Patterns of temporal variation in goshawk reproduction and prey resources

To investigate whether Northern Goshawk (Accipiter gentilis) reproduction is food-limited, we evaluated the reproductive output from 401 goshawk breeding opportunities on the Kaibab Plateau, Arizona during 1999¨C2002. Concurrently, we estimated densities of 10 goshawk prey species (seven birds, three mammals) using distance sampling. We then assessed the relationship between goshawk productivity (number of fledglings produced) and prey density within and among years by relating the contribution of individual prey species and total prey density to goshawk productivity. We also estimated the proportion of total diet and biomass for each species that contributed iÝ3% of all prey items. Total prey density was highly correlated with variation in goshawk productivity (r2 = 0.98, P = 0.012). Red squirrel (Tamiasciurus hudsonicus) density explained more variation in goshawk productivity than any other species (r2 = 0.94, P = 0.031), but density could not be estimated for every predominant prey species in goshawk diets. However, only red squirrels had a positive and significant relationship to goshawk productivity in terms of frequency (r2 = 0.97, P = 0.014) and biomass (r2 = 0.95, P = 0.033). Northern Flickers (Colaptes auratus) and cottontail rabbits (Sylvilagus spp.), which contributed the greatest frequency and biomass, respectively, to goshawk diets, showed no relationship with goshawk productivity. Even though goshawks on the Kaibab Plateau have a diverse diet and will readily switch to alternate prey species, goshawk productivity showed significant interannual variation. Our results suggest that the magnitude of goshawk productivity was determined by total prey density and annual variation was driven by differences in the densities of critical prey species.

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