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Although adult plumage in Cooper's Hawks (Accipiter cooperii) exhibits little of no sexual dichromatism, iris color reportedly changes from yellow or light orange in younger birds to shades of orange of 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 a signal of age and hence reproductive fitness and thus offer a basis for nonrandom mating in Cooper's Hawks. In this study we examine the relationships between eye color and age, sex, and reproductive output for Cooper's Hawks in two breeding populations in British Columbia and North Dakota, 1999-2002, and compare these results to those previously published for a breeding population 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 a given eye color displayed variation in known and relative ages in British Columbia and Wisconsin. There was no significant relationship between the eye color of males and their brood sizes in any of these three populations, and therefore no discernable support for the premise that male eye color per se signals male fitness, or functions as a sexual 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 relationship, especially in [...]

From an analysis of nest site habitat data at 24 Sharp-shinned Hawk (Accipiter striatus) and 52 Cooper's Hawk (A. cooperii) nests in Wisconsin, we conclude that Cooper's Hawks tend to nest in stands with lower densities of taller and larger trees than do Sharp-shinned Hawks, and that Cooper's Hawks also tend to nest in sites with a greater proportion of hardwood cover than Sharp-shinned Hawks. Significant interspecific differences were found in combined habitat types (hardwoods, mixed conifer-hardwoods, and conifer plantations) for nest tree height and nest tree DBH (diameter at breast height); nest height; nest height relative to tree height: canopy height: canopy cover; tall shrub density: tree density; and mean DBH. Nest sites of the two species were similar in terms of understory canopy cover, ground cover, low shrub index, understory tree density, basal area, distance to nearest forest opening, and distance to water. We detected few significant intraspecific differences in nest site habitat, and these only in the Cooper's Hawk., Although the Sharp-shinned Hawk (Accipiter striatus) and the Cooper's Hawk (A. cooperii) breed sympatrically in many parts of the United States and southern Canada, their nest site habitats have been [...]

Trichomoniasis, a digestive tract disease of raptors caused by ingesting the protozoan Trichomonas gallinae of infected columbid prey, recently was reported to be common in and an important agent of mortality among urban nestling and fledgling Cooper's Hawks (Accipiter cooperii) in Arizona. However, the prevalence of T. gallinae in wild raptor populations is poorly documented. During 2000 we detected T. gallinae in only three (2.7%) among 110 nestling Cooper's Hawks at 48 nests in both urban and rural habitats in Wisconsin, North Dakota, and British Columbia. Mortality attributable to trichomoniasis among 5- to 22-month-old Cooper's Hawks was zero in the three combined study areas. Prevalence of infection by T. gallinae seems to vary greatly among nesting Cooper's Hawks across this raptor's continental breeding range., Boal et al. (1998) reported that trichomoniasis, as contracted from consuming columbid prey infected with Trichomonas gallinae (a flagellated protozoan), was an important source of mortality among nestling and fledgling [...]

Robert N. Rosenfield [1,4] John Bielefeldt [2] Sarah A. Sonsthagen [1] Travis L. Booms [3] ABSTRACT.--Some birds that attempt to nest in habitats such as conifer plantations may experience lower [...]