Prey and weather factors associated with temporal variation in Northern Goshawk reproduction in the Sierra Nevada, California
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We studied the association between Northern Goshawk (Accipiter gentilis) reproduction and annual variation in prey and weather factors in the Lake Tahoe region of the Sierra Nevada, California, during 1992-1995. The proportion of Northern Goshawk breeding territories occupied varied between years although differences were not statistically significant. However, annual variation was observed in the proportion of Northern Goshawk territories with active nests, successful nests, and in the number of young produced per successful nest. Annual variation in reproduction was associated with variation in late-winter and early-spring temperatures and Douglas squirrel (Tamiasciurus douglasii) abundance (February-April). Douglas squirrel abundance, and their frequency and biomass in diets of Northern Goshawks during the breeding period, varied annually in concordance with cone crop production. Northern Goshawk reproduction was greatest in 1992 following both abundant late-winter and early-spring Douglas squirrel populations, which resulted from high cone crop production the previous autumn, and mild late-winter and early-spring temperatures. These results are consistent with the prediction that carnivorous birds require increased energy before breeding in order to reproduce successfully. In the high elevations of the Sierra Nevada, prey availability is reduced during the late winter and early spring because of the migration and hibernation patterns of important prey species and temperatures are near or below the lower critical temperature for Northern Goshawks during this period. In contrast to other prey species, Douglas squirrels are active throughout the year and are available during this period. Thus, our results suggest that forest management and restoration strategies adopted to enhance Northern Goshawk foraging areas should consider management of conifer tree size distributions and species compositions to enhance seed production in terms of frequency over time, number of seeds per crop, and energetic value of seeds by tree species, as these are important habitat elements and ecological processes influencing Douglas squirrel populations. Autecological studies of focal species of concern such as the Northern Goshawk are necessary to provide the basic ecological knowledge required to integrate species level concerns with landscape and ecosystem management perspectives to advance conservation science and improve land management.
