Ecology of the Northern Goshawk in the Sierra Nevada, California

This dissertation describes the migration status, annual home ranges, nest-site habitat characteristics, and the association of biotic and abiotic factors with annual variation in reproduction of northern goshawks (Accipiter gentilis) in the Lake Tahoe region of the Sierra Nevada, California, during 1991-1995. Northern goshawk reproduction was greatest in years with abundant late-winter/early-spring Douglas squirrel populations (Tamiasciurus douglasii), following high cone crop production the previous autumn, and mild late-winter/early-spring temperatures. These results are consistent with the prediction that carnivorous bird species exhibit an increased demand energetic strategy and require increased energy prior to breeding in order to successfully reproduce. Northern goshawks are year round residents in the study area. Breeding period home ranges (95% adaptive kernel estimates) based on radio-telemetry were 2016 ha (sd = 1690) for females and 2698 ha (sd = 1043) for males. Nonbreeding period home ranges were 5555 ha (sd = 3289) for females and 9379 ha (sd = 6821) for males. Home ranges did not differ between sex during either the breeding or nonbreeding periods. Nonbreeding period home ranges were significantly larger versus the breeding period for both males and females. Northern goshawk nest-sites had significantly greater canopy cover (>5m height), live trees in the>60-100 cm and>100 cm diameter-at-breast-height (dbh) classes, and dead trees in the>60-100 cm dbh class. Correspondingly, random sample points had significantly greater herbaceous, rock, and shrub/sapling cover, and live trees in the 5-30 cm dbh class compared to nest sites. Logistic regression analysis indicated that canopy cover, live trees>100 cm dbh, and shrub/sapling cover were the best predictors of northern goshawk nest-site habitat. -- Adapted from abstract.