Sandhill Cranes from 2015 to 2017 (x–/km), average proportional use from 2015 to 2017 (x– Prop.), and trends in use from 2002 to 2017 (Trend) within bridge segments 1–11 in response to habitat variables. Coefficients include change in unobstructed channel width between 1938 and 2016 (DUOCW), change in maximum unobstructed channel width between 1938 and 2016 (DMUCW), and percent meadow-prairie land cover in 2016 (% Meadow-Prairie). An ordered logistic regression model with a cumulative “probit” link function was used to predict “Trend” ( 1 = negative, 0 = no trend, − – – 1 = positive), and generalized linear models with Gaussian distributions were used to predict “x/km” and “x Prop.” Dependent Log

variable Coefficient B SE t/z likelihood AICc delta weight – x/km DUOCW 5840.2*** 818.7 7.1 −84.1 177.7 0.00 0.81 DMUCW 5590.5*** 988.4 5.7 82.1 178.8 1.14 0.10 – − x Prop. DUOCW 0.41*** 0.07 5.5 18.2 −26.9 0.00 0.48 % Meadow-Prairie 0.66*** 0.12 5.4 18.0 −26.6 0.30 0.41 Trend % Meadow-Prairie 1.27* 0.64 1.9 −7.6 24.7 0.00 0.42 *P < 0.05 **P < 0.01 ***P < 0.001 be used to assess Sandhill Crane relative abun- be important to consider the potential impacts dance and distribution in the CPRV (assum- of changing survey methods on the integrity of ing that detection probability is relatively con- the USFWS’s long-term data set as well as stant). Our results indicated that the average harvest regulation management. day of peak Sandhill Crane abundance in the Distributions of Sandhill Cranes that were CPRV was 25 March, one day earlier than the developed during the peak of migration (e.g., estimate provided by Pearse et al. (2015). We Kinzel et al. 2006, Dubovsky 2016) may under- documented peak counts as early as 8 March estimate eastern segments, which our findings and as late as 8 April across 14 survey years. indicate peaked in use 1–2 weeks ahead of Pearse et al. (2015) estimated peak counts western segments (Table 3, Fig. 5). Krapu et between 13 March and 3 April from 2001 to al. (2014) found some evidence for this under- 2007 using data from Sandhill Cranes tracked estimation, because WC–A and EC–M breed- with platform transmitting terminals (PTTs). ing affiliation Sandhill Cranes, which are pre- From 2001 to 2007, between 71% and 94% of dominantly Greater Sandhill Cranes, are more PTT-marked Sandhill Cranes were present likely to roost in eastern segments (EC–M = within the CPRV during the USFWS spring 97.8% and WC–A = 76.8% east of Shelton). coordinated survey, suggesting that the yearly They also tend to have shorter total migra- population indices reflected varying proportions, leaving the CPRV 3–12 days earlier tions of the MCP in the CPRV to a greater than WA–S and NC–N breeding affiliation degree than real changes in the population Sandhill Cranes, which comprise mostly Lesser (Pearse et al. 2015). Our results documented Sandhill Crane subspecies (Krapu et al. 2014). even wider temporal variation in Sandhill Crane Our findings also suggest that the early arrival peak abundance than the USFWS survey did. of significant proportions of the MCP to the As Ferguson et al. (1979) recommended, con- CPRV is associated with seasonal weather ducting photo-corrected spring surveys weekly trends on the wintering grounds and south- over a period of at least 3 weeks would likely ern stopover locations (Tables 6, 7). This trend reduce fluctuation in the USFWS population seems particularly pronounced for EC–M, abundance index. However, doing so would WC–A, and (to some degree) NC–N breeding need to be weighed against financial costs and affiliations of Sandhill Cranes that use the the prospective value of finer-resolution but Texas Coastal Plain, the Texas Panhandle, south- less frequent data. One solution could be to western Oklahoma, and central Kansas for conduct surveys across 3 weeks every 3 years, their wintering grounds and migration routes which would equate to roughly the same effort (Krapu et al. 2011, 2014; Fig. 2). Our models and may produce a more robust abundance indicated that an increase in the mean winter index. Another option may be to conduct repli- temperatures of a key wintering region by 1 °F cate surveys in only the densest Sandhill Crane (0.56 °C) was associated with an increase in roosting areas annually; a combination of both the abundance index of Sandhill Cranes in the alternative strategies could also work. It will CPRV by tens of thousands of cranes in early 60 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

0.5 A

0.4

0.3

0.2 % Meadow-Prairie 2016 0.1

0.0 -1 0 1 Trend B

-0.2

-0.4

MUCW 1938–2016 -0.6 D

-0.8 -1 0 1 Trend 1.5 C

1.0

0.5 No. Active Channels 1938–2016 No.

D 0.0

-1 0 1 Trend

Fig. 11. Habitat parameters by bridge segments with statistically significant positive (1), stable (0), or negative (−1) trends (Trend) in the proportional use by Sandhill Cranes across survey years 2002–2017. Black horizontal lines denote median values, while the top and bottom of boxes denote the upper and lower interquartile ranges (75th and 25th percentiles), respectively. Extending “whiskers” denote values of 1.5 times the interquartile range; areas outside of this range constitute outliers and are marked with points. (Caption continued on page 61.) CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 61

D 0.3

0.2 % Woodland 2016 % Woodland

0.1

-1 0 1 Trend E

0.6

0.4

0.2 % Conservation 2016

0.0 -1 0 1 Trend

Fig. 11. Continued. A, Proportion of meadow-prairie land cover within 800 m of the main channel of the Platte River in 2016 (% Prairie-Meadow 2016). B, Percent change in maximum unobstructed channel width from 1938 to 2016 on the main channel of the Platte River (ΔMUCW 1938–2016). C, Percent change in the number of active channels of the Platte River from 1938 to 2016 (Δ No. of Active Channels 1938–2016). D, Proportion of land cover within 800 m of the main channel of the Platte River classified as woodland in 2016 (% Woodland 2016). E, Proportion of land within 800 m of the main channel of the Platte River owned or managed by conservation organizations in 2016 (% Conservation 2016). The highest median value for conservation ownership is where trends are flat.

March (Tables 6, 7). This association suggests (Runkle et al. 2017; mean temperatures in that relatively small temperature changes Texas have already increased 1 °F). Our results could result in large temporal shifts in the also demonstrated that a one-unit increase in timing of the MCP’s spring migration, espe- PDSI values at some wintering locations (such cially considering that average temperatures as Coastal Texas) was associated with an increase in Texas are projected to increase by ∼4 °F of over 10,000 Sandhill Cranes in the CPRV (2.22 °C; low-emissions scenario) to ∼10 °F in early March (Tables 6, 7). Coastal Texas is (5.56 °C; high-emissions scenario) by 2100, projected to receive between 5% and 10% compared to mean values from 1901–1960 less annual precipitation by 2050 (Runkle et 62 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 al. 2017). Along with temperature and recent and 1.4 days per year (Table 2, Fig. 4). For available water content in soils, precipitation instance, counts of over 125,000 Sandhill Cranes is a major determinant of soil moisture bal- advanced during our study, while becoming ance and therefore drought indices such as scarcer later in the migration season across PDSI (Hayes et al. 2007). Consistent increases years, demonstrating that trends in early arrival in temperature, along with decreases in pre- were not the result of population growth but cipitation, will increase the risk for drought likely of a temporal shift in migration (Table and decrease available wetland area that 2, Fig. 4). From 1942 to 2016, Whooping Sandhill Cranes depend on throughout their Cranes advanced their spring and fall migra- wintering range (Hayes et al. 2007, Harner et tion dates by approximately 21 and 22 days, al. 2015, Reese and Skagen 2017, Runkle et respectively, in the central Great Plains (Jor- al. 2017). gensen and Brown 2017), whereas Common A number of studies indicate that shorter- Cranes (Grus grus) in France have advanced distance migrants, like those in the WC–A their spring migration by about 20 days over a and EC–M breeding affiliations, are more period of 30 years (Filippi-Codaccioni et al. flexible and responsive to local conditions in 2011). The first reported sightings of Sand- their migration timing and routes (Temple hill Cranes submitted to the Nebraska Bird and Cary 1987, Adamík and Pietruszková 2008, Review from 1914 to 2013 have advanced from Palm et al. 2009, Swanson and Palmer 2009). approximately late March to early February Given the relatively shorter migration dis- (Harner et al. 2015). Our data suggest that the tance of Greater Sandhill Cranes compared to MCP has advanced its migration by between Lesser Sandhill Cranes, it is possible that the 18 and 23 days over the last 16 years. Despite former disproportionately comprises increases the different temporal scopes of the data sets in early arrivals and potentially in overwinter- used to model changes in the migratory chro- ing occurrences (Harner et al. 2015). Mean nology of crane species, the various data sets winter temperatures and drought conditions each achieve a very similar result. The Com- on the Coastal Plain of Texas, predominantly a mon Crane, Whooping Crane, and Sandhill wintering location for Greater Sandhill Cranes, Crane have all advanced their spring migra- appeared to be a factor influencing early arrivals tion in the Northern Hemisphere over histor- of significant numbers of cranes (Table 7). ically recorded dates (Alonso et al. 2008, Concurrently, Krapu et al. (2014) found that Prange 2012, Harner et al. 2015, Jorgensen departure dates from the CPRV were corre- and Brown 2017). Research suggests that cli- lated with daily ambient temperatures in late mate in the CPRV during the last few decades March and early April for Greater Sandhill has been anomalous compared to the climate Cranes but not for Lesser Sandhill Cranes. record of the last 150 years, being more vari- Our research indicated a decline in Sandhill able and showing a rapid warming trend Cranes remaining in the CPRV through the (Hughes 2000, Mann et al. 2016, Pittock 2017, first week of April (survey week 8, 02–08 April; Runkle et al. 2017). Therefore, it is possible Table 2). It is possible that this decline is influ- that a large portion of these migration advances enced by Greater Sandhill Cranes departing have taken place over the last 2 decades, and the CPRV earlier in recent years. Krapu et al. that the increased climatic variation may also (2014) also noted that staging length in the result in wider variation in the timing of spring CPRV was negatively correlated with arrival staging in the CPRV (Harner et al. 2015, Pit- dates, suggesting that early arrivals tend to tock 2017). Increasing spring temperatures have stay longer. A high number of Sandhill Cranes been related to advancing migration chronol- extending the period in which they stage at ogy (Filippi-Codaccioni et al. 2011, Jorgensen the CPRV will likely put additional pressure and Brown 2017); our results corroborate these on agricultural foraging resources (Pearse et findings in that warmer temperatures at key al. 2010, Salvi 2012), as well as increase the wintering and early spring stopover locations disease risk for cranes and other waterbird explained variation in arrival dates and migra- species that overlap in wetland habitat use tion chronology of the MCP to the CPRV. (Vogel et al. 2013, Bertram et al. 2017). Despite the consistency of advancement in All metrics of migration chronology demon- migration chronology across various metrics strated an advancement of between 1.1 days in our data, it is important to note that research CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 63 from the late 1970s and early 1980s described 2018). However, the replacement of native grass- the Sandhill Crane migration as spanning from lands and wetlands with agricultural lands pro- late February to mid-April with a peak in vided Sandhill Cranes with a greater carrying abundance most often in late March (Fergu- capacity at a range-wide scale, particularly by son et al. 1979, USFWS 1981, Krapu et al. improving foraging opportunities on the winter- 1982). Our data demonstrate that the majority ing grounds and at stopover locations, because of the migration still occurs in March, with corn provides more fat per ounce than native the peak often occurring in late March. How- plant resources (Krapu et al. 1984, Iverson et ever, the tail of the distribution has switched al. 1987, Pearse et al. 2010, Gerber et al. from April to February in our data, with more 2014). Sandhill Cranes and Common Cranes in individuals arriving early in February in recent Europe, two of the world’s most granivorous years, and fewer staying past peak abundance crane species, have likely become more spa- into mid-April. For example, from 2002 to 2005, tially and temporally flexible in wintering habi- counts from week 9 (09–15 April) consistently tat use in response to the increased produc- exceeded counts from survey week 4 (05–11 tion of cereal grains, including corn and rice March), and from 2014 to 2017, the opposite (Oryza sativa) (Alonso et al. 1994, 2008, Miene was true, with variation in the intervening and Archibald 1996, Pearse et al. 2010, Prange years. In 2016 and 2017, week 3 (26 Febru- 2012). Research indicates that the wintering ary–04 March) exceeded week 9 for the first distribution of Common Cranes in Europe has time in our data, which suggests that Sandhill shifted north as a result of warmer winters and Cranes are likely coming earlier to the CPRV agricultural waste grain availability (Alonso et and staying longer, but that their stopover is al. 1994, 2008, Prange 2012). The advances in now less commonly extending into mid-April. Sandhill Crane migration described during this Long-term changes in wind and storm pat- study are probably the interactive result of terns associated with climate change have also recent above-average winter temperatures asso- been linked to shifting avian migration pat- ciated with climate change, periods of relative terns (Adamík and Pietruszková 2008, Min- drought, and the availability of waste grain on gozzi et al. 2013). Cranes migrate primarily by key wintering grounds and stopover locations. thermal soaring and therefore are dependent Parsing out the influences of these particular on favorable wind conditions (Swanberg 1987, factors is beyond the scope of this study, but it Volkov et al. 2016). Shifts in the spring wind should be a consideration when interpreting regime (Catto et al. 2014) or precipitation pat- our results, and an area for future research. terns (Trenberth 2011) that provide moisture Spatial Dynamics to basin wetlands, which are important to the MCP in the southern plains, could result in Our findings demonstrated increasing pro- further temporal or spatial shifts in spring portional use and densities in eastern seg- migration (La Sorte et al. 2019). Drought con- ments (Tables 4, 5). These increases suggest ditions at wintering and early spring stopover the continuation of a trend associated with locations, particularly in Coastal Texas and habitat loss that began prior to the 1938 aerial southwestern Oklahoma, predicted MCP arrival imagery used in this study (Eschner et al. metrics in the CPRV, suggesting that weather 1983, O’Brien and Currier 1987). Channel patterns aside from temperature are also impor- width losses had already been noted from 1938 tant (Table 7). Periods of extended drought in to 1965 near Cozad, Nebraska, an area aban- the southern plains may have been a major doned by Sandhill Cranes well before CPRV factor in irregular wintering distributions of conservation efforts began in the 1970s (Walkin- both Whooping Cranes and Sandhill Cranes shaw 1956, Williams 1978, O’Brien and Cur- (Wright et al. 2014, Harner et al. 2015). rier 1987, Krapu et al 2014). Sandhill Cranes Sandhill Cranes require flooded herbaceous have been moving east since at least the 1950s, emergent wetlands for breeding; agricultural and they have abandoned much of the west- expansion led to the regional extirpation of ern CPRV (Walkinshaw 1956, Faanes and Le Sandhill Cranes from significant portions of Valley 1993, Buckley 2011). Faanes and Le Val- their former breeding range, particularly within ley (1993) noted a negative trend (−0.5% per the Great Plains (Walkinshaw 1949, Baker et al. year) in the density of Sandhill Cranes between 1995, Gerber et al. 2014, Silcock and Jorgensen Kearney and Wood River, and a positive trend 64 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

(+0.7% per year) farther east from Wood River segment 8 from 1998 to 2016, we found that to HWY 34. We demonstrated a continuation meadow-prairie cover increased 12.1% but still and acceleration of this trend with a 2.3% per totaled only 13.3% in 2016, which is well below year increase in the total proportion of Sand- the cover associated with segments 1 (20.7%) hill Cranes detected from Wood River to Chap- and 3 (49.6%) where Sandhill Crane use man, Nebraska, and a 2.0% per year decrease in appears to be increasing most significantly the total proportion of Sandhill Cranes detected (Tables 5, 8). Further concentration of Sandhill from Kearney to Wood River, Nebraska (Table 5). Crane densities along the CPRV promotes a Buckley (2011) found that segment 1 (HWY potential increase in disease risk for Sandhill 34 to Chapman) had low Sandhill Crane use Cranes and other organisms, including Whoop- from 2002 to 2010 despite having some of the ing Cranes (Lu et al. 2013, Bertram et al. 2017, best habitat in the CPRV, presumably because Fenton et al. 2018). Additionally, increased it was isolated east of major roosting densities. densities of Sandhill Cranes in fewer reaches of Segment 1 now exemplifies one of the strongest the CPRV escalates the potential risk for mass positive trends in proportional use per year mortality incidents resulting from extreme (Table 5). Segments 1 and 2 supported about weather events like hail and ice storms (Hig- 13.8% of the Sandhill Cranes from 2002 to gins and Johnson 1978, Lingle 1997, Narwade 2010, and supported 22.4% from 2013 to 2017 et al. 2014). Increasing the spatial footprint of (Fig. 8). However, these segments have <1% of habitat restoration efforts may encourage dis- the total area within 800 m of the main channel persal of the MCP throughout the CPRV. of the Platte River in conservation ownership Our spatial model demonstrated that large or management, such as easements. Gaining proportional declines in total unobstructed chan- protections from development on these lands nel width (UOCW) and maximum unobstructed should be a top priority for ensuring the eco- channel width (MUCW) per segment from 1938 logical integrity of migratory Sandhill Crane to 2016 were associated with reduced propor- habitat in the CPRV (Tables 4, 5; Figs. 1, 6; tional use and densities of Sandhill Cranes from Faanes and Le Valley 1993). Central segments 2015 to 2017, despite generally wide channel still hold densities of Sandhill Cranes compara- widths in 2016 in some locations (Tables 9, ble to eastern segments on average (Fig. 7b); 10). The percent of channel width loss in the nevertheless, significant declines in the propor- main channel of the Platte River from 1938 tional use per year may be concerning for con- to 2016 may serve as an effective proxy for servation managers (Tables 4–5; Figs. 6–8). Our multiple dimensions of habitat change associ- findings suggest that significant conservation ated with woodland accretion and channel mor- ownership in particular western and central seg- phology alteration (Schumm 1963, Williams ments may be responsible for maintaining sta- 1978, Johnson 1994, Horn et al. 2012). Sand- ble roost densities adjacent to unmanaged areas hill and Whooping Cranes use the Platte River of declining density (Fig. 11e). This is exempli- in great part to take advantage of quality sand- fied by segment 7, where over 60% of the land bar roosting habitat characteristic of braided within 800 m of the river is under conservation rivers (Krapu et al. 1982, Kinzel 2009). How- management. We estimated little change in ever, the Platte River has been transitioning crane use in this river segment, despite it being from a braided river to a more anabranching bordered to the east by a segment declining in river system (sections where the channel is crane use and having low conservation owner- split by stabilized islands) as a result of reduc- ship (14%) and to the west by a segment with tions in sediment load and discharge (Williams low densities (Tables 4, 5, 8; Figs. 6, 7). How- 1978, Eschner et al. 1983, O’Brien and Cur- ever, recent increases in conservation owner- rier 1987, Horn et al. 2012). ship in some western and central segments and Change in channel width from 1938 to 2016 the associated habitat restoration efforts have is likely a top explanatory variable because it not significantly redistributed densities of Sand- reflects that segments which have changed the hill Cranes, potentially because the extent of least from historic widths, have maintained a wet meadow and braided river restorations may more braided nature than reaches which have not have been large enough to make an impact seen large percent losses of channel width; (Tables 4, 5, 8). For instance, following conser- these least-changed segments have become vation land purchases and restorations within more anabranching (O’Brien and Currier 1987, CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 65

Horn et al. 2012). Our findings demonstrate Kearney to Odessa (segment 9), an area that that segments that exhibited the greatest losses has experienced continued declines in Sandhill in UOCW from 1938 to 2016 also had the Crane use (Faanes and Le Valley 1993; Tables highest levels of woodland-forest land cover 4, 5), provides a clear example of drastic chan- in 2016 and saw the largest increases in the nel loss; UOCW averaged over 1 km in 1938 number of active channels from 1938 to 2016 and has declined by over 80% as of 2016. (Fig. 10; Appendixes 2, 3). Percent change in The other factor that best predicted the the number of active channels and the per- proportion of Sandhill Cranes using individ- cent cover of woodland-forest were negatively ual segments and trends therein was the pro- correlated with Sandhill Crane proportional use portion of meadow-prairie land cover within and densities per segment from 2015 to 2017 half a mile of the river. Faanes and Le Valley (Table 9). Heavily wooded and anabranched (1993) suggested that Sandhill Crane habitat reaches of the Platte River have more stabi- in the CPRV was limited by the availability of lized banks (O’Brien and Currier 1987, John- appropriately wide channels and wet meadow son 1994); have comparatively incised (steeper habitat. Sidle et al. (1993) also found a corre- banks and deeper channels), fragmented, and lation between selected roost sites and dis- sinuous channels (Schumm 1963, Williams 1978, tance to wet meadow. These results are fur- Eschner et al. 1983, Horn et al. 2012); have ther corroborated by Sparling and Krapu (1994), less exposed sandbar habitat (Kinzel 2009, Horn who used flight distances to particular habitat et al. 2012); and are therefore lower-quality resources as a proxy for their importance. Spar- Sandhill and Whooping Crane roosting habi- ling and Krapu (1994) found that distances to tat, despite occasionally having UOCWs in 2016 roost sites were highest followed by distances appropriate for crane roosting (Eschner et al. to wet meadows. We found that meadow-prairie 1983, O’Brien and Currier 1987, Farmer et land cover was the best predictor of a posi- al. 2005, Pearse et al. 2017). These reaches tive annual trend in proportional use per seg- also have less meadow-prairie or agricultural ment from 2002 to 2017. It was also strongly habitat adjacent to the river for pre-roost associated with mean proportional use and aggregations important to both the pair bond- density from 2015 to 2017, providing evidence ing and the safety of Sandhill Cranes (Currier that meadows (particularly those within 800 m 1982, Johnsgard 1983, Tacha 1988). Addition- of the main channel of the Platte River) rep- ally, mature cottonwoods adjacent to the river resent a habitat resource influencing the dis- provide quality habitat for Bald Eagles (Hali- tribution of Sandhill Cranes in the CPRV. aeetus leucocephalus), which have increased Our top models suggested that the distri- significantly in abundance in the CPRV over bution of Sandhill Cranes in the CPRV, and the last 3 decades and pose a potential depre- therefore their distributional shifts from west dation risk to Sandhill Cranes (Silcock and to east, are best explained by the availability of Jorgensen 2017, Caven et al. 2018). limited habitat resources, specifically meadow- Though the rate of woodland-forest devel- prairie habitat and channels that have changed opment in the former riverbed stabilized by the least in width and character since 1938. the early 1970s, the abundance of vegetated Despite increases in meadow-prairie land cover islands within the existing high banks of the in most western segments and mean UOCWs channel and the number of anabranches have in all western segments from 1998 to 2016 continued to increase from 1984 to 2009 (Cur- (Table 8, Appendix 2), Sandhill Cranes con- rier 1982, O’Brien and Currier 1987, Johnson tinued to shift east from 2002 to 2017. This 1994, Horn et al. 2012). These hydro-ecological suggests that improvements made since 1998 changes have not occurred uniformly through- may not be large enough in scale to redistrib- out the CPRV, because sediment load, flow ute densities of Sandhill Cranes. Sandhill Cranes regime, and the intensity of active manage- move, on average, 5.7 km between roost sites ment for conservation purposes differ through- from night to night, and contiguous areas of out the CPRV (Williams 1978, Eschner et al. quality habitat allow for a wider selection of 1983, O’Brien and Currier 1987, Pfeiffer and important resources (Sparling and Krapu et al. Currier 2005, Rapp et al. 2012). These changes 1994, Krapu et al. 2014) and consistently support have been most pronounced in the western higher densities of cranes (Davis 2003, Buckley portion of the CPRV (Table 8; Figs. 3, 8, 9). 2011). Nonetheless, our findings demonstrated 66 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 declines in some segments that include areas distances to waste grain foraging sites. Depen- of high-quality riverine and meadow habitat. dence upon market-driven products, such as Buckley (2011) argues that Sandhill Crane particular cultivated grains, poses a risk to declines in western segments where quality wildlife populations at various spatial scales habitat exists may be reflective of roosting (Krapu et al. 2004, Salvi 2012). For instance, habitat isolation. Although not a variable in top Salvi (2012) found that decreases in corn pro- models, the median longitude was significantly duction in favor of rapeseed (Brassica napus) correlated with the proportional use of seg- were associated with negative trends in Com- ments increasing from west to east (Table 9). mon Crane abundance at some historic win- Seager et al. (2018) recently documented the tering areas in southern France. Increases in eastern shift of the climactic conditions his- soybean (Glycine max) cultivation in the CPRV torically associated with the 100th meridian, could pose similar risks for Sandhill Cranes, which demarcates the longitudinal start of because they do not forage on it regularly and the arid west on the North American conti- soybeans are nutritionally deficient (Krapu et nent, to what is about the 98th meridian. This al. 2004, 2005b). Pearse et al. (2010) suggest broad shift in climate could negatively impact that efforts to improve riverine habitat to basin wetlands and the water birds that redistribute Sandhill Cranes could decrease depend on this habitat in the southern plains competition for waste grain resources near (Covich et al. 1997, Reese and Skagen 2017). high-quality sites and help Sandhill Cranes Pearse et al. (2018) recently documented an energetically by decreasing the distance from eastern shift of about 1.2 km per year in the riverine roosting areas to agricultural forag- migratory corridor of the Whooping Crane ing areas. over the last 8 decades. It is possible that Management Implications eastward shifts in the distribution of Sandhill Cranes since the 1950s, which we continued To promote the redistribution of Sandhill to document in our research, are reflective Cranes throughout the CPRV, managers could not only of habitat change in the CPRV, but restore lowland tallgrass prairie and wet also of increasingly arid conditions in the meadow within 800 m of the main channel of western portion of the traditional Sandhill the Platte River (Pfeiffer 1999, Riggins et al. Crane migration corridor (Covich et al. 1997, 2009, Meyer et al. 2010). As much of these Reese and Skagen 2017). lands are wooded, reducing woodland-forest Though our study did not critically evalu- cover (particularly areas of more recent wood- ate the effect of waste corn availability on the land accretion dominated by invasive species) distribution of Sandhill Cranes in the CPRV, and restoring it to meadow-prairie habitat research suggests that cornfields nearer to sig- could prove an effective strategy. Also, restor- nificant complexes of wet meadow and lowing croplands to native habitats adjacent to land tallgrass prairie receive more use (Spar- existing tracts of prairie-meadow habitat could ling and Krapu 1994, Anteau et al. 2011). maximize the footprint of contiguous herba- Concurrently, research indicates that waste corn ceous land cover and protect its ecological availability in the CPRV, which Sandhill Cranes integrity (Rowe et al. 2013). Russian olive and depend on, has declined as a result of har- eastern redcedar are problematic invasive vest efficiency and competition with growing species that did not become established to a numbers of Snow Geese (Chen caerulescens) significant extent in the CPRV until the 1950s, and other waterfowl (Krapu et al. 2004, Pearse and they can negatively impact the structure et al. 2010). Interestingly, Pearse et al. (2010) and function of riverine and prairie ecosystems found that competition for waste corn resources (Currier 1982, Huddle et al. 2011, Coppedge was highest in the eastern portion of the et al. 2001a, Nagler et al. 2011). Areas with CPRV, where we demonstrated increased Sand- high densities of these species provide a tar- hill Crane relative abundance and density from get for riverine meadow-prairie habitat restora- 2002 to 2017. Krapu et al. (2005a) suggest that tion efforts focused on improving Sandhill Crane fat storage in Sandhill Cranes may have declined habitat. Though there is some debate regard- within the CPRV from the late 1970s to the ing the historic density of eastern cottonwood mid-1990s, likely as a result of additional energy and peachleaf willow trees in the CPRV (Cur- expenditure associated with increased flight rier 1982, Currier and Davis 2000, Johnson CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 67 and Boettcher 2000), it is clear that woodland- CPRV’s riparian woodlands, and that most forest cover far exceeds that from before the migrant species demonstrated weight loss large-scale development of Nebraska’s water during stopover periods between 1998 and resources beginning in the late 1800s (Williams 2001. However, Scharf et al. (2008) argue that 1978, Currier 1982, Eschner et al. 1983, O’Brien these habitats provide an important forest and Currier 1987, Johnson 1994). Restoration stepping stone for migrating woodland birds efforts will be most acceptable if they maximize moving through the central Great Plains that the benefit to Sandhill Cranes and other native is superior to the surrounding grassland habi- prairie species while minimizing the financial tats. Furthermore, species of regional concern, costs of such an endeavor and the risks to such as the Red-bellied Snake (Storeria occip- species of concern that utilize woodlands. itomaculata), rely upon eastern cottonwood– Sandhill Cranes are an effective “umbrella dominated riparian forests along the Platte species” in the CPRV because their habitat River (Geluso and Harner 2013, Tye et al. preferences reflect the historic structure of the 2017). Strategically reducing the density of ecosystem (Currier 1982, Currier and Davis woodland-forests (particularly areas dominated 2000, Davis 2003), mirror the needs of a num- by invasive species) to increase meadow-prairie ber of species of concern (Faanes et al. 1992, land cover where quality Sandhill Crane roost- Kirsch 1996), and delineate an ecologically sig- ing habitat can be restored would leave significant area (Caro and O’Doherty 1999, Suter nificant habitat to meet the needs of wood- et al. 2002). Whooping Cranes and Least Terns land species. Many side channels of the Platte (Sterna antillarum) both prefer wide unob- River, mostly north of the main channel, have structed channel widths (Faanes et al. 1992, been completely replaced by woodland-forest Farmer et al. 2005, Kirsch 1996); Regal Fritil- habitat in the last century and reflect a local- lary (Speyeria idalia) populations can become ized ecological regime shift, ensuring the con- isolated in prairies fragmented by woodland tinued presence of woodland in the CPRV edges (Ries and Debinski 2001, Caven et al. (Williams 1978, Currier 1982, O’Brien and Cur- 2017); and woody encroachment also limits rier 1987, Johnson 1994, Bunn and Arthington habitat suitability for grassland birds (Grant 2002, Biggs et al. 2009). Grassland birds are et al. 2004, Ellison et al. 2013). The Bobolink the fastest declining avian community in con- (Dolichonyx oryzivorus), Henslow’s Sparrow tinental North America (Rosenberg et al. (Ammodramus henslowii), Upland Sandpiper 2016). Ellison et al. (2013) demonstrated that (Bartramia longicauda), Greater Prairie-Chicken the removal of linear tree rows from frag- (Tympanuchus cupido), and several other grass- mented prairies increased bird and nesting land endemic avian species need large con- densities for Henslow’s Sparrows and Bobo - tiguous areas of prairie (Winter and Faaborg links, as well as nesting densities for Eastern 1999, Grant et al. 2004). Samson et al. (2004) Meadowlarks (Sturnella magna) in Wisconsin. estimated that only about 4.4% of the original Where appropriate, Sandhill Cranes and a extent of “central tallgrass prairie” remains, host of prairie and braided river endemic and Samson and Knopf (1994) estimated that species could likely benefit from targeted only 2% of Nebraska’s tallgrass prairie remains. efforts to restore herbaceous habitats in place Habitat restoration efforts focused on improv- of linear woodlands and areas dominated by ing habitat for Sandhill Cranes could poten- invasive tree species near the main channel tially create a network of tallgrass prairies of the Platte River (e.g., Farmer et al. 2005, and wet meadows adjacent to the Platte River Caven et al. 2017). that could benefit a host of regionally declin- Our findings suggest that large-scale efforts ing species (Rosenberg et al. 2016, Caven et to maintain wide channels within the CPRV al. 2017). will have positive habitat consequences for Riparian woodlands in the CPRV provide Sandhill Cranes. An extensive body of research breeding and migratory habitat for a diversity demonstrates that mechanical river manage- of avifauna; however, the dominant species are ment improves and maintains quality Sand- widespread forest-edge and woodland gener- hill Crane roosting habitat (Faanes and Le alists (Davis 2005a, 2005b). Furthermore, Davis Valley 1993, Currier 1997, Pfeiffer and Cur- (2005b) indicates that productivity and recruit- rier 2005, Kinzel 2009). Our results suggest ment for breeding birds is relatively low in that these efforts may need to be expanded 68 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76 and accelerated to improve contiguous areas names is for descriptive purposes only and of quality habitat large enough to counter does not imply endorsement by the U.S. Gov- landscape-level trends. Conservation organi- ernment. The findings and conclusions in this zations should continue to increase limited article are those of the authors and the U.S. habitat resources, such as wet meadow habi- Geological Survey and do not necessarily rep- tat and wide channels, with the intention of resent the views of the U.S. Fish and Wildlife redistributing densities of Sandhill Cranes into Service or the U.S. Forest Service. larger areas of connected high-quality habitat. Resource managers should continue to moni- LITERATURE CITED tor the impacts of restoration efforts on the distribution of roosting Sandhill Cranes in the ADAMÍK, P., AND J. PIETRUSZKOVÁ. 2008. Advances in spring but variable autumnal trends in timing of inland CPRV to determine whether objectives are wader migration. Acta Ornithologica 43:119–128. being met and resources are being expended ALONSO, J.A., J.C. ALONSO, AND G. NOWALD. 2008. Migra- judiciously. Unprotected relict meadow-prairie tion and wintering patterns of a central European and quality riverine roosting habitats showing population of Common Cranes Grus grus. Bird Study increased crane use, particularly east of HWY 55:1–7. ALONSO, J.C., J.A. ALONSO, AND L.M. BAUTISTA. 1994. 281 (segments 1 and 2), where <1% of the Carrying capacity of staging areas and facultative land is currently safeguarded from development, migration extension in Common Cranes. Journal of should be targeted for strategic conservation Applied Ecology 31:212–222. efforts, such as conservation easements, to pro- ANTEAU, M.J., M.H. SHERFY, AND A.A. BISHOP. 2011. Loca- tion and agricultural practices influence spring use of tect the habitats’ ecological integrity (Theobald harvested cornfields by cranes and geese in Nebraska. 2003). Conservation organizations should also Journal of Wildlife Management 75:1004–1011. plan for larger numbers of Sandhill Cranes BAKER, B.W., B.S. CADE, W.L. MANGUS, AND J.L. arriving earlier and staying longer within the MCMILLEN. 1995. Spatial analysis of Sandhill Crane nesting habitat. Journal of Wildlife Management 59: CPRV, given recent advances in arrival dates. 752–758. Earlier arrivals reinforce the need to expand BARTON, K. 2016. MuMIn: multi-model inference. R contiguous areas of quality habitat, as ear- package version 1.15.6. https://CRAN.R-project.org/ lier and longer stays could mean increased package=MuMIn pressure on the CPRV ecosystem, as well as BENNING, D.S., AND D.H. JOHNSON. 1987. Recent improvements to Sandhill Crane surveys in Nebraska’s Cen- increased risks posed to Sandhill Cranes by tral Platte Valley. Pages 10–16 in J.C. Lewis, editor, disease agents and extreme weather events. Proceedings 1985 Crane Workshop. Platte River Whooping Crane Habitat Maintenance Trust and U.S. ACKNOWLEDGMENTS Fish and Wildlife Service, Grand Island, NE. 415 pp. BERTRAM, M.R., G.L. HAMER, B.K. HARTUP, K.F. SNOW- DEN, M.C. MEDEIROS, AND S.A. HAMER. 2017. Haemo- We thank the Crane Trust, the USFWS, sporida prevalence and diversity are similar in endan- and the Rainwater Basin Joint Venture, in gered wild Whooping Cranes (Grus americana) and particular Andy Bishop, for providing fund- sympatric Sandhill Cranes (Grus canadensis). Para- ing for this research; Craig Davis and Karine sitology 144:629–640. BIGGS, R., S.R. CARPENTER, AND W.A. BROCK. 2009. Turn- Gil for their work during the earlier iterations ing back from the brink: detecting an impending of this program; Jacob Salter, Ross McLean, regime shift in time to avert it. Proceedings of the and Evan Suhr for their support in conduct- National Academy of Sciences of the United States ing aerial surveys and managing data; Mike of America 106:826–831. Fritz for consultation; and Lonnie Logan, Daniel BOWMAN, T.D. 2014. Aerial observer’s guide to North American waterfowl: identifying and counting birds Kinzie, Sage Platt, Dan Nabb, and Holly Poul- from the air. Technical Report FW6003, U.S. Fish son for their expert job in flying aerial survey and Wildlife Service, Denver, CO. 79 pp. routes. We also thank Roger Grosse for his BRADLEY, N.L., A.C. LEOPOLD, J. ROSS, AND W. H UFFAKER. help in procuring aerial imagery and provid- 1999. Phenological changes reflect climate change in Wisconsin. Proceedings of the National Academy ing advice regarding spatial analyses. Aerial of Sciences of the United States of America 96: survey work was conducted under Nebraska 9701–9704. Game and Parks Commission Scientific and BRAND, L.A., J.C. STROMBERG, D.C. GOODRICH, M.D. Education Permit number 1059. We greatly DIXON, K. LANSEY, D. KANG, D.S. BROOKSHIRE, AND appreciate the rigorous reviews provided by D.J. CERASALE. 2011. Projecting avian response to linked changes in groundwater and riparian flood- James A. Dubovsky and one other anonymous plain vegetation along a dryland river: a scenario reviewer. Any use of trade, firm, or product analysis. Ecohydrology 4:130–142. CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 69

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APPENDIX 1. Comparisons of density per bridge segment and river reach using one-way analyses of variance (ANOVAs). TABLE 1A. Comparison of mean Sandhill Crane density per kilometer by reach of the Central Platte River: East (bridge segments 1–4), Central (bridge segments 5–7), and West (bridge segments 8–11). Variable df Sum of squares Mean square F P Reach 2 3.23e+10 1.62e+10 27.77 3.17e−8*** Residuals 39 2.27 +10 +10 e 5.87e ***P < 0.001

TABLE 1B. Tukey’s Honest Significance Difference Test to assess significance of differences between pairs of individual river reaches (East, Central, West) regarding Sandhill Crane density. Comparison Difference 95% CI lower limit 95% CI upper limit P East–Central 279.29 −249.76 808.36 0.411 West–Central −1146.75 −1675.81 −617.69 <0.0001*** West–East −1426.05 −1955.11 −896.99 <0.0001*** ***P < 0.001

TABLE 1C. Comparison of mean Sandhill Crane density per kilometer across survey bridge segments. Variable df Sum of squares Mean square F P Bridge segment 10 155,855,630 15,585,563 22.13 2e−16*** Residuals 143 100,712,703 704,285 ***P < 0.001 CAVEN ET AL. ♦ TEMPOROSPATIAL SHIFTS IN SANDHILL CRANE STAGING 75 SD min–max – = 164 measurements of UOCW and SD min–max x – SD min–max x – SD min–max x – x 0.54 268 114 80–509 272 117 79–511 224 99 73–391 579 142 488–743 0.39 224 84 116–372 216 80 110–373 201 71 129–318 3640.50 171 170 76 229–762 66–360 153 68 63–290 143 60 58–256 3360.03 175 3.3 1.0 148–762 1–4 3.3 1.0 1–4 3.3 1.0 1–40.21 3.4 239 54 1.0 161–359 231 1–4 53 160–365 221 48 153–343 3040.16 40 196 76 218–349 94–359 183 75 91–355 190 65 84–343 2340.21 81 3.5 1.1 92–336 2–5 3.5 1.1 2–5 3.5 1.1 2–50.15 4.4 323 76 0.7 206–449 322 3–5 79 208–447 281 66 184–418 3770.11 106 287 87 268–536 169–449 296 85 208–447 222 65 112–304 3230.17 133 2.0 0.0 218–536 2–2 2.0 0.0 2–2 2.0 0.0 2–20.55 2.4 275 101 0.9 137–423 268 2–4 98 136–416 201 89 70–331 6080.48 263 216 81 129–1010 107–352 203 79 103–337 150 75 62–284 416 203 92–775 0.56 298 42 247–379 289 47 231–385 264 31 193–309 6790.70 192 188 51 360–895 138–283 188 53 111–274 151 58 68–286 6270.11 193 1.3 0.5 358–895 1–2 1.3 0.5 1–2 1.3 0.5 1–20.32 1.5 305 44 0.8 232–373 300 1–3 37 230–356 254 33 212–330 4490.37 65 261 67 306–524 98–336 255 61 100–315 186 56 83–255 4110.04 103 2.0 0 158–513 2–2 2.0 0.0 2–2 2.0 0.0 2–20.70 2.1 191 65 0.3 64–276 197 2–3 67 63–282 183 75 62–372 6330.68 359 138 49 252–1185 64–248 138 51 63–248 114 39 60–191 425 314 99–1144 0.82 201 74 125–369 205 72 125–365 168 57 91–266 11100.79 121 159 45 902–1286 103–254 160 46 105–258 124 48 63–218 776 308 185–1190 0.68 264 38 194–316 262 38 195–314 228 46 152–292 8290.66 236 215 68 286–1183 93–316 214 67 93–312 177 58 92–254 638 228 270–1109 0.69 186 79 115–373 192 82 122–415 135 41 80–229 6020.49 210 166 68 144–879 67–310 166 66 65–312 103 28 57–165 327 227 43–740 0.59 260 91 64–509 258 90 63–511 223 84 62–449 6390.57 310 205 86 129–1286 64–509 201 87 63–511 161 74 57–391 473 271 43–1190 − − − − − − − − − − − − − − − − − − − − − − − − − − − − 0.06 0.38 1.7 0.5 1–2 1.7 0.5 1–2 1.8 0.6 1–3 1.2 0.4 1–2 0.04 1.4 2.4 0.6 1–3 2.4 0.6 1–3 2.6 0.7 1–4 1.0 0.0 1–1 0.04 0.39 2.9 0.7 1–4 3.0 0.8 1–4 3.1 0.8 1–4 2.1 0.8 1–4 0.02 0.12 2.2 1.0 1–5 2.2 1.0 1–5 2.2 1.0 1–5 2.0 1.2 1–5 − − − − 2. Total unobstructed channel width (UOCW) and maximum unobstructed channel width (MUCW) for the main channel of the Platte River, as well the total number of unobstructed channel width (UOCW) and maximum (MUCW) for the main of Platte River, 2. Total PPENDIX MUCW 0.20 CHAN 0 0.14 1.1 0.4 1–2 1.1 0.4 1–2 1.1 0.4 1–2 1.0 0.0 1–1 MUCH 0.19 CHAN 0 MUCH 0.03 CHAN 0 MUCH 0.30 CHAN 0 MUCW 0.45 CHAN MUCW 0.24 CHAN 0 MUCW 0.40 CHAN 0 MUCW 0.21 CHAN 0 0.42 2.6 1.0 1–5 2.7 0.9 2–5 2.6 1.0 1–5 1.9 0.9 1–3 MUCW 0.28 CHAN MUCW 0.21 CHAN 0 0.22 1.6 0.8 1–3 1.6 0.8 1–3 1.6 0.8 1–3 1.3 0.5 1–2 MUCW 0.61 CHAN MUCW 0.28 CHAN % CHG 16 2016 2015 1998 1938 ______A 98 38SEG-VAR 1 UOCW 0.09 0.53 363 77 198–509 363 83 207–511 332 84 133–449 776 157 601–904 2 UOCW 0.11 3 UOCW 0.08 4 UOCW 0.15 5 UOCW 0.37 7 UOCW 0.20 8 UOCW 0.04 9 UOCW 0.20 11 UOCW 0.37 active channels per bridge segment (CHAN) from aerial imagery taken in 1938, 1998, and 2016. For description of segments, see Fig. 1. n description of segments, see Fig. active channels per bridge segment (CHAN) from aerial imagery taken in 1938, 1998, and 2016. For MUCW per year for 1998, 2015, and 2016, n = 136 1938. TTL total. 6 UOCW 0.13 10 UOCW 0.16 TTL UOCW 0.17 76 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST (2019), VOL. 11, PAGES 33–76

APPENDIX 3. Pearson’s product-moment correlation coefficients for relationships between land cover and channel width metrics collected at bridge segments within the Central Platte River Valley using aerial imagery from 1938, 1998, and 2016. All significant correlations (P < 0.05) are bolded. For site descriptions, see Fig. 1. Variablea 1 2 3 4 5 6 7 8 9 10 11 12 LON1 1 0.66 0.47 −0.51 −0.43 −0.53 0.56 0.43 −0.61 0.53 −0.18 −0.26 X38UOCW2 0.66 1 0.91 −0.78 −0.25 −0.28 0.47 0.61 −0.86 0.82 0.06 0.42 X38MUCW3 0.47 0.91 1 −0.65 −0.01 −0.06 0.32 0.57 −0.67 0.78 0.01 0.58 X38CHAN4 −0.51 −0.78 −0.65 1 0.29 0.25 −0.5 −0.47 0.74 −0.68 −0.24 −0.32 X98UOCW5 −0.43 −0.25 −0.01 0.29 1 0.88 −0.16 0 0.43 −0.47 −0.42 −0.01 X98MUCW6 −0.53 −0.28 −0.06 0.25 0.88 1 −0.13 0.03 0.4 −0.53 −0.11 0.02 UOCW20167 0.56 0.47 0.32 −0.5 −0.16 −0.13 1 0.9 −0.37 0.34 −0.06 −0.14 MUCW20168 0.43 0.61 0.57 −0.47 0 0.03 0.9 1 −0.37 0.42 −0.1 0.14 PWOOD169 −0.61 −0.86 −0.67 0.74 0.43 0.4 −0.37 −0.37 1 −0.81 −0.27 −0.45 PMEAD1610 0.53 0.82 0.78 −0.68 −0.47 −0.53 0.34 0.42 −0.81 1 0.15 0.62 X98PMEAD11 −0.18 0.06 0.01 −0.24 −0.42 −0.11 −0.06 −0.1 −0.27 0.15 1 0.35 12 PCONS −0.26 0.42 0.58 −0.32 −0.01 0.02 −0.14 0.14 −0.45 0.62 0.35 1 aVariable definitions 1: Median longitude. 2: Change in unobstructed channel width from 1938 to 2016. 3: Change in maximum unobstructed channel width from 1938 to 2016. 4: Change in the number of active channels of the Platte River from 1938 to 2016. 5: Change in unobstructed channel width from 1998 to 2016. 6: Change in maximum unobstructed channel width from 1998 to 2016. 7: Unobstructed channel width in 2016. 8: Maximum unobstructed channel width in 2016. 9: Proportion of land cover designated as woodland-forest within 800 m of the Platte River in 2016. 10: Proportion of land cover designated as meadow-prairie within 800 m of the Platte River in 2016. 11: Change in meadow-prairie land cover within 800 m of the Platte River from 1998 to 2016. 12: Percent of land within 800 m of the main channel of the Platte River owned or managed through easement by conservation organization. INFORMATION FOR AUTHORS

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Volume 11 2019 ISSN 1545-0228

CONTENTS

Sandhill Crane use of riverine roost sites along the central Platte River in Nebraska, USA ...... David M. Baasch, Patrick D. Farrell, Andrew J. Caven, Kelsey C. King, Jason M. Farnsworth, and Chadwin B. Smith 1 Adult Whooping Crane (Grus americana) consumption of juvenile channel catfish (Italurus punctatus) during the avian spring migration in the Central Platte River Valley, Nebraska, USA ...... Andrew J. Caven, Jenna Malzahn, Keith D. Koupal, Emma M. Brinley Buckley, Joshua D. Wiese, Rick Rasmussen, and Carol Steenson 14 First description of a Bald Eagle (Haliaeetus leucocephalus) attempting depredation on an adult Whooping Crane (Grus americana) of the Aransas-Wood Buffalo population ...... Matthew R. Rabbe, Andrew J. Caven, and Joshua D. Wiese 24 Temporospatial shifts in Sandhill Crane staging in the Central Platte River Valley in response to climatic variation and habitat change ...... Andrew J. Caven, Emma M. Brinley Buckley, Kelsey C. King, Joshua D. Wiese, David M. Baasch Greg D. Wright, Mary J. Harner, Aaron T. Pearse, Matt Rabbe, Dana M. Varner, Brice Krohn, Nicole Arcilla Kirk D. Schroeder, and Kenneth F. Dinan 33