THE WILSON BULLETIN A QUARTERLY JOURNAL OE ORNITHOLOGY Published by the Wilson Ornithological Society VOL 115, NO. 3 September 2003 PAGES 225-356 Wilson Bull, 115(3), 2003, pp. 225-230 COMPARATIVE RELATIONSHIPS AMONG EYE COLOR, AGE, AND SEX IN THREE NORTH AMERICAN POPULATIONS OF COOPER’S HAWKS ROBERT N. ROSENFIELD,!^ JOHN BIELEFELDT,^ LAURA J. ROSENFIELD,^ ANDREW C. STEWART,^ ROBERT K. MURPHY, ^ DAVID A. GROSSHUESCH,^ AND MICHAEL A. BOZEK^ ABSTRACT. — Although adult plumage in Cooper’s Hawks (Accipiter cooperii) exhibits little or no sexual dichromatism, iris color reportedly changes from yellow or light orange in younger birds to shades of orange or red in older birds, especially in males. However, there is little quantitative data on this phenomenon. It has been suggested that male eye color may serve as asignal of age and hence reproductive fitness and thus offer abasisfornonrandommatinginCooper’sHawks.Inthisstudyweexaminetherelationshipsbetweeneyecolor andage,sex,andreproductiveoutputforCooper’sHawksintwobreedingpopulationsinBritishColumbiaand NorthDakota,1999-2002,andcomparetheseresultstothosepreviouslypublishedforabreedingpopulation in Wisconsin, 1980-1995. Cooper’s Hawks in British Columbia and North Dakota appear to acquire darker orange or red irides more frequently and more quickly than their counterparts at known and relative ages in Wisconsin. Females in all study sites are slower and less likely than males to acquire the darkest eye colors. Eye color is not a reliable predictor of age in individual male and female Cooper’s Hawks, for researchers and perhaps for the birds themselves, because individual hawks of agiven eye color displayed variation in known and relative ages in British Columbia and Wisconsin. There was no significant relationship between the eye colorofmalesandtheirbroodsizesinanyofthesethreepopulations,andthereforenodiscernablesupportfor the premise that male eye color per se signals male fitness, or functions as asexual trait for assortative mating in this species. Received 20 January 2003, accepted 3 June 2003. In the Cooper’s Hawk {Accipiter cooperii), a species with little or no sexual dichromatism in adult plumage (Rosenfield and Bielefeldt 1993), iris color shows an age dependent re- lationship, especially in males, with irides typ- ‘ Dept, of Biology, Univ. of Wisconsin, Stevens Point, WI 54481, USA. 2Park Planning, Racine County Public Works Div., Sturtevant,WI53177,USA. 3 Wisconsin Coop. Fishery Unit, Univ. of Wiscon- sin, Stevens Point, WI 54481, USA. BC Environment, P.O. Box 9344, STN Govt., Vic- toria, BC V8W 9M1, Canada. ^ U.S. Fish and Wildlife Service, Des Lacs National Wildlife Refuge, Kenmare, ND 58746, USA. ^ 5426 Juniata St., Duluth, MN 55804, USA. ’Corresponding author; e-mail: rrosenfi@uwsp.edu ically changing from yellow or light orange in younger birds to shades of orange or red in older birds (Rosenfield et al. 1992, Rosenfield and Bielefeldt 1997). Other researchers, al- thoughlacking multiyear data on eye color of individually marked birds of known age, have postulated that eye color may be a character- istic (Snyder and Snyder 1974) or one of a suite of characteristics (Boal 2001) that might signal reproductive fitness and offer the op- portunity for assortative mating, but we found no correlation between male eye color and clutch or brood sizes as indices of fitness in a population in Wisconsin (Rosenfield et al. 1992, Rosenfield and Bielefeldt 1997). How- ever,therelationshipsbetweeneyecolorand 225