University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
USGS Northern Prairie Wildlife Research Center US Geological Survey
2012
Conservation Implications When the Nest Predators Are Known
Frank Thompson USDA Forest Service Northern Research Station, [email protected]
C.A. Ribic Wisconsin Cooperative Wildlife Research Unit, [email protected]
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Thompson, Frank and Ribic, C.A., "Conservation Implications When the Nest Predators Are Known" (2012). USGS Northern Prairie Wildlife Research Center. 260. https://digitalcommons.unl.edu/usgsnpwrc/260
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Abstract. Conservation and management of pas nest predator in southern latitudes. Differences in serines has largely focused on habitat manipula the importance of predator species or groups were tion or restoration because the natural commu likely the result of both their geographic patterns nities on which these birds depend have been of distribution and habitat preferences. Some destroyed and fragmented. However, productivity direct and indirect predator control measures is another important aspect of avian conservation, developed for waterfowl management potentially and nest predation can be a large source of nest could be used to benefit passerine productivity. ing mortality for passerines. Recent studies using We reviewed three examples-cowbirds, snakes in video surveillance to identify nest predators allow shrublands, and ground squirrels in grasslands-to researchers to start evaluating what methods could illustrate how different predator control strategies be used to mitigate nest predation to help passer may be needed in different situations. Mitigation ines of conservation concern. From recent studies, of passerine nest predation will need to be based we identified latitudinal and habitat-related pat on knowledge of predator communities to be terns in the importance of predator groups that effective. This requires large samples of predation depredate passerine nests. We then reviewed how events with identified predators; video technology knowledge of specific nest predators can benefit is essential for this task. conservation of bird species of concern. Mammals were the dominant predator group in northern Key Words: conservation, cowbirds, forest, grass grasslands. Snakes were the dominant predator land, mesopredators, nest predators, passerines, group in southern habitats. Fire ants were only a shrubland, snakes, video technology.
any bird species have suffered loss, degra birds is not totally determined by habitat factors. One dation, and fragmentation of the habitats particularly important aspect of avian ecology is pro on which they depend (e.g., Askins et al. ductivity, and nest predation can be a large source of 2007). As a result, conservation efforts on behalf of nesting mortality, especially for passerines (Ricklefs these birds have largely focused on habitat manipu 1969; Martin 1988, 1995; Newton 1998; Thompson lations or restoration. However, population status of 2007). Not knowing who the nest predators actually
Thompson, F. R., III, and C. A. Ribic. 2012. Conservation implications when the nest predators are known. Pp. 23-34 in C. A. Ribic, F. R. Thompson III, and P. J. Pietz (editors). Video surveillance of nesting birds. Studies in Avian Biology (no. 43), University of California Press, Berkeley, CA.
23 are can affect not only our understanding ofthe ecol can knowledge of specific nest predators benefit ogy of birds but also conservation and management conservation of bird species of concern? To address actions (Nelson 2001, Thompson 2007, Lahti 2009). the first question, we reanalyze data from some Integrating knowledge of predation into manage existing studies in various parts of the midwestern ment has been addressed in reviews for harvested and southern United States to make direct com bird species (e.g., waterfowl, Sovada et aI. 2001; parisons of the risk of predation by predator groups ground-nesting gamebirds, Jimenez and Conover (not possible with the original results) which illus 2001); however, the only passerine group to be con trate larger-scale patterns. For the second question, sidered so far are neotropical migrant songbirds we review different strategies for managing nest (Heske et aI. 2001). One issue raised by Heske et aI. predation based on work done on waterfowl, and (2001) for this group was the diversity of potential then present examples relevant to passerines: two in predator species and how to manage them. shrubland and forest habitats and one in grasslands. Recent studies using video surveillance on pas serine nests have made significant contributions PATTERNS IN IMPORTANCE OF PASSERINE toward identifying nest predators, which has NEST PREDATORS IN THE MIDWESTERN started to help us assess the relative importance of different predator taxa in different habitats in AND SOUTHERN U.S. North America (Benson et al. 2010, Reidy and Methods Thompson, chapter 11, this volume, Ribic et aI., chapter 10, this volume). These findings provide We used five data sets from camera studies of pas us the opportunity to start evaluating how this serine nest predation in the midwestern and south information can help us with large-scale conser ern U.S. that spanned a wide geographic range, vation and management. In other words, given represented multiple habitats, and were among the what we know about the nest predators, what is largest studies published (Fig. 2.1). Because of this the potential to mitigate nest predation to help post-study selection and stratification, geography passerines of conservation concern? and habitat are somewhat confounded, but never Unfortunately, comparison of predation among theless we believe the comparisons are useful. The studies to illustrate larger-scale patterns is ham data sets represent North Dakota and Minnesota pered by the way many results have been reported. (hereafter North Dakota) grasslands, Wisconsin Most studies documenting the identity of nest pred grasslands, Missouri shrubland and forest, Texas ators present counts of predation events by species shrubland and forest, and Georgia forest. The or groups of species. Counts of predation events can North Dakota grassland data were from 1996-2000 be compared within a study to assess the relative and consisted of 1,187 camera nest-days (Pietz importance of predators but are not directly compa et al., chapter 1, this volume; Granfors, pers. comm.). rable across studies because of differences in cam Dominant nesting species were Clay-colored era nest-days (i.e., the number of days active nests Sparrow (Spizella pallidal and Savannah Sparrow were monitored with video surveillance systems). (Passerculus sandwichensis); other common species Another limitation is that counts of predation events included Bobolink (Dolichonyx oryzivorus) , Vesper do not represent actual nest mortality rates. One Sparrow (Pooecetes gramineus) , and Chestnut alternative to the use of counts is the use of multi collared Longspur (Calcanus ornatus). Dominant nomial logistic regression models to estimate the habitats were native and planted grasslands, con daily probability of predation events resulting from sisting of grasses, forbs, and small shrubs. The different predators (Thompson and Burhans 2003). Wisconsin grasslands data were from 1998-2008 These probabilities are analogous to daily, cause and consisted of 2,404 camera nest-days (Pietz specific mortality rates and can be directly compared et al., chapter 1, this volume). Dominant nesting spe across studies to illustrate larger-scale patterns in cies were Eastern Meadowlark (Sturnella magna), predator dominance or even absolute mortality rates Grasshopper Sparrow (Ammodramus savannarum), by specific predators. Bobolink, and Savannah Sparrow. Dominant habi In this paper we address two questions: (1) Are tats were continuously grazed pasture consisting of there geographic and habitat-related patterns in the cool-season grasses, primarily Kentucky bluegrass importance of nest predator species or groups for (Poa pratensis); Conservation Reserve Program fields passerines of conservation concern; and (2) how consisting of cool-season grasses, primarily smooth
24 STUDIES IN AVIAN BIOLOGY NO.43 Ribic, Thompson, and Pietz North Dakota/Minnesota (Pietz et aI., chapter 1, this volume)
(Conner et al. 2010)
Texas (Reidy and Thompson, chapter 11, this volume)
Figure 2.1. Locations of studies that used video surveillance of passerine nests in the midwestern and southern United States. We reanalyzed data from these studies to compare daily predation rates of passerine nests in grasslands, shrublands, and forests by five predator groups. brome (Bromus inennis); and remnant prairie con (Toxostoma rufom) , Eastern Towhee (Pipi!o erythr sisting of native warm-season grasses, such as little phtha!mus) , Blue Grosbeak (Guiraca caeru!ea) , and bluestem (Schizachyrium scoparium) , big bluestem Indigo Bunting. Dominant habitats were longleaf (Andropogon gerardii) , needle (porcupine) grass (Pinus pa!ustris), slash (P. elliottii), and loblolly (P. (Heterostipa spartea), and Indian grass (Sorghastrum taeda) pine forest and mixed pine and hardwood nutans). The Missouri shrubland and forest data forest. were from 1997-1999 and consisted of 1,500 camera The data consisted of an observation for each nest-days (Thompson and Burhans 2003). Dominant nest-day represented by six possible fates: pre nesting species were Field Sparrow (Spizella pusilla) , dation by (1) raptors/corvids, (2) Brown-headed Indigo Bunting (Passerina cyanea) , Kentucky Cowbirds (Mo!othrus ater) , (3) fire ants (all Warbler (Oporornis formosus) , Worm-eating Warbler most likely the imported red fire ant, So!enopsis (Helmitheros vennivorus) , Ovenbird (Seiurus auro invicta) , (4) mammals, or (5) snakes; and (6) capillus), Acadian Flycatcher (Empidonax virescens) , other (i.e., success, failure due to other causes, and Wood Thrush (Hylocichla muste!ina). Dominant or unknown). A nest could suffer multiple fates habitats were old fields consisting of grasses, forbs, involving partial predation by one or more pred and shrubs and oak (Quercus spp.)-hickory (Carya ators, so predation rates should be interpreted spp.) forest. The Texas shrubland and forest data as the probability of partial or complete preda were from 1997-2009 and consisted 0[3,346 camera tion by the specific predator group. We used the nest-days (Reidy and Thompson, chapter 11, this vol "other" fate category as our reference category ume). Dominant nesting species were Black-capped because our interest was to estimate rates ofloss Vireo (Vireo atricapilla) and Golden-cheeked Warbler to specific predator groups. For each data set, (Setophaga chrysoparia). Dominant habitats were we fit a multinomial regression model (PROC short scrubland dominated by oaks (Quercus spp.) Logistic, SAS ver. 9.2, SAS Institute, Cary, NC) and Ashe juniper Uuniperus ashei)-oak woodlands. with response categories representing the six The Georgia forest data were from 2003-2006 and fates to estimate the daily probability and 95% consisted of 2,486 camera nest-days (Conner et al. confidence interval of each fate (Thompson and 2010). Dominant nesting species were Northern Burhans 2003). This approach was only possi Cardinal (Cardina!is cardina!is) , Brown Thrasher ble because of video monitoring; we knew the
PREDATORS AND BIRD CONSERVATION 25 0.05 North Dakota Wisconsin grassland grassland 0.04 .~ Cii t 0 0.03 E -""'10 0.02 0 0.01
0.00 0.05 Missouri shrubland Missouri forest 0.04 .~ Cii t 0 0.03 E -""'10 0.02 0 0.01
0.00 0.05 Texas shrubland Texas forest 0.04 .~ Cii t 0 0.03 E -""'10 0.02 0 0.01
0.00
Georgia forest
Figure 2.2. Daily nest mortality (95% CI) among habitats and predator groups in the midwestern and southern United States. Data are 1,187 and 2,404 camera nest-days in North Dakota/Minnesota and Wisconsin, respectively (Pietz et aI., chapter 1, this volume); 3,346 camera nest-days in Texas (Reidy and Thompson, chapter 11, this volume); 1,500 camera nest-days in Missouri (Thompson and Burhans 2003); and 2,486 camera nest-days in Georgia (Conner et al. 2010).
fates of each nest on each day it was monitored_ Results and Discussion The models for Missouri and Texas included an intercept and covariate for habitat type (forest or We estimated the daily probability of predation by shrubland); for North Dakota, Wisconsin, and each of the five predator groups for each data set Georgia the model only included an intercept There were patterns in the probability of predation because the data were from only one general by raptors/corvids, Brown-headed Cowbirds, fire habitat type (grasslands or forest)_ We did not ants, mammals, and snakes that emerged from our examine more complex relationships with cov analysis (Fig. 2.2). Several patterns appeared related ariates because these are addressed by the origi to latitude. Mammals were the dominant preda nal individual studies, and our objective was to tor group in northern grasslands (North Dakota, examine regional patterns in the average prob Wisconsin). Snakes were the dominant predator ability of predation by each of the five predator group in southern habitats (Texas, Georgia). Fire groups_ ants were a unique and important predator in the
26 STUDIES IN AVIAN BIOLOGY NO.43 Ribic, Thompson, and Pietz southern latitudes. The southern latitudes also had forests. These differences all relate to habitat pref a more even distribution among predator groups erences of the predators. Snakes, especially rat than the northern latitudes (Fig. 2.2). snakes, prefer edge because of benefits for ther The decrease in the importance of snakes as a moregulation (Blouin-Demers and Weatherhead nest predator in northern latitudes is consistent 2001). The habitat heterogeneity of Missouri and with geographic patterns in snake distributions. Texas shrublands and adjacent woodland edges While numbers of all species tend to decrease are likely ideal habitats for ratsnakes; hence their with latitude in North America, numbers of ecto great importance as predators there. The impor therms decline more steeply than endotherms, tance of ratsnakes as predators in the Texas juni and numbers of snake species are negatively per forests suggests these are good snake habi correlated with numbers of mammal and bird tats as well. This might be explained by the low species (Schall and Pianka 1978). The most com stature of juniper trees; snakes bask in the top of monly detected snake group was the ratsnake the canopy in these forests, which have the same (Pantherophis spp.); this group is at its north properties that make edges attractive to snakes ern range limit in Wisconsin (Christoffel et al. (Sperry et al. 2009). The increased importance of 2000) and does not occur on the Wisconsin study fire ants in shrubland versus forest within Texas sites or in North Dakota (Trepanowski 2003). In is consistent with their habitat preference for Wisconsin, snakes such as the western foxsnake open disturbed areas (Allen et al. 2004). (Mintonius vulpinus) and gartersnake (Thamnophis The importance of specific predator species spp.) were responsible for grassland bird nest pre likely results from both geographic patterns in dation, but at a much lower rate than mammals. the distribution of those species and from their Fire ants were only a nest predator in southern habitat preferences. For example, most species of latitudes. The imported red fire ant was intro ground squirrels [e.g., thirteen-lined ground squir duced in Alabama in the 1930s and has spread rel (Ictidomys tridecemlineatus) and Richardson's throughout most of the southern U.S. (Allen et al. ground squirrel (Urocitellus richardsonii)] occur 2004). Hence, the pattern in their importance as a in the western U.S. in open habitats. The most nest predator reflects their distribution. frequent nest predator in Wisconsin and North Another geographic pattern relates to cowbirds. Dakota grasslands was the thirteen-lined ground Cowbirds were predators at all sites except for squirrel (Pietz et al., chapter 1, this volume). No forests in Missouri and Georgia (Fig. 2.2). This ground squirrels occurred at the Missouri or Texas is somewhat consistent with patterns in their sites because only woody habitats were studied breeding abundances; they are most abundant there. Tree squirrels occurred in the shrubland in the Great Plains and Midwest, with decreas and forest habitats in Missouri, Texas, and Georgia ing abundance moving coastward. This pattern but they were not dominant predators. in abundance likely reflects their historic distri Our point here, somewhat obvious but until bution in the Great Plains and expansion across recently not investigated (e.g., Ribic et al. 2009), is North America (Thompson et al. 2000). However, that the importance of predator groups and spe the geographic pattern in the importance of cow cies clearly differs geographically and by habitat. birds as brood parasites (and, by extension, nest Therefore, different approaches to understanding predators) may be confounded by habitat (Igl and and potentially mitigating predation will be needed Johnson 2007). in different geographic regions and habitats. Other patterns were related to habitat. In the northern grasslands, predation rates by group PREDATOR SPECIFIC APPROACHES were very similar for the two areas. At mid-lati TO UNDERSTANDING OR MITIGATING tudes (Missouri), the most important predator NEST PREDATION group shifted from snakes in shrublands to mam mals in forests, largely due to greater predation by Mesopredators and Waterfowl Nest Predation raccoons (Procyon lotor) in forests. The biggest dif ference among habitats in Texas was the increased Waterfowl researchers have for a long time worked importance of cowbirds and fire ants in shrub on identifying nest predators (e.g., with time land; however, in contrast to Missouri, snakes lapse photography; Sargeant et al. 1998) and on were equally or even slightly more important in understanding the interaction of nest predators
PREDATORS AND BIRD CONSERVATION 27 southern latitudes. The southern latitudes also had forests. These differences all relate to habitat pref a more even distribution among predator groups erences of the predators. Snakes, especially rat than the northern latitudes (Fig. 2.2). snakes, prefer edge because of benefits for ther The decrease in the importance of snakes as a moregulation (Blouin-Demers and Weatherhead nest predator in northern latitudes is consistent 2001). The habitat heterogeneity of Missouri and with geographic patterns in snake distributions. Texas shrublands and adjacent woodland edges While numbers of all species tend to decrease are likely ideal habitats for ratsnakes; hence their with latitude in North America, numbers of ecto great importance as predators there. The impor therms decline more steeply than endotherms, tance of ratsnakes as predators in the Texas juni and numbers of snake species are negatively per forests suggests these are good snake habi correlated with numbers of mammal and bird tats as well. This might be explained by the low species (Schall and Pianka 1978). The most com stature of juniper trees; snakes bask in the top of monly detected snake group was the ratsnake the canopy in these forests, which have the same (Pantherophis spp.); this group is at its north properties that make edges attractive to snakes ern range limit in Wisconsin (Christoffel et al. (Sperry et al. 2009). The increased importance of 2000) and does not occur on the Wisconsin study fire ants in shrubland versus forest within Texas sites or in North Dakota (Trepanowski 2003). In is consistent with their habitat preference for Wisconsin, snakes such as the western foxsnake open disturbed areas (Allen et al. 2004). (Mintonius vulpinus) and gartersnake (Thamnophis The importance of specific predator species spp.) were responsible for grassland bird nest pre likely results from both geographic patterns in dation, but at a much lower rate than mammals. the distribution of those species and from their Fire ants were only a nest predator in southern habitat preferences. For example, most species of latitudes. The imported red fire ant was intro ground squirrels [e.g., thirteen-lined ground squir duced in Alabama in the 1930s and has spread rel (Ictidomys tridecemlineatus) and Richardson's throughout most of the southern U.S. (Allen et al. ground squirrel (Urocitellus richardsonii)] occur 2004). Hence, the pattern in their importance as a in the western U.S. in open habitats. The most nest predator reflects their distribution. frequent nest predator in Wisconsin and North Another geographic pattern relates to cowbirds. Dakota grasslands was the thirteen-lined ground Cowbirds were predators at all sites except for squirrel (Pietz et al., chapter 1, this volume). No forests in Missouri and Georgia (Fig. 2.2). This ground squirrels occurred at the Missouri or Texas is somewhat consistent with patterns in their sites because only woody habitats were studied breeding abundances; they are most abundant there. Tree squirrels occurred in the shrubland in the Great Plains and Midwest, with decreas and forest habitats in Missouri, Texas, and Georgia ing abundance moving coastward. This pattern but they were not dominant predators. in abundance likely reflects their historic distri Our point here, somewhat obvious but until bution in the Great Plains and expansion across recently not investigated (e.g., Ribic et al. 2009), is North America (Thompson et al. 2000). However, that the importance of predator groups and spe the geographic pattern in the importance of cow cies clearly differs geographically and by habitat. birds as brood parasites (and, by extension, nest Therefore, different approaches to understanding predators) may be confounded by habitat (Igl and and potentially mitigating predation will be needed Johnson 2007). in different geographic regions and habitats. Other patterns were related to habitat. In the northern grasslands, predation rates by group PREDATOR SPECIFIC APPROACHES were very similar for the two areas. At mid-lati TO UNDERSTANDING OR MITIGATING tudes (Missouri), the most important predator NEST PREDATION group shifted from snakes in shrublands to mam mals in forests, largely due to greater predation by Mesopredators and Waterfowl Nest Predation raccoons (Procyon lotor) in forests. The biggest dif ference among habitats in Texas was the increased Waterfowl researchers have for a long time worked importance of cowbirds and fire ants in shrub on identifying nest predators (e.g., with time land; however, in contrast to Missouri, snakes lapse photography; Sargeant et al. 1998) and on were equally or even slightly more important in understanding the interaction of nest predators
PREDATORS AND BIRD CONSERVATION 27 nest success because, as video nest surveillance Direct control of snakes will not be desirable or revealed, other predators were more important feasible in many circumstances. Efficient removal (fire ants, snakes, and birds; Conner et al. 2010). of snakes is probably not possible, and snakes are These two examples highlight how knowing the a desired component of many ecosystems and are identities of the nest predators can lead to success protected in many states. However, the cumula (cowbirds) and not knowing can lead to failure tive work in this area suggests a possible use of (mesopredators) in direct removal efforts and the indirect habitat manipulation strategies. A plau challenge of dealing with a diverse predator com sible management hypothesis for this system is munity versus a single dominant predator. that reducing edge will reduce snake activity or abundance in an area and, thereby, lower nest Potential Indirect Control of Snakes predation. Klug et al. (2010) speculated that only a small reduction in shrub cover might signifi A plethora of research on snakes in the Midwest cantly reduce predation risk on grassland birds. has occurred at least partially because of the con Additional questions are whether this action firmation by video surveillance that snakes are would lower snake abundance in a landscape or important nest predators (Thompson et al. 1999). simply shift snake activity elsewhere, to other Recent studies have addressed habitat selection prey and potentially other birds. Because reduc and activity patterns by ratsnakes, racers (Coluber ing edge and fragmentation is a common conser spp.), and prairie kingsnakes (Lampropeltis vation recommendation for addressing low nest calligaster calligaster) (Blouin-Demers and survival of passerines in fragmented habitats, Weatherhead 2001, Sperry et al. 2009, Carfagno there may already be programs in place that could and Weatherhead 2006, Richardson et al. be used to evaluate this hypothesis. 2006, Klug et al. 2010), and some studies have directly investigated links between snake activ Thirteen-lined Ground Squirrels and ity and habitat use and nest predation (Sperry Passerine Nest Predation et al. 2008, 2009; Weatherhead et al. 2010). Other research in the same or nearby areas has found Agriculture (including grazing by nonnative that nest predation is higher in areas with greater ungulates), fire suppression, and urbanization are fragmentation or amounts of edge (Robinson continuing to change the areas of North America et al. 1995, Suarez et al. 1997, Heske et al. 2001, originally composed of prairie into a patchwork of Peak 2007, Reidy et al. 2009). In general, these land uses (Askins et al. 2007). Currently, planted studies have shown that snakes are an important or surrogate grasslands dominate the remain predator of songbird nests, snakes prefer edges, ing eastern grassland systems (Askins et al. and nest mortality is often high near edges. Snake 2007); these surrogate grasslands are composed preference for edges is thought, in part, to be due of Eurasian grass and forb species, are typically to the convenience of the mix of sun and shade for associated with agriculture, and include hay thermoregulation. Klug et al. (2010) investigated fields, pastures, fallow, and old fields (Sample nest success of grassland birds and habitat selec et al. 2003). In Wisconsin, Sample and Mossman tion by bird-eating snakes; they found snakes pre (1997) developed a heuristic Bird Conservation ferred shrub habitats and that nesting success of Area model to argue that restoring and enhanc grassland birds decreased with increasing shrub ing both small and large grassland patches within cover. However, two other studies that simulta landscapes that already have a high proportion neously studied snakes and nest success found a of grassy habitat and little or no woodland would snake preference for edge but did not find lower likely have the greatest benefit for grassland birds. nest success there (Sperry et al. 2009, Weatherhead Under this model, there was an implicit assump et al. 2010); this suggests the need for further work tion that woody-edge predators were driving the in this area or alternative hypotheses. In addition dynamics of grassland bird populations on small to the thermal benefits referred to above, the high patches (D. Sample, pers. comm.). Removing structural heterogeneity of shrub habitat compared woody patchesjfencerows in the landscape and to grassland may provide snakes with higher prey enlarging grassy habitat patches, then, would densities (Carfagno et al. 2006) or protection from reduce a major source of predators and, thus, ben predators (Wilgers and Horne 2006). efit grassland birds by lowering predation risk.
PREDATORS AND BIRD CONSERVATION 29 However, at the time this model was developed, predation. Besides thinking about birds, we need there was little information about the identities of to remember to take the "predators' view of the grassland bird nest predators. The first work in landscape" (Kuehl and Clark 2002:1233). This Wisconsin on pastures (Renfrew and Ribic 2003) approach is leading to the developing perspective seemed to confirm that woody-edge predators that for songbird conservation, we will need to (specifically raccoons) indeed were responsible take a broader system perspective to understand for a high proportion of nest predation events; or mitigate songbird nest predation (Thompson however, there was some indication that a species 2007, Lahti 2009). that lived in the same grassy habitats as the grass The development of video monitoring technol land birds, specifically the thirteen-lined ground ogy and its growing application to nest predation squirrel, was also an important nest predator. studies is finally providing the empirical evidence Investigation into the nest predators in a differ to demonstrate the need for a broad perspective. ent grassy habitat, Conservation Reserve Program Only a few songbird studies have attempted to fields (Ribic et al., chapter 10, this volume), pro relate songbird nest predation by predator spe vided further evidence of the importance of the cies or predator groups to environmental vari thirteen-lined ground squirrel as a major predator ables (Benson et al. 2010, Reidy and Thompson, on grassland bird nests. chapter 11, this volume). Examination of models How does this new information impact the for particular species or groups of predators has Bird Conservation Area model of Sample and helped discover temporal, habitat, and landscape Mossman (1997)? In this case, removal of edges effects that are often masked by lumping preda to enlarge habitat for grassland birds will also tors. However, for most birds in most landscapes increase habitat for a major grassland nest preda it is not known if productivity is even limiting tor. Thus, the assumed benefit of increased pro population growth, and hence if mitigating nest ductivity, expected from an assumed decrease predation should even be considered. We still in a major predator associated with woody habi have many unanswered questions for passerines. tat, may not be realized. While the patch size Where nest predation is considered unacceptably needed for grassland bird populations has been high, mitigation of nest predation based on hypoth considered (e.g., more than 1,000 ha for prairie eses that reflect knowledge of predators should fragments; Herkert et al. 2003), the question we be most effective. What tool to use-exclusion, may want to· ask is what patch size is needed to removal, dilution or a combination-should reflect dilute the effect of the nest predator associated knowledge of both birds and their nest predators. with grassy habitat? We could also consider direct We are not advocating predator control but simply predator control; thirteen-lined ground squir recognizing it is an option that has been locally rels are not protected in Wisconsin, and targeted successful when appropriately targeted. We believe control might reduce nest predation effects for a that using knowledge of nest predators and their season or two, allowing a pulse of grassland bird habitat requirements will be more effective than productivity. However, we do not know the effects direct predator control in managing habitats for direct predator control may have on species that passerines, whether this is called indirect preda feed on the ground squirrel. Thinking about tor control or just good bird habitat management. grassland bird management may need to broaden Knowledge of the importance of predators, and in into thinking about multispecies management of what circumstances nest predation is pathologi the grassland ecosystem. cally high, leads to credible and testable hypotheses about what has changed in habitats and landscapes Concluding Thoughts and how it can be approached from a broader con servation perspective. Developing credible man Beginning with the articulation of specific agement hypotheses and evaluating these in an hypotheses concerning predators and edge and adaptive management approach may be helpful fragmentation effects on songbirds (Heske et al. in resolving some of these unknowns and moving 2001, Thompson et al. 2002) and investigations forward with effective conservation. This approach into the identities of nest predators of songbirds is currently being used by the u.S. Fish and (Pietz et al., chapter 1, this volume), it has become Wildlife Service in Strategic Habitat Conservation clear that we cannot broadly generalize about nest (U.S. Fish and Wildlife Service 2009). Finally,
30 STUDIES IN AVIAN BIOLOGY NO.43 Ribic, Thompson, and Pietz investigation of hypotheses concerning nest preda Christoffel, R., R. Hay, and L. Ramirez. 2000. Snakes tion will be more informative if it takes a systems of Wisconsin. PUB-ER-100 00. Wisconsin Depart approach and addresses specific aspects of preda ment of Natural Resources, Bureau of Endangered tor ecology (Teunissen et al. 2008). Large samples Resources, Madison, WI. of predation events for which we know the identity Conner, L. M., J. C. Rutledge, and L. L. Smith. 2010. of predators will be required; video technology is Effects of mesopredators on nest survival of shrub nesting songbirds. Journal of Wildlife Management essential for this task. 74:73-80. Emery R. B., D. W. Howerter, L. M. Armstrong, ACKNOWLEDGMENTS M. G. Anderson, J. H. Devries, and B. L. Joynt. Our thanks to research collaborators R. B. Renfrew, J. 2005. Seasonal variation in waterfowl nesting suc L. Nack, M. J. Guzy, T. J. Anderson, K. S. Ellison, D.W. cess and its relation to cover management in the Sample, J. L. Reidy, M. M. Stake, D. E. Burhans, and Canadian prairies. Journal of Wildlife Management L. M. Conner. We thank all the field technicians who 69:1181-1193. helped collect the data used in this paper and the private Garrettson, P. R., and F. C. Rohwer. 2001. Effects of land owners who allowed us onto their properties. mammalian predator removal on production of Camera systems were built by J. Christensen, J. upland-nesting ducks in North Dakota. Journal of Dadisman, and K. S. Ellison. We thank P. J. Pietz, D. Wildlife Management 65:398-405. Donner, K. Bakker, and two anonymous reviewers for Granfors, D. A., P. J. Pietz, and L. A. Joyal. 2001. Fre their comments on previous versions of this manu quency of egg and nestling destruction by female script. Mention of trade names or commercial products Brown-headed Cowbirds at grassland nests. Auk does not constitute endorsement for use by the U.S. 118:765-769. government. Financial support was provided by the Greenwood, R. J., and M. A. Sovada. 1996. Prairie U.S. Army, U.S. Forest Service, U.S. Geological Survey, duck populations and predation management. U.S. Fish and Wildlife Service Region 3 Nongame Bird Transactions of the North American Wildlife and Conservation Program, The Nature Conservancy, and Natural Resources Conference 61:31-42. the Wisconsin Department of Natural Resources. We Herkert, J. R., D. L. Reinking, D. A. Wiedenfeld, thank the Department of Forest and Wildlife Ecology, M. Winter, J. L. Zimmerman, W. E. Jensen, E. J. University of Wisconsin, Madison, for assistance with Finck, R. R. Koford, D. H. Wolfe, S. K. Sherrod, M. publication expenses. A. Jenkins, J. Faaborg, and S. K. Robinson. 2003. Effects of grassland fragmentation on the nesting success of breeding birds in the mid-continental LITERATURE CITED United States. Conservation Biology 17:587-594. Allen, C. R., D. M. Epperson, and A. S. Garmestani. Heske, E. J., S. K. Robinson, and J. D. Brawn. 2001. 2004. Red imported fire ant impacts on wildlife: a Nest predation and neotropical migrant songbirds: decade of research. American Midland Naturalist piecing together the fragments. Wildlife Society 152:88-103. Bulletin 29:52-61. Askins, R. A., F. Chavez-Ramirez, B. C. Dale, C. A. Hoekman, S. T., L. S. Mills, D. W. Howerter, J. H. Haas, J. R. Herkert, F. L. Knopf, and P. D. Vickery. Devries, and I. J. Ball. 2002. Sensitivity analyses of 2007. Conservation of grassland birds in North the life cycle of midcontinent Mallards. Journal of America: understanding ecological processes in dif Wildlife Management 66:883-900. ferent regions. Ornithological Monographs No. 64. IgI, L. D., and D. H. Johnson. 2007. Brown-headed Benson, T. J., J. D. Brown, and J. C. Bednarz. 2010. Cowbird, Molothrus ater, parasitism and abundance Identifying predators clarifies predictors of nest in the northern Great Plains. Canadian Field-Natu success in a temperate passerine. Journal of Animal ralist 121:239-255. Ecology 79:225-234. Jimenez, J. E., and M. R. Conover. 2001. Ecological Blouin-Demers, G., and P. J. Weatherhead. 2001. Hab approaches to reduce predation on ground-nesting itat use by black rat snakes (Elaphe obsolete obsolete) gamebirds and their nests. Wildlife Society Bulletin in fragmented forests. Ecology 82:2882-2896. 29:62-69. Carfagno, G. L. F., E. J. Heske, and P. J. Weatherhead. Klug, P. E., S. L. Jackrel, and K. A. With. 2010. Linking 2006. Does mammalian prey abundance explain snake habitat use to nest predation risk in grass forest-edge use by snakes? Ecoscience 13:293-297. land birds: the dangers of shrub cover. Oecologia Chodachek, K. D., and M. J. Chamberlain. 2006. 162:803-813. Effects of predator removal on upland nesting Kostecke, R. M, S. G. Summers, G. H. Eckrich, and ducks in North Dakota grassland fragments. Prairie D. A. Cimprich. 2005. Effects of Brown-headed Naturalist 38:25-37.
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