Physical characteristics of nest sites are thought to influence their use by birds and the outcome of breeding attempts. The presence of preexisting nesting structures can also be an important factor influencing reuse patterns and reproductive success. We examined the relationships between nest-site physical characteristics, patterns of nest reuse, and reproductive output in Rough-legged Hawks (Buteo lagopus) breeding in the Canadian High Arctic. Because this species is a rodent specialist, we also examined the effect of variation in lemming density, their primary prey, on reproductive output. We monitored 109 nesting attempts over a 9 yr period and sampled physical characteristics of 87 known nesting sites in 53 territories on Bylot Island, Nunavut, Canada. The probability that a nest was used by a breeding pair increased with the distance to the nearest breeding conspecific, for nests sheltered by an overhang, and with lemming density. Hawk nests inaccessible to foxes and facing away from the north were associated with high reproductive success. Reproductive success was also positively related to summer lemming density and negatively related to rainfall. Our results suggest that nest-site physical characteristics that offer a favorable microclimate and protection from weather and predators provide the highest-quality sites. We observed a high level of reuse of preexisting nests and previously occupied territories, which is indicative of site fidelity by individuals. Hawks that bred in a newly built nest had similar clutch size and hatching date compared to those that used preexisting nests, which suggests that building a new nest entailed no short-term reproductive costs. Reproductive success was nonetheless reduced for birds breeding in new nests located in newly occupied territories compared to those nesting in previously occupied ones, which suggests that a high proportion of pairs in new territories were young birds or that some of these sites were of low quality.
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Vol. 133 • No. 4