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Communal Roosting and Foraging Behavior of Staging Sandhill Cranes

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Communal Roosting and Foraging Behavior of Staging Sandhill Cranes Wilson Bull., 106(l), 1994, pp. 62-71 COMMUNAL ROOSTING AND FORAGING BEHAVIOR OF STAGING SANDHILL CRANES DONALD W. SPARLINGJ’ AND GARY L. KRAPU ’ AssraAcr.-Each spring more than 300,000 Sandhill Cranes (GAS canadensis)roost communally at night in river channels in the Platte River Valley of Nebraska and disperse at dawn to forage in agricultural fields. Cranes with central roosts had activity ranges double the size of those with peripheral roosts; 42% of the birds changed activity ranges prior to the onset of migration. Minimum daily flight distance generally increased during the staging period. Cranes used native grassland and planted hayland more often than expected, relative to their percentage of occurrence, and fed longest there; cornfields were under-utilized. These differences probably reflect, in part, (1) limited distribution of grasslands and haylands resulting in a greater energy expenditure to acquire protein in the form of macroinvertebrates and (2) wider distribution of cornfields with adequate energy-rich foods but limited protein. Cranes probably forage more efficiently and conserve energy by following conspecifics from communal roosts to local feeding grounds, by settling in fields where foraging flocks are already present, and by establishing diurnal activity centers. Alert behavior varied with flock size but not as predicted from group size, presumably because predation of staging adult cranes is inconsequential. Received4 Jan. 1993, accepted15 June 1993. Sandhill Cranes of the midcontinent population acquire nutrient re- serves for migration and reproduction while on staging areas along the Platte and North Platte rivers in Nebraska (Krapu et al. 1985). Cranes roost communally at night in shallow waters of wide river channels or other wetlands and spend the days foraging in flocks on surrounding uplands (Krapu et al. 1984, Folk and Tacha 1990). Little is known con- cerning the specific functions of communal roosting and flocking to staging cranes beyond a probable advantage of reducing the risk of predation through increased predator detection (sensu Pulliam 1973, Caraco 1979) or evasive behaviors unique to groups. In some species of birds, com- munal roosts may serve as information centers, increasing foraging effi- ciency (Ward and Zahavi 1973; Waltz 1982, 1987; but see Bayer 1982; Stutchbury 1988 for contrary opinions). Foraging efficiency can also be increased through local enhancement (Hinde 1961) or use of alternative diurnal roosts (Caccamise and Morrision 1986, 1988). In the Platte River Valley, loss of communal roosting habitat has re- sulted in high densities of staging cranes (Krapu et al. 1984) prompting a need to evaluate spatiotemporal use of existing habitat and choice of foraging sites. In this paper, we examine (1) characteristics and use of I Present address: U.S. Fish and Wildlife Service, Northern Prairie Wildlife Research Center, P.O. Box 2096, Jamestown, North Dakota 58402. 2 U.S. Fish and Wildlife Service, Patuxent Wildlife Research Center, 115 10 American Holly Dr., Laurel, Maryland 20708. 62 Sparfing and Krapu l SANDHILL CRANE BEHAVIOR 63 activity ranges relative to energy expenditure and food availability, (2) the role of communal roosts in foraging behavior, (3) habitat preferences, and (4) influence of group size and predation on foraging and other be- haviors. We also discuss social factors as they pertain to staging behavior in Sandhill Cranes. STUDY AREA AND METHODS The studyarea encompassed253-km* alongthe Platte River betweenKearney and Shelton, Nebraska. Habitat composition within the study area was 49.8% cornfields, 17.3% native grassland, 10.8% planted hayland (primarily alfalfa and planted grasses),11.6% riverine, and 10.5% other (roads, homesteads,plowed fields). Interstate 80 (I-80) is adjacent to the north channel of the Platte River. A more complete description is in Krapu et al. (1984). Radiotelemetry.-During March and April 1978 and 1979, craneswere live-trapped with rocket and cannon-projectednets positioned near groupsof mounted crane decoysin crop- land and pastures. To minimize disturbance, nets were fired from blinds located several hundred m from decoys. Each captured bird was weighed, aged as juvenile or adult by head plumage (Lewis 1979) and banded. Battery-operated transmitters weighing approxi- mately 40 g (< 2% of body weight) were attached to 13 cranesin 1978 and 23 in 1979, using a neck and body loop and backpackharness. Ten birds in 1978 (8 adult, 2 juvenile) and 14 in 1979 (13 adult, one juvenile) were located frequently enough to permit statisticallyvalid analysesof activity rangesand movements. When feasible, each radio-equipped crane was located at hourly intervals during daylight and once eachjust after dusk and before dawn. Locations were determined by triangulation from ground vehicles to the nearest 100 m. Fixed-wing aircraft were used when birds could not be located from the ground. No individual was followed in more than one year. Distances and angles between locations were calculated along straight lines. These dis- tances represent minimal distances becausecranes meandered between radio fixes. Birds that were located in the same habitat on two sequentialobservations were assumedto have remained in that habitat for the interval, and those located in different sites were assumed to have visited only those in which they were recorded. Cranes center their activities within definable, undefended areasthat may changein size and location through time. These “activity ranges” differ from conventional home ranges in that they are transient and occupiedonly for roostingand feeding. Areas of activity ranges were estimated with a harmonic means method (Dixon and Chapman 1980) including the 95% closestpoints. When cranes used two or more discrete (as determined by non-over- lapping clustersof locations separatedby at least 2 km) activity ranges,separate areas were calculatedfor each. Time budgets.-Diurnal time budgets were developed within each of the habitats by recording activities of individual cranes at 12-set intervals throughout the stagingperiod. Time spent observingin each habitat was proportional to the percent of study area covered by that habitat. Individuals were selected arbitrarily by locating a group of cranes in a spotting scope’s field of view and, after looking away and slightly moving the scope hori- zontally and vertically, observingthe individual nearestto the intersectionof the crosshairs. Observationson an individual lasted from a few set to 5 min. Observationsoccurred from 06:OOto approximately 18:00 h CST. Behavioral categoriesincluded “resting” (sitting or standing still), “feeding”, “alert”, “calling”, “courting”, “aggression” (fighting or being at- tacked), “locomotion” (walking), and “comfort” (preening). An “unknown” categorycon- tained observationsthat could not be classifiedas one of the other behaviors. 64 THE WILSON BULLETIN l Vol. 106, No. 1, March 1994 Statistical analyses.-Unless otherwise stated, all values are means + SD. Statistical treatment of distancesflown, either among habitats or within a day, were complicated both by missingcells in some casesand repeatedobservations from marked individuals. We used repeated-measuresanalysis of variance (Milliken and Johnson 1984) on thesedata. Angular statisticsand correlationswere conductedfollowing Batschelet(198 1). Behavioral frequen- ciesbased on countswere analyzedwith G-tests(Sokal and Rohlf 1969). Sequentialbehaviors may not have been truly independent, but we believe that observations were adequately dispersedover birds, groups, and contexts to warrant this type of statistical treatment. RESULTS Characteristicsand use of activity ranges.-Fifty-seven percent of the nocturnal roosts were centrally located within the activity ranges, 37% were at the periphery, and 6% had too few observations to determine type of activity range (Table 1, Fig. 1). Fourteen birds maintained one activity range, nine had two distinct activity ranges (crane 78-10 had two ranges but only one with enough observations for statistical purposes), and one had three ranges. Among cranes with peripheral roosts, 73% had activity ranges south of their nocturnal roosts. We tested whether activity range size was dependent on type of roost, year, or interaction between type and year. Because accuracy of estimating activity range size is related to the number of observations for an activity range, we weighted the analysis by multiplying the squared residual for each observation by the square root of the number of observations within each range (SAS 1987). Size of activity range differed between roost types (P = 0.049). The weighted mean activity range area for individuals with central roosts (39.6 ? 64.3 km2) exceeded that of peripheral roosts (17.4 * 29.0 km2). The unweighted mean for central roosts was 38.7 + 17.1 km2 and for peripheral roosts was 17.4 +- 8.3 km2. There were no dif- ferences between years or in interaction between roost type and year (P > 0.11). To test whether the type of roost affected travel distances to foraging locations, we compared the means of distances for first flights in the morning from roosts to feeding locations between years and type of roost. If a crane had two activity ranges of the same type, a combined mean was obtained by weighting each separate mean by its sample size. Means of actual distances from roosts to initial feeding areas did not differ be- tween activity ranges with peripheral roosts (1285 * 388 m) and those with central roosts (1408 k 407 m)
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