Comparative diets of nesting Golden Eagles in the Columbia Basin between 2007-2013 and the late 1970s.
Habitat in the Columbia Basin is characterized by open expanses of shrub-steppe and grasslands along major rivers (Vander Haegen and others 2000). Fruit orchards and irrigated farmland surround much of the native habitat used by eagles, especially where terrain is level enough to be farmed. Golden Eagles most often nest along cliff faces above basins and forage along open hillsides and in draws (Watson and others 2014). These eagles also nest in mature and over-mature Ponderosa Pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) trees at higher elevations along the foothills away from the drainage basins.
We collected prey remains in and below the Golden Eagle nests throughout the Columbia Basin (Fig. 1). Prey were collected once at each nest in 2007-2013, from July to August, after young eagles fledged. When prey were limited or absent in the nest on the initial visit, we visited and resampled the nest in a subsequent year. Marr and Knight (1983) used a similar sampling protocol in the 1970s and visited nests once per season, with some nests visited multiple years. We also resampled 2 nests where we installed trail cameras (Reconyx Rapidfire) for 1 season between May and July to test their efficacy in assessing diets and to compare frequencies of relatively large (>1 kg) prey and small prey documented by the 2 methods. Large prey may be overrepresented in raptor diets that are assessed by collecting prey during only 1 nest visit (Marti and others 2007). We prioritized visits to nests sampled by Marr and Knight (1983), most of which were in north-central Washington (Fig. 1), but were only able to sample nests on 5 of the same territories. Most territories (68%) studied in the 1970s were unoccupied during our study, and 34% have been unoccupied since the 1980s (Washington Department of Fish and Wildlife, Wildlife Survey Data Management Database, G Blatz, 600 Capitol Way N, Olympia, WA 98501). The rest of the territories were not visited primarily because of a lack of permission to trespass, or unsafe climbing conditions. Thus, we limited prey comparisons between the 2 studies to a general comparison of frequencies of major taxa with a chi-square contingency test rather than testing for prey changes on specific territories. From each prey sample, which included prey remains and pellets, we derived the minimum number of individuals down to the most specific taxa possible through identification of major bones and bone fragments (Searfoss 1995; Elbroch 2006) matched with fur and feathers (Moore and others 1974; Scott and McFarland 2010). We computed prey biomass derived from frequency and published mass values for adult and young prey species. For consistency, we used the same values of mass reported in Marr and Knight (1983) to compute biomass estimates and consulted Sibley (2000) for avian mass where necessary. Also, biomass of prey was not estimated for adult deer (Odocoileus spp.), Coyotes (Canis latrans), and Bighorn Sheep (Ovis canadensis) that were too large for entire carcasses to be transported to nests, thus having indeterminate mass.
From 2007-2013, we collected prey at 36 Golden Eagle nests representing 24 territories (12 nests visited twice) including the 2 video camera samples. We pooled camera samples because we did not find a difference in the frequency of relatively large prey and small prey between camera and prey samples (P = 0.967). Analysis of 250 prey remains ([bar.x] = 7.3 individuals/sample, s = 5.4) showed that mammals constituted the highest proportion of diets by frequency (56.4%) and biomass (79.0%), and were predominantly Yellow-bellied Marmots, Coyotes, and deer (Table 1). Coyote pups and deer fawns were particularly prevalent. These prey types were also distributed most widely among eagle territories (Table 1). Birds were less important by frequency (41.2%) and biomass (20.6%), and predominated by species in the family Phasianidae. Prey species diversity was relatively high among birds compared to mammals, with Chukar Partridge (Alectoris chukar), tetraonids, Great Horned Owls (Bubo virginianus), and corvids most abundant. Reptiles (1.6%) and fish (0.4%) were uncommon as prey.
Frequencies of prey among the 4 most abundant and widely-distributed taxa, relative to all other prey, were different ([chi square] = 179.96, df = 4, P< 0.0001) from prey analyzed in the 1970s (Marr and Knight 1983). Resident eagles delivered greater-than-expected numbers of deer and Coyotes to nests in our study (standardized residuals = 9.78 and 8.06, respectively), and conversely captured fewer-than-expected phasianids and sciurids (standardized residuals = -3.38 and -3.36, respectively). Specifically, in our study eagles ate fewer Yellow-bellied Marmots (27.6 versus 40.3%), Chukar Partridge (4.4 versus 11.7%), and Blue Grouse (Dendragapus obscurus) (2.2 versus 13.0%).
Similar prey collection protocols between our study and Marr and Knight (1983), with nests visited once per season, with some nests visited multiple years, gave validity to this temporal comparison of diets, but we acknowledge that only 13% of territories from the 1970s were revisited in our study. Orthophoto interpretation (National Agriculture Imagery Program 2013, http://gis.apfo.usda.gov/arcgis/services, last accessed 15 May 2014) <5 km from sampled nests on the other territories revealed a high consistency in type and condition of habitats between studies. In both studies, all territories ranged from lower-elevation foothills down to lower-elevation shrub-steppe, with a moderate to sparse number of trees. Only 4 territories had recent evidence of agricultural conversion or disturbance, and these territories had been occupied by eagles since the 1980s (Washington Department of Fish and Wildlife, Wildlife Resource Data System). Thus, there were no obvious dissimilarities between general habitats on most territories sampled in the 2 studies that might point to underlying differences in prey available to eagles and account for dietary differences. Because we did not sample prey availability, however, we do not know how localized prey distribution on specific territories might have influenced the diet comparison.
Prominence of Yellow-bellied Marmots as prey of nesting Golden Eagles in the Columbia Basin was not unexpected because of their association with open slopes surrounded by talus and rugged terrain (Van Vuren 2001). Whereas marmots were also prominent as Golden Eagle prey during the late 1970s (Knight and Erickson 1978; Marr and Knight 1983), the comparative higher use of Coyote pups and deer fawns and lower use of upland birds and squirrels in our study may represent a dietary shift. We were unable to test for temporal changes in diets at the same territories sampled in earlier years largely because of the low, recent occupancy of eagles on those territories. Although we suspect reduced occupancy is related to prey, the cause remains unclear.
In both studies there was low occurrence of both jackrabbit species and 2 endemic ground squirrels (for example, Urocitellus washingtoni and Urocitellus townsendii) in the eagle's diet, which is consistent with long-term declines in these prey species and their resulting Candidate status in Washington. In the Yakima Valley, the central region where eagle diets were sampled in both studies (Fig. 1), the Black-tailed Jackrabbit (Lepus californicus) population in the 1950s was cyclic, large enough to generate public "drives" to reduce rabbit numbers, and was hunted legally until 2000 when the species' long-term decline became apparent (Washington Department of Fish and Wildlife, unpubl. data). Although leporids and sciurids predominate in the nesting diets of Golden Eagles throughout North America (Kochert and others 2002), our analysis shows that some eagle pairs in Washington are able to persist on diets predominated by other prey. The invasive California Ground Squirrel (Otospermophilus beecheyi) was the main ground squirrel found in our study in the Yakima Valley region but was not present in nests in the same region in the late 1970s. It is unclear if this frequency is related to range expansion of this squirrel, but they have become a relatively important component of eagle diets in south-central Washington.
Our comparison of prey frequencies by biomass grouping between prey- and camera-based samples would have benefited from a larger sample and systematic collection of prey. Collopy (1983) found no significant bias in species composition of Golden Eagle prey between direct observations and nest prey collected 2 d later, and Marti and others (2007) recommended multiple prey collections at each nest during the season to reduce potential bias resulting from longer persistence of larger prey items. On the other hand, prey items secured by adult Golden Eagles as carrion, such as ungulate carcasses (Sanchez-Zapata and others 2010), may be underrepresented in nesting diets because such items are often not transported to nests. In eastern Washington, ungulate carrion resulting from wounding loss late in the hunting season and from winter-kill is available to eagles in the early part of the nesting season in January and February when carcasses persist in cooler temperatures (J Watson, pers. obs). Coyotes are hunted legally throughout the year by the public and federal control agents, and there is organized contest hunting of Coyotes in late winter, all of which make Coyote carcasses available to foraging eagles (J Watson, pers. obs.). Consumption of Coyote and ungulate carrion would be important to document because these prey items are likely sources of lead ingestion and subsequent toxicosis in eagles in the Pacific Northwest (Stauber and others 2010).
Key words: Aquila chrysaetos, Columbia Basin, diet, Golden Eagle, nesting, prey remains, Washington
Acknowledgments.--Support for this research was through the Wildlife Program of the Washington Department of Fish and Wildlife, and the Partners for Wildlife Program of the Woodland Park Zoo, Seattle. The Naches Ranger District of the US Forest Service funded climbing equipment. We thank V Marr for providing historical eagle diet data and nest locations. A Turner, P Wik, W Moore, M Vekasy, J Bernatowicz, D Anderson, and T Pitz provided vital assistance with data collection. We thank J Hilaire, US Forest Service, and R Fischer, US Army Corps of Engineers, for logistical assistance. We thank M. Collopy, E Craig, M. Kochert, and M Vander Haegen for critical reviews that improved drafts of this note.
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TABLE 1. Prey of nesting Golden Eagles in eastern Washington collected on 24 territories in 2007-2013 (tr = trace; empty cell = insufficient information for calculation; ad. = adult). Distribution % Common name Scientific name territories MAMMALS Nuttall's Cottontail Syvilagus nuttalii 17.4 Black-tailed Jackrabbit Lepus californicus 4.3 White-tailed Jackrabbit Lepus townsendii 4.3 Yellow-bellied Marmot Marmota flaviventris 56.5 Hoary Marmot Marmota caligata 4.3 Columbian Ground Squirrel Urocitellus columbianus 4.3 California Ground Squirrel Otospermophilus beecheyi 34.8 Tree Squirrel Sciurus spp. 4.3 Unidentified Vole Microtus spp. 4.3 Bushy-tailed Woodrat Neotoma cinerea 4.3 Northern Pocket Gopher Thomomys talpoides 4.3 Striped Skunk Mephitis mephitis 13.0 Coyote (immature) Canis latrans 39.1 Coyote (ad. carrion) 13.0 Deer fawn Odocoileus spp. 73.9 Deer (ad. carrion) 17.4 Bighorn Sheep (ad. carrion) Ovis canadensis 4.3 Unidentified large mammal 8.7 TOTAL MAMMALS Birds Red-tailed Hawk Buteo jamaicensis 8.7 Chukar Partridge Alectoris chukar 39.1 Pheasant Phasianus colchicus 13.0 Blue Grouse Dendragapus obscurus 17.4 Ruffed Grouse Bonasa umbellus 13.0 Wild Turkey Meleagris gallopavo 17.4 Rock Dove Columba livia 4.3 Mourning Dove Zenaida macroura 4.3 Great Horned Owl Bubo virginianus 47.8 Short Eared Owl Asio flammeus 8.7 Western Screech Owl Otus kennicottii 13.0 Barn Owl Tyto alba 4.3 Northern Flicker Colaptes auratus 8.7 Hairy Woodpecker Picoides villosus 8.7 White-headed Woodpecker Picoides albolarvatus 4.3 Lewis's Woodpecker Melanerpes lewis 4.3 Clark's Nutcracker Nucifraga Columbiana 4.3 Stellar's Jay Cyanocitta stelleri 8.7 American Magpie Pica hudsonia 26.1 American Crow Corvus brachyrhynchos 21.7 Common Raven Corvus corax 30.4 European Starling Sturnus vulgaris 4.3 Western Bluebird Sialia mexicana 4.3 Unidentified passeriformes 30.4 TOTAL BIRDS Reptiles Gopher Snake Pituophis melanoleucus 4.3 Unidentified snake 21.7 TOTAL REPTILES Fish Largescale Sucker Catostomus macrocheilus 4.3 TOTAL PREY % Total Biomass Common name Frequency frequency (g) MAMMALS Nuttall's Cottontail 4 1.6 514 Black-tailed Jackrabbit 1 0.4 1379 White-tailed Jackrabbit 1 0.4 2042 Yellow-bellied Marmot 53 21.2 2797 Hoary Marmot 1 0.4 6300 Columbian Ground Squirrel 3 1.2 451 California Ground Squirrel 11 4.4 727 Tree Squirrel 1 0.4 533 Unidentified Vole 1 0.4 49 Bushy-tailed Woodrat 1 0.4 310 Northern Pocket Gopher 1 0.4 104 Striped Skunk 3 1.2 4500 Coyote (immature) 17 6.8 2043 Coyote (ad. carrion) 3 1.2 Deer fawn 32 12.8 3623 Deer (ad. carrion) 5 2.0 Bighorn Sheep (ad. carrion) 1 0.4 Unidentified large mammal 2 0.8 TOTAL MAMMALS 138 56.4 Birds Red-tailed Hawk 2 0.8 1056 Chukar Partridge 11 4.4 602 Pheasant 4 1.6 1138 Blue Grouse 5 2.0 907 Ruffed Grouse 10 4.0 557 Wild Turkey 5 2.0 5800 Rock Dove 2 0.8 332 Mourning Dove 1 0.4 120 Great Horned Owl 11 4.4 1505 Short Eared Owl 2 0.8 348 Western Screech Owl 3 1.2 150 Barn Owl 1 0.4 460 Northern Flicker 3 1.2 155 Hairy Woodpecker 2 0.8 66 White-headed Woodpecker 1 0.4 61 Lewis's Woodpecker 1 0.4 115 Clark's Nutcracker 1 0.4 130 Stellar's Jay 3 1.2 125 American Magpie 12 4.8 170 American Crow 6 2.4 384 Common Raven 8 3.2 1200 European Starling 1 0.4 82 Western Bluebird 1 0.4 29 Unidentified passeriformes 7 2.8 100 TOTAL BIRDS 101 41.2 Reptiles Gopher Snake 1 0.4 250 Unidentified snake 4 1.6 250 TOTAL REPTILES 1.6 Fish Largescale Sucker 1 0.4 454 TOTAL PREY 244 99.4 Total % Estimated Common name biomass (g) total biomass MAMMALS Nuttall's Cottontail 2056 0.5 Black-tailed Jackrabbit 1379 0.3 White-tailed Jackrabbit 2042 0.5 Yellow-bellied Marmot 148,241 35.0 Hoary Marmot 6300 1.5 Columbian Ground Squirrel 1353 0.3 California Ground Squirrel 7997 1.9 Tree Squirrel 533 0.1 Unidentified Vole 49 tr Bushy-tailed Woodrat 310 0.1 Northern Pocket Gopher 104 0 Striped Skunk 13,500 3.2 Coyote (immature) 34,731 8.2 Coyote (ad. carrion) Deer fawn 115,936 27.4 Deer (ad. carrion) Bighorn Sheep (ad. carrion) Unidentified large mammal TOTAL MAMMALS 334,531 79.0 Birds Red-tailed Hawk 2112 0.5 Chukar Partridge 6622 1.6 Pheasant 4552 1.1 Blue Grouse 4535 1.1 Ruffed Grouse 5570 1.3 Wild Turkey 29,000 6.9 Rock Dove 664 0.2 Mourning Dove 120 tr Great Horned Owl 16,555 3.9 Short Eared Owl 696 0.2 Western Screech Owl 450 0.1 Barn Owl 460 0.1 Northern Flicker 465 0.1 Hairy Woodpecker 132 tr White-headed Woodpecker 61 tr Lewis's Woodpecker 115 tr Clark's Nutcracker 130 tr Stellar's Jay 375 0.1 American Magpie 2040 0.5 American Crow 2304 0.5 Common Raven 9600 2.3 European Starling 82 tr Western Bluebird 29 tr Unidentified passeriformes 700 0.2 TOTAL BIRDS 87,369 20.6 Reptiles Gopher Snake 250 0.1 Unidentified snake 750 0.2 TOTAL REPTILES 1000 0.3 Fish Largescale Sucker 454 0.1 TOTAL PREY 423,354 99.9
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|Title Annotation:||GENERAL NOTES|
|Author:||Watson, James W.; Davies, Robert W.|
|Publication:||Northwestern Naturalist: A Journal of Vertebrate Biology|
|Date:||Mar 22, 2015|
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