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Bald eagle (Haliaeetus leucocephalus) predation of arctic ground squirrels (Urocitellus parryii).

Arctic Ground Squirrels (Urocitellus parryii) may be a keystone species in northern environments (Wheeler and Hik 2013). Their burrows provide shelter to a host of other species, and their burrowing may be an important ecosystem process at the microsite level, likely qualifying them as ecosystem engineers. Moreover, they are an important part of the seasonal diet of a host of avian and mammalian predators.

Ground-dwelling squirrels are reported prey for eagles. For example, Golden Eagles (Aquila chrysaetos) are a noted predator of Rock Squirrels (Otospermophilus variegatus; Oaks and others 1987), Columbian Ground Squirrels (Urocitellus columbianus; Elliott and Flinders 1991), and Mohave Ground Squirrels (Xerospermophilus mohavensis, Best 1995). Evans and Holdenried (1943) stated that they were also a predator of California Ground Squirrels (Otospermophilus beecheyi), but provided no evidence. Bailey (1975) reported the remains of Arctic Ground Squirrels from the nest of a Golden Eagle in southwestern Alaska.

There are comparatively few records of Bald Eagles (Haliaeetus leucocephalus) preying on squirrels, their main prey species being fish, waterfowl, and lagomorphs. Bald Eagles, however, have been noted to prey on Richardson's Ground Squirrels (Urocitellus richardsonii; Caton and others 1992) and Rock Squirrels (Grubb 1995). Garcelon (1988) hypothesized that the Bald Eagle population on Santa Catalina Island, California, became extirpated after scavenging on the carcasses of California Ground Squirrels that were poisoned with strychnine and sodium monofluroacetate (compound 1080). Tree-dwelling Fox Squirrels (Sciurus niger) have also been observed in the diet of Bald Eagles (Southern 1964; Mabie and others 1995), and there are videos of this available on the internet. To the best of my knowledge, there is only 1 recorded instance of Arctic Ground Squirrels as a food item of Bald Eagles: Based on an analysis of nest contents, Murie (1940) reported Bald Eagles on Kavalga Island (Aleutian Islands), Alaska consuming Arctic Ground Squirrels that were introduced to provide a food source for an exploited population of Arctic Fox (Vulpes lagopus). However, it is important to note that Murie (1940) did not report whether Bald Eagles on Kavalga Island killed or scavenged ground squirrels. Here, I report an observation of a Bald Eagle killing and feeding on an Arctic Ground Squirrel.

On 24 May 2014, I observed a Bald Eagle in adult plumage killing and eating an Arctic Ground Squirrel at the Yukon Wildlife Preserve (60.874[degrees]; -135.331[degrees]), near Whitehorse, Yukon, Canada. At 12:13 PDT, I observed the eagle (sex unknown), land in a short-grass paddock within an enclosure inhabited by American Bison (Bison bison). The eagle used its talons to hold and subdue the squirrel and spread and flapped its wings to maintain balance while the squirrel struggled. After 3-4 min, the squirrel appeared dead and the eagle closed its wings, lowered its head, and fed on the squirrel. I observed the eagle feeding on the squirrel until 12:36 PDT, and then ceased my observation. I could not ascertain the sex of the ground squirrel, but it was most likely an adult, as juveniles in southwestern Yukon do not emerge until 6-7 June at the earliest (Boonstra and others 2001).

This observation confirms Bald Eagle predation of Arctic Ground Squirrels. Murie's (1940) observation of Bald Eagles on a single island in the Aleutian Islands, Alaska, consuming--but not necessarily killing--Arctic Ground Squirrels is the only other record of this interaction, suggesting that these encounters may be rare or localized. Long-term research on Arctic Ground Squirrels in southwestern Yukon has not previously noted predation by Bald Eagles (CJ Krebs, University of British Columbia, pers. comm.; R Boonstra, University of Toronto, pers. comm.). However, Bald Eagle predation of Arctic Ground Squirrels had previously been observed at the Yukon Wildlife Preserve. In spring 2012, a mature Bald Eagle was observed preying on an Arctic Ground Squirrel at the preserve (J Paleczny, Yukon Wildlife Preserve, pers. comm.), and Bald Eagles have been observed perched near short-grass paddocks (R Boonstra, University of Toronto, pers. comm.), far from a water body.

Arctic Ground Squirrels are abundant at the Yukon Wildlife Preserve and they may provide a locally important food source to Bald Eagles early in the breeding season when access to open water and preferred prey species (fish and waterfowl) is limited. It is not known if ground squirrel predation is a local phenomenon (learned behavior within a local population), or if it is more prevalent among Bald Eagles in the region. Detailed studies of Bald Eagle diets in the region are necessary to determine the relative importance of Arctic Ground Squirrels as prey.

Key words: Arctic Ground Squirrel, Bald Eagle, Haliaeetus leucocephalus, predation, Urocitellus parryii, Yukon

Acknowledgments.--DF Clyde assisted with this observation. I thank J Paleczny for sharing his observation of Bald Eagles preying on Arctic Ground Squirrels at the Yukon Wildlife Preserve. DS Hik and an anonymus reviewer kindly provided helpful comments on an earlier draft of this note.

LITERATURE CITED

Bailey EP. 1975. Discovery of a golden eagle nest on the Alaska Peninsula. Condor 77:207-208.

Best TL. 1995. Spermophilus mohavensis. Mammalian Species 509:1-7.

Boonstra R, Boutin S, Byrom A, Karels T, Hubbs A, Stuart-Smith K, Blower M, Antpoehler S. 2001. The role of red squirrels and Arctic ground squirrels. In: Krebs CJ, Boutin S, Boonstra R, editors. Ecosystem dynamics of the boreal forest: the Kluane project. New York, New York: Oxford University Press, p 179-214.

Caton E, McClelland BR, Patterson DA, Yates RE. 1992. Characteristics of foraging perches used by breeding bald eagles in Montana. Wilson Bulletin 104:136-142.

Elliot CL, Flinders JT. 1991. Spermophilus columbianus. Mammalian Species 372:1-9.

Evans FC, Holdenried R. 1943. A population study of the Beechy ground squirrel in central California. Journal of Mammalogy 24:231-260.

Garcelon DK. 1988. The reintroduction of bald eagles on Santa Catalina Island, California, [thesis]. Areata, CA: Humboldt State University.

Grubb TG. 1995. Food habits of bald eagles breeding in the Arizona desert. Wilson Bulletin 107:258-274.

Mabie DW, Merendino MT, Reid DH. 1995. Prey of nesting bald eagles in Texas. Journal of Raptor Research 29:10-14.

Murie OJ. 1940. Food habits of the northern bald eagle in the Aleutian Islands, Alaska. Condor 42:198-202.

Oaks EC, Young PJ, Kirkland GL, Schmidt DF. 1987. Spermophilus variegatus. Mammalian Species 272:1-8.

Southern WE. 1964. Additional observations on winter bald eagle populations: including remarks on biotelemetry techniques and immature plumages. Wilson Bulletin 76:121-137.

Wheeler HC, Hik DS. 2013. Arctic ground squirrels Urocitellus parryii as drivers and indicators of change in northern ecosystems. Mammal Review 43:238-255.

Yukon Department of Environment, Whitehorse, YT Y1A 2C6 Canada; thomas.jung@gov.yk.ca. Submitted 5 September 2014, accepted 18 November 2014. Corresponding Editor: Robert Hoffman.
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Title Annotation:General Notes
Author:Jung, Thomas S.
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Date:Sep 22, 2015
Words:1104
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