Robert N. Rosenfield1, Andrew C. Stewart2, William E. Stout3, Sarah A. Sonsthagen4, and Paul N. Frater5
1 Corresponding author: Department of Biology, University of Wisconsin-Stevens Point, Stevens Point, Wis., U.S.A 54481; email: email@example.com
2 1921 Doran Road, Cobble Hill, B.C., Canada VOR 1L5
3 W2364 Heather Street, Oconomowoc, Wis., U.S.A. 53066
4 U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage,
Alaska, U.S.A. 99508
5 School of Engineering and Natural Sciences, University of Iceland, Dunghagi 5, Reykjavik, Iceland
Abstract: Our investigations of the morphology of live breeding Cooper’s Hawks (Accipiter cooperii) in southeastern Vancouver Island, British Columbia, Canada, and central and southeastern Wisconsin, United States, 2005–2009, revealed small differences in absolute toe lengths in birds between study sites, but that relative to body size (mass) in both sexes, smaller British Columbia hawks had longer middletoes than did larger Wisconsin hawks. Our findings support a predator-prey principle that the smaller size of British Columbia hawks reflects selection for an optimum body size that enhances agility when pursuing their dominant prey, small to mid-sized songbirds. Further, relatively longer toes may impart a longer reach that enhances capture of agile avian prey. Less agile mammalian prey are more prevalent in the diet of Wisconsin birds, which may suggest less selective pressure for longer toes in this eastern population.
Key words: morphology, Cooper’s Hawk, Accipiter cooperii, intraspecific geographic variation, middle toe length, body mass, predator prey relationships, British Columbia, Wisconsin.
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