Developing comprehensive conservation strategies requires knowledge of factors influencing population growth and persistence. Although variable prey resources are often associated with fluctuations in raptor demographic parameters, the mechanisms of food limitation are poorly understood, especially for a generalist predator like the northern goshawk (Accipiter gentilis). To determine the reproductive responses of goshawks to variable prey populations, we evaluated 823 goshawk breeding opportunities on the Kaibab Plateau, Arizona, USA, during 1994–2002. Concurrently, density was estimated for 4 prey species (2 avian, 2 mammalian). We explored the relationship between goshawk reproduction and prey density at one temporal scale (year) and 2 spatial scales (study area, forest type). Prey density for all 4 species combined accounted for 89% of the variation in goshawk reproduction within the entire study area (P < 0.001), 74% in mixed conifer forest (P = 0.003) and 85% in ponderosa pine (Pinus ponderosa) forest (P < 0.001). We found that an incremental increase in prey density resulted in a greater increase in goshawk reproduction in ponderosa pine forest than in mixed conifer forest, suggesting that the denser structural conditions of mixed conifer forest may have reduced prey availability. Red squirrel (Tamiasciurus hudsonicus) density explained more annual variation in goshawk reproduction within the study area (r2 = 0.87, P < 0.001), mixed conifer forest (r2 = 0.80, P = 0.001), and ponderosa pine forest (r2 = 0.85, P < 0.001) than did any other individual species. Although certain prey species were more strongly correlated with fluctuations in goshawk reproduction than were others, the high model selection uncertainty and the strong relationship between total prey density and number of goshawk fledglings produced indicated that alternate prey species were readily substituted for one another. Therefore, conservation strategies concerned with the status of goshawk populations should incorporate forest management practices that increase the abundance, diversity, and availability of prey resources.
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Vol. 71 • No. 7