Are there Gloger tail bands in British Columbia Cooper’s Hawks?

Robert N. Rosenfield1, William E. Stout2, Andrew C. Stewart3, Timothy G. Driscoll4, Sarah A. Sonsthagen5, and John P. Seibel6

1 Corresponding author: Department of Biology, University of Wisconsin-Stevens Point, Wis., U.S.A. 54481; email:
2 W2364 Heather Street, Oconomowoc, Wis., U.S.A. 53066
3 1921 Doran Road, Cobble Hill, B.C., Canada VOR 1L5
4 Urban Raptor Research Project, Grand Forks, N.D., U.S.A. 58201
5 Department of Biology, University of Wisconsin-Stevens Point, Wis., U.S.A. 54481
6 3534 N. 94th Street, Milwaukee, Wis., 53222 U.S.A

Abstract: Our novel investigations of the width of melanic, dark tail bands of live breeding Cooper’s Hawks (Accipiter cooperii) in southeastern Vancouver Island, British Columbia, Canada, and eastern North Dakota and southern Wisconsin, United States, 2004-2009, revealed that tail-band width in both sexes was on average 2 mm greater in British Columbia hawks vs. counterparts in other study sites. Relative to body size (mass) and tail length in both sexes, smaller British Columbia hawks had significantly wider tail bands than larger North Dakota birds and the largest hawks in Wisconsin. Our metrics on extent of plumage colour concord with Gloger’s rule: that breeding birds in environments of high relative humidity (British Columbia) where bacteria thrive have darker feathers than conspecifics in areas of lower relative humidity because darker feathers resist bacterial degradation better than light feathers. Our data augment our earlier findings of genetic and morphological divergence among these breeding populations of Cooper’s Hawks and provide further statistical support for resurrection of the subspecies A. c. mexicanus for Pacific Northwest Coopers’ Hawks. However, we do not support reuse of subspecific status in part because the geographical extent of British Columbia-type Cooper’s Hawks is unknown. Describing Cooper’s Hawks generally throughout North America should acknowledge significant regional variation in form of this species.

Key words: tail-band widths, tail feathers, melanin, Cooper’s Hawk, Accipiter cooperii, Gloger’s rule, morphology, subspecies, A.c. mexicanus, British Columbia, North Dakota, Wisconsin.

PDFicon Rosenfield, R. N., W. E. Stout, A. C. Stewart, T. G. Driscoll, S. A. Sonsthagen, and J. P. Seibel. 2021. Are there Gloger tail bands in British Columbia Cooper’s Hawks? British Columbia Birds 31:51–57.

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