Short Communications Raptor Res. 26(3):189-191 J. © 1992 The Raptor Research Foundation, Inc. Eye Color of Cooper’s Hawks Breeding in Wisconsin Robert N. Rosenfield Department ofBiology, University of Wisconsin, Stevens Point, WI 54481 John Bielefeldt Park Planning, Racine County Public Works, Sturtevant, WI 53177 Kenneth R. Nolte Caesar Kleberg Wildlife Research Institute, Campus Box 218, TX Texas A<irl University, Kingsville, 78363 Several authors have noted the progressive changes in Captures ofmales atagivennesting area are alsodivided eye color with age in North American accipiters (Gross- between the initial individual trapped on an area {N = man and Hamlet 1964, Meng 1951, Snyder and Snyder 82) and subsequent individuals (replacement males) 1974). These changes are believed to proceed from shades trapped on the same area in later years (N = 31). We of yellow in younger birds to shades of orange or red in assumed that eye colors in our sample of initial male older birds. Snyder and Snyder (1974) suggested thatthese individuals on these 82 different nesting areas represented changes in eye color are also associated with gender and the actual proportion of various eye colors within the speculated about the function of eye color. Here we use breWeeding population in Wisconsin. determined the predominant color over the major the eye color recorded during 377 captures of breeding part of the iris using the following categories in the color Cooper’s Hawks (Accipiter cooperii) in Wisconsin (1980- chart in Palmer (1962): yellow, orange-yellow, orange, 91) to examine eye color in relation to age and gender. scarlet-orange, and scarlet. Hereafter we regard orange- We also discuss our results in relation to breeding quality yellow, scarlet-orange, and scarlet as light orange, dark in males. orange, and red color categories, respectively. In calculat- Study Area and Methods We trapped breeding Cooper’s Hawks at 105 separate Table 1. Iris color ofknown-age Cooper’s Hawks breed- ing in Wisconsin. nesting areas (Rosenfield and Bielefeldt 1992) throughout We Wisconsin during 1980-91. caught 253 different in- dividuals (113 males and 140 females) and retrapped some Eye Color Categories A.ge of the same birds a total of 124 times in subsequent years. Most Cooper’s Hawks were trapped near their nests in IN Light Dark mist nets (3 x 12 m) using a live Great-Horned Owl Years Yellow Orange Orange Orange Red We {Bubo virginianus) as a decoy (Hamerstrom 1963). also caught hawks in bow nets baited with European Star- Males {N = 29 captures) lings {Sturnus vulgaris) and Ring Doves {Streptopelia ri- 1 2 8 0 0 0 soria). 2 0 3 5 0 0 Forty-two captures involved birds of known age: 1 yr 3 0 2 3 1 0 old individuals in predominantly brown plumage {N = 4 0 0 2 2 0 17); 2 yr old individuals (all males) with two generations 5 0 0 0 0 1 of gray feathers, plus a few retained brown feathers on rump and/or scapular areas {N = 4); 2 or 3 yr old indi- Females {N = 13 captures) vBiiedlueaflesldotri1g9i9n2a;llNy m=ar5k);edanads 1n6esrtelcianpgstur(eRsosoefnftiheeldabaonvde 21 06 21 00 00 00 individuals in later years. The remaining 335 captures involved 227 individuals of unknown initial age (ASY 3 0 1 0 0 0 birds >2 yr ofage) and 108 recaptures ofthese individuals 4 1 0 1 0 0 (A3Y, A4Y, etc.) in subsequent years. 5 0 0 0 1 0 189 190 Short Communications VoL. 26, No. 3 Table 2. Iris color of “relative-age” Cooper’s Hawks breeding in Wisconsin. Relative-age birds are after second year birds (ASY), and recaptures of these individuals (A3Y, A4Y, etc.) in subsequent years. Eye Color Categories Mean 2 4 Eye 1 Light 3 Dark 5 Color Relative Age Yellow Orange Orange Orange Red Total Score Males {N — 155 captures) ASY 0 14 37 26 19 96 3.5 A3Y 0 1 13 18 4 36 3.7 A4Y 0 0 4 5 6 15 4.1 A5Y 0 0 0 1 4 5 4.8 A6Y 0 0 0 0 2 2 4.7 A7Y 0 0 0 1 0 1 Females {N = 180 captures) ASY 7 43 58 21 2 131 2.8 A3Y 0 7 12 4 1 24 3.0 A4Y 0 3 4 2 1 10 3.1 ASY 0 0 5 2 0 7 3.3 A6Y 0 0 5 1 0 6 3.3 A7Y 0 0 1 1 0 2 mgmean eye color scores for hawks in a given age category, likely to have light orange eyes. Conversely, it seemed that we assigned corresponding numerical scores of 1-5 for most males did not attain their red eyes until >5 yr of yellow through red eyes, respectively. Clutch size was age. determined in 1980-91 by climbing to nests in mid-late In both sexes, detectable changes in eye color appeared incubation (typically about mid-May in Wisconsin). to cease once the birds were about 5 yr old (Table 2). Asymptotes seem to occur at the same relative age for both Results and Discussion sexes, but females reached their asymptote at markedly In both sexes eye color showed a progressive change lower scores than did males (3.3 vs. 4.7; Table 2). Evi- from lighter shades of yellow in younger birds to darker dently, some females did not achieve red eyes, at least at shades of orange or red in older birds (Tables 1, 2). How- ages up to A7Y. ever, 1 yr old males had significantly darker eyes, usually Our data on male ages and eye colors were also useful light orange, than 1 yr old females did, which are usually in examining the possibility that preferential mating oc- yellow (x^ = 6.51, P < 0.05). Although our data on females curs on the basis of eye color in males, as suggested by did not allow further statistical comparisons of eye color Snyder and Snyder (1974). Our information from ASY between the sexes for known-age hawks, at relative ages captures suggests that about 20% of breeding male Coo- from ASY through A7Y males always showed a markedly per’s Hawks in Wisconsin had red eyes (Table 2). How- higher mean eye color score than females of the same ever, this sample of ASY birds may have been skewed relative age (Table 2). Thus the initial gender disparity toward males with lighter eyes because it included re- in eye color in yearling hawks not only persisted in older placement males at some nesting areas; such replacements birds, but also increased with relative age (Table 2). could predominantly have been younger males with lighter Males that were 2-5 yr old were very unlikely to have eyes. We thus compared eye color in individual males yellow eyes, while males that were 4-5 yr old were not initially captured on 82 nesting areas with eye color in Table 3. Iris color of initial and replacement male Cooper’s Hawks at 82 nesting areas in Wisconsin. Eye Color Categories Mean 2 4 Eye 1 Light 3 Dark 5 Color Yellow Orange Orange Orange Red Total Score Initial 1 18 24 21 18 82 3.5 Replacement 1 7 17 5 1 31 2.9 September 1992 Short Communications 191 Table 4. Clutch size by eye color category for breeding aves de mas edad. Los machos tuvieron ojos mas obscures male Cooper’s Hawks in Wisconsin. que las hembras de la misma edad. No hemos encontrado justificacion a la hipotesis que sostiene que el vigor de los N machos esta asociado con el color de los ojos. No hubo Color Category X Range diferencias en los tamanos de las nidadas que estuvieran Yellow 1 4.0 NA en relacion con categorias en el color de los ojos. [Traduccion de Eudoxio Paredes-Ruiz] Light Orange 12 4.25 3-6 Orange 45 4.40 2-5 Acknowledgments Dark Orange 28 4.28 2-6 It is with great pleasure that we acknowledge the guid- Red 19 4.68 4-5 ance and support of Frances and Frederick Hamerstrom, for it was primarily because of their strong urging that we began trapping breeding Cooper’s Hawks 12 yr ago. replacement males at 31 of these areas. Initial males did Wethus regard results herein as a tributetotheirforesight. show a greater proportion of red eyes than replacement Many organizations and individuals have helped in fi- males (22% vs. 3%) and initial males had higher mean eye nancial and other ways with our research. We especially color scores than their replacements (Table 3). Therefore, thank the Wisconsin Department of Natural Resources, we regard the 227o figure as the actual proportion of red- the University ofWisconsin at Stevens Point, the Madison, eyed males in the breeding population. Milwaukee and Lakeland Audubon Societies, and C. and Snyder and Snyder (1974) hypothesized that red eyes M. Nelson. R.K. Anderson and R. Jurewicz have provided “could be a good indicator” of the age and quality of a long-term support for our study. This manuscript was male and that “hunting skills may improve progressively improved by the comments of K. Bildstein and D. Evans, over the lifetime of a bird.” They postulated that birds and the statistical advice of E. Anderson. D. Snyder typed with the darkest red eyes might have the greatest success various drafts of this manuscript. The Personnel Devel- in obtaining mates. If older, darker-eyed males are indeed opment Committee atthe University ofWisconsin-Stevens higher quality mates and more proficient hunters than Point provided support for publication. younger, lighter-eyed males, there should be fitness dif- ferences related to eye color. Literature Cited We suggest that a revealing test of male quality should be clutch size. Males provide virtually all the food to their Grossman, M.L. and H. Hamlet. 1964. Birds of prey matesinthe pre-incubation period (Rosenfield et al. 1991), of the world. Bonanza Books, New York. a time in which both prey abundance and vulnerability Hamerstrom, F. 1963. The use of Great Horned Owls are presumably at their lowest levels during the breeding in catching marsh hawks. Proc. Internal. Ornithol. Congr cycle. This role as principal supplier of prey, as well as 13:866-869. the males’ relative hunting skills in the pre-incubation Meng, H.K. 1951. The Cooper’s Hawk Accipiter coo- period, should be manifest in the pair’s clutch size. Al- though males with red eyes had the largest mean clutch perii. Ph.D. thesis. Cornell University, Ithaca, NY. size (Table 4), there was no statistically significant dif- Palmer, R.S. [Ed.]. 1962. Handbook of North Amer- ference in clutch size among the eye color categories (me- ican birds. Vol. 1. Loons through Flamingos. Yale dian test = 5.02, df = 3, P > 0.05). Therefore we found University Press, New Haven, CT. no support for the premise that male fitness is associated Rosenfield, R.N., Bielefeldt and Cary. 1991. J. J. with male eye color. Finally, if the darkest red eyes were Copulatory and other pre-incubation behaviors of associated with male quality, we could expect selection for Cooper’s Hawks. Wilson Bull. 103:656-660. a proportion of red-eyed males much higher than the 22% AND Bielefeldt. 1992. Natal dispersal and we found in the Wisconsin breeding population. J. — inbreeding in the Cooper’s Hawk. Wilson Bull. 104: Resumen. Hemos examinado las relaciones entre edad, 182-184. sexo, vigor de los machos (i.e., numero de huevos en el Snyder, N.F.R. and H.A. Snyder. 1974. Function of nido), y el color de los ojos, en 253 gavilanes de la especie Accipitercooperii, durante el ciclo de reproduccion, en Wis- eye coloration in North American accipiters. Condor consin, desde 1980 a 1991. En ambos sexos el color de los 76:219-222. ojosmostroun cambio progresivo queva del amarillo claro, en gavilanes jovenes, al anaranjado obscuro 6 al rojo, en Received 23 March 1992; accepted 26 May 1992