Urocitellus Ground Squirrels Biology of Ground Squirrels (Urocitellus Spp.) As Prey of Golden Eagles (Aquila Chrysaetos) in the Western United States
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Urocitellus Ground Squirrels Biology of Ground Squirrels (Urocitellus spp.) as Prey of Golden Eagles (Aquila chrysaetos) in the Western United States Belding’s ground squirrel (Urocitellus beldingi) in northern California. Photo Credit,deLight Sky Urocitellus Ground Squirrels Biology of Ground Squirrels (Urocitellus spp.) as Prey of Golden Eagles (Aquila chrysaetos) in the Western United States U.S. Fish and Wildlife Service Regions 1, 2, 6, and 8 Western Golden Eagle Team Front Matter Date: January 31, 2018 Disclaimer The reports in this series have been prepared by the U.S. Fish and Wildlife Service (Service) Western Golden Eagle Team (WGET) for the purpose of proactively addressing energy-related conservation needs of golden eagles in Regions 1, 2, 6, and 8. The team was composed of Service personnel, sometimes assisted by contractors or outside cooperators. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service. Suggested Citation Bedrosian, G., D.L. Hansen, and R. Spaul. 2018. Biology of Ground Squirrels (Urocitellus spp.) as Prey of Golden Eagles (Aquila chrysaetos) in the Western United States. Unpublished report prepared by the Western Golden Eagle Team, U.S. Fish and Wildlife Service, Regions 1, 2, 6, and 8. Available online at: https://ecos.fws.gov/ServCat/Reference/Profile/97773 Acknowledgements This report was authored by Geoffrey Bedrosian, Dan L. Hansen, and Robert Spaul. The authors are grateful to the following reviewers (in alphabetical order): Colleen Lenihan, Katie Powell, Rebecca Smith, Beatrice Van Horne, Hillary White, and Brian Woodbridge. Eric Yensen provided feedback on a previous version of this report. Urocitellus ground squirrels—i Overview Holarctic ground squirrels (Genus Urocitellus, hereafter “ground squirrels”) are members of the squirrel family of rodents (Sciuridae), which generally live on or in the ground. Ground squirrels are typically grayish or brownish, with the dorsal hair darker than the ventral, a light-colored eye ring, cylindrical bodies, large eyes, and chisel-shaped incisors. Their forelimbs have sharp claws for digging and their longer hind limbs allow for rapid movement. Ground squirrels are smaller-bodied than prairie dogs (Cynomys spp.), marmots (Marmota spp.), and rock squirrels (Otospermophilus spp.). This report reviews the role of ground squirrels as prey for golden eagles (Aquila chrysaetos), as well as relevant aspects of their natural history, ecology, and management. We discuss seven species of ground squirrels that occur within the range of the golden eagle in the conterminous western United States (U.S.) and which have been documented as golden eagle prey: Belding’s (U. beldingi), Columbian (U. columbianus), Merriam’s (U. canus), Piute (U. mollis), Richardson’s (U. richardsonii), Uinta (U. armatus), and Wyoming (U. elegans) ground squirrels. This account does not address other ground squirrel genera (e.g., Otospermophilus spp., see rock squirrel account). Most ground squirrel species are found at relatively low proportions in breeding season diets of golden eagles, compared to hares and rabbits. However, these ground squirrel species may play an important role in buffering golden eagle food resources in years with low hare and rabbit availability. In contrast, Belding’s ground squirrels can be found at very high densities in irrigated alfalfa (Medicago sativa) fields in northern California where they are the dominant prey species used by nesting golden eagles. It is likely that the abundance of Belding’s ground squirrels influences the high density, occupancy rates and reproductive performance of nesting golden eagles observed in that area. The degree to which golden eagles feed on ground squirrels is likely influenced by the availability of ground squirrel populations, as well as the availability of preferred prey species (hares and rabbits). Ground squirrels occupy a variety of shrub-steppe, grassland, and montane habitats in the northwestern U.S. Ground squirrels are diurnal and considered semi-fossorial; they spend their nights and take refuge in burrows, but forage above ground. During the active season, ranging from February–March through May–September, burrows provide ground squirrels with cover from severe weather and extreme temperatures, protection from predators, and shelter for rearing young. Ground squirrel diets consist of a variety of grasses and forbs, as well as flowers and seeds, when available. Additional food items include insects, berries, and occasionally carrion of conspecifics. Contemporary ground squirrel populations typically occur in relatively small local populations within geographically restricted ranges, except where they are exceptionally abundant. Population size and density can fluctuate annually depending on availability of suitable habitat and food. Other influences on ground squirrel abundance include parasites, disease, weather, predators, control programs, and habitat modification. Across the western U.S., anthropogenic habitat modification is thought to be responsible for creating discontinuous populations of many ground squirrel species. Many western shrub-steppe Urocitellus Ground Squirrels—ii ground squirrel populations have experienced extensive declines across their historical ranges yet most have low national conservation rankings. Regions where habitat alteration is most pervasive and negatively influencing ground squirrels include the Great Basin ecosystem and the Columbia Plateaus of Washington and Oregon. The U.S. Fish and Wildlife Service recognizes prey densities as potential limiting factors for golden eagles. Furthermore, poorly managed livestock grazing, invasive vegetation, wildfire-caused habitat conversion, and climate change can cause the loss of foraging habitat. Conversion of native grasslands to agriculture has limited suitable habitat for western shrub-steppe species of ground squirrels while providing other species, such as the Belding’s ground squirrel, with new food sources. Conversion of native shrub-steppe to agriculture may have a particularly negative impact on ground squirrels when it occurs on the arable, deep soil communities most favored by ground squirrels. Agricultural areas in shrub-steppe ecosystems may be able to support greater numbers of ground squirrels through management practices that maintain or increase natural cover, thereby enhancing the spatial and functional heterogeneity of the landscape. Many areas of the West are now locked in a grass/fire cycle with the potential to negatively impact golden eagle prey habitat. Following major wildfires, loss of shrub cover, and subsequent reductions in populations of hares and rabbits, golden eagles may shift their diet to eating more ground squirrels. However, conversion of shrublands to exotic annual grasses can cause wider annual fluctuations in ground squirrel numbers, lower population densities, and lower body masses compared with shrub and native grass habitats. Invasive annual grasses, intense wildfire, and continued habitat conversion are likely to further isolate and restrict ground squirrel populations. Thus, conservation of ground squirrel populations will largely depend on preservation of occupied habitat, particularly for the sagebrush-steppe species that exist as a series of small, isolated populations. Population control of ground squirrels on agricultural fields is a source of lead contamination to golden eagles that feed on shot carcasses and may also be a source of anticoagulant rodenticide (AR) poisoning. Lead toxicosis and AR poisoning can directly result in mortality, and can also increase the risk to golden eagles of accidental trauma and the likelihood of a fatal collision with vehicles or structures. Lead exposure to golden eagles from shot ground squirrels could be reduced by collecting and disposing of carcasses or by using non-lead ammunition. Secondary exposure of golden eagles to ARs could likewise be reduced by removing poisoned ground squirrel carcasses and generally avoiding the use of poisons that pose a risk to non-target wildlife. Urocitellus Ground Squirrels—iii Table of Contents Front Matter .............................................................................................................................................. i Disclaimer .......................................................................................................................................................... i Suggested Citation ........................................................................................................................................... i Acknowledgements ........................................................................................................................................ i Executive Summary .......................................................................... Error! Bookmark not defined. Table of Contents .................................................................................................................................... 1 Importance to Golden Eagles ............................................................................................................. 2 Occurrence in Golden Eagle Diets............................................................................................................. 2 Ground Squirrels in Breeding Season Diets of Golden Eagles ................................................................ 2 Individual Ground Squirrel