Printer Friendly

Comparative diets of nesting Golden Eagles in the Columbia Basin between 2007-2013 and the late 1970s.

In 1991, the Golden Eagle (Aquila chrysaetos) was identified as a candidate for state listing in Washington as a result of concerns about this species' population status. Concurrent with the designation has been a continued widespread conversion of shrub-steppe habitats to agriculture (Vander Haegen and others 2000) and potential for associated changes in eagle prey. In the late 1970s, Marr and Knight (1983) examined food habitats of Golden Eagles on 38 territories in eastern Washington and found nearly equal numbers of avian and mammalian prey, <1% of jackrabbits (Lepus californicus and Lepus townsendi) and ground squirrels (Urocitellus spp.), and 42% Yellow-bellied Marmots (Marmota flaviventris), which was unusually high (Knight and Erickson 1978). It is unknown whether these species are representative of current diets of nesting Golden Eagles in the Columbia Basin. In this paper we provide a dietary analysis of nesting eagles based on prey collected throughout the Columbia Basin from 2007-2013 and compare the results to diets from 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.

LITERATURE CITED

COLLOPY MW. 1983. A comparison of direct obervations and collections of prey remains in determining the diet of Golden Eagles. Journal of Wildlife Management 47:360-368.

ELBROCH M. 2006. Animal skulls--a guide to North American species. Mechanicsburg, PA: Stackpole Books. 727 p.

KNIGHT RL, ERICKSON AW. 1978. Marmots as a food source of Golden Eagles along the Columbia River. Murrelet 59:28-30.

KOCHERT MN, STEENHOF K, MCINTYRE CL, CRAIG EH. 2002. Golden Eagle (Aquila chrysaetos), Account 684. In: Poole A, Gill F, editors. The birds of North America. Philadelphia, PA and Washington, DC: The Academy of Natural Sciences and The American Ornithologists' Union. 44 p.

MARR NV, KNIGHT RL. 1983. Food habits of Golden Eagles in eastern Washington. Murrelet 64:73-77.

MARTI CD, BECHARD M, JAKSIC FM. 2007. Food habits. In: Bird DM, Bildstein KL, editors. Raptor research and management techniques. Blaine, WA: Hancock House Publishers, p 129-149.

MOORE TD, SPENCE LE, DUGNOLLE CE, HEPWORTH WG. 1974. Identification of the dorsal guard hair of some mammals of Wyoming. Cheyenne, WY: Wyoming Game and Fish Department Bulletin No. 14.177 p.

SANCHEZ-ZAPATA JA, EGUIA S, BLAQUES M, MOLEON M. 2010. Unexpected role of ungulate carcasses in the diet of Golden Eagles Aquila chrysaetos in Mediterranean mountains. Bird Study 57:352-360.

SCOTT SD, MCFARLAND C. 2010. Bird feathers--a guide to North American species. Mechanicsburg, PA: Stackpole Books. 357 p.

SEARFOSS G. 1995. Skulls and bones--a guide to the skeletal structures and behavior of North American mammals. Mechanicsburg, PA: Stackpole Books. 277 p.

SIBLEY DA. 2000. The Sibley guide to birds. New York, NY: Alfred A. Knopf, Inc. 544 p.

STAUBER EN, FINCH N, TALCOTT PA, GAY JM. 2010. Lead poisoning of Bald (Haliaeetus leucocephalus) and Golden (Aquila chrysaetos) Eagles in the US inland Pacific Northwest region--an 18-year retrospective study: 1991-2008. Journal of Avian Medicine and Surgery 24:279-287.

VANDER HAEGEN WM, DOBLER FC, PIERCE DJ. 2000. Shrubsteppe bird response to habitat and landscape variables in eastern Washington, USA. Conservation Biology 14:1145-1160.

VAN VUREN DH. 2001. Predation on Yellow-bellied Marmots (Marmota flaviventris). American Midland Naturalist 145:94-100.

WATSON JW, DUFF AA, DAVIES RW. 2014. Home range and resource selection by GPS-monitored adult Golden Eagles in the Columbia Plateau Ecoregion: Implications for wind power development. Journal of Wildlife Management 78:1012-1021.

Washington Department of Fish and Wildlife, 600 Capital Way N, Olympia, WA 98501 USA; james. watson@dfw.wa.gov (JWW, RWD). Submitted 30 November 2013, accepted 27 June 2014. Corresponding Editor: Joan Hagar.

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
COPYRIGHT 2015 Society for Northwestern Vertebrate Biology
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2015 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:GENERAL NOTES
Author:Watson, James W.; Davies, Robert W.
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Article Type:Report
Geographic Code:1USA
Date:Mar 22, 2015
Words:3106
Previous Article:New records of seven Cusk-eels (Ophidiidae) and brotulas (Bythitidae) in coastal waters of British Columbia, Canada.
Next Article:Breeding status of Ancient Murrelets attending gathering grounds near Langara Island, British Columbia, 1970-1971.
Topics:

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters