Color and body size vary substantially across North American range (Appendix 2; 47, 42, 2). Using museum specimens often of unknown breeding status and geographic origin, Whaley and White (47) reported that the largest birds occur in the southwest, the smallest in northwest, and eastern birds were intermediate in size. They also indicated that eastern birds had longer toes perhaps due to a greater proportion of avian prey in the diet. Based on large samples of known breeding birds from northern populations in British Columbia, western and eastern North Dakota, and southern Wisconsin, Rosenfield et al. (2) found that British Columbia and western North Dakota birds were similar in size (i.e., body mass), eastern North Dakota birds were larger than birds in these two populations and the largest hawks of these four populations occurred in Wisconsin (Wisconsin birds were similar in size to the 'large' southwestern birds reported by Whaley and White ). Indices of size dimorphism between sexes did not vary among these four northern populations (2). Although genetic differences were found between birds in British Columbia versus the three other northern populations (16), body size differences among these four populations were attributed to the principle that smaller size is favored for hunting smaller, agile prey (predominately birds) that comprised the diet of British Columbia and western North Dakota birds versus the larger hawks in eastern North Dakota and Wisconsin that hunted larger prey (birds and mammals such as chipmunks; 2, 16). In contrast to the study by Whaley and White (47), which lacked comprehensive diet data for sampling sites, Rosenfield (48) found that toe length (as indexed by middle-toe length) was longer in proportion to body size in British Columbia versus Wisconsin, in accord with enhancing the reach of British Columbia hawks hunting mostly agile, small avian prey versus likely less agile larger (often mammalian) prey of Wisconsin hawks (49, 50).
Rosenfield et al. (2) also found that wing chords were longer in more migratory northern populations than in nonmigratory adults in British Columbia (longer wings provide more efficient flight at lower energetic costs).
Cooper's Hawks in British Columbia and North Dakota acquire darker orange or red irides more frequently and more quickly than their counterparts at known and relative ages in Wisconsin birds (42). Females in all three study sites were slower and less likely than males to acquire darkest eye colors (dark orange and red; 42).
The underparts of adults in northwestern populations average darker red than that of their eastern counterparts.