Aspects of the Breeding Biology of Rough-Legged Hawks Along the Colville River, Alaska

I studied the breeding performance of rough-legged hawks (Buteo lagopus) nesting on cliffs along 350 km of the Colville River in northern Alaska in 1985 and 1987- 1995. Territories were visited by boat or foot twice each year during the late incubation/early brood-rearing and late brood-rearing/fledging periods. The number of pairs occupying breeding territories varied from 40-103 (x̄= 81.8, CV = 26.1%) among years. In most years, pairs were spaced an average of about 1 km apart where nest sites were not limited, but a lack of suitable nesting cliffs in much of the study area limited the distribution and spacing of rough-legs. Among years, mean brood size varied from 1.7- 3.5 young per nest, and the proportion of pairs that successfully raised young to the late brood-rearing stage varied from 34-84%. The cause of most nest failures was unknown, but 25% of failures were caused by nests or cliffs disintegrating and falling. Additionally, nests subjectively considered to be accessible to mammalian predators were more likely to fail (27.0% failed, n = 237) than those considered inaccessible (19.2% failed, n = 317, X2 = 4.68, d.f. = 1, P = 0.03). Median egg-laying dates varied from 17-27 May among years, and median laying dates were negatively correlated with average daily minimum temperatures in the 20 d period prior to egg-laying (rs = -0.85, P = 0.002). When undisturbed pairs were compared to those that we disturbed by entering their nests or interrupting their incubation/brooding for up to 90 minutes, there were no significant differences in nest failure rates (Fisher\u27s exact test, one-tailed, P = 0.69), brood sizes (X2 = 1.67, d.f. = 4, P = 0.80), or reoccupancy of territories in the subsequent year (Fisher\u27s exact test, one-tailed, P = 0.18). Prey remains in nests included 1067 individuals of 32 species; by number, 62.3% were microtine rodents, 30.0% were birds, and the remainder (7.7%) were arctic ground squirrels (Spermophilus parryii) and snowshoe hares (Lepus americanus)