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Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii) are important predators of birds in North America, but little is known about their natural history during the winter. Even basic survival information is not well documented in these species and is generally unknown during the winter. Therefore, we examined survivorship and causes of mortality among wintering Cooper's and Sharp-shinned hawks. We radio-tracked 27 Cooper's and 40 Sharp-shinned hawks during 5 winters from 1999 to 2004. Neither species nor sex was a significant covariate of survivorship, but the probability of adult survival (75.4%) over 110 days was significantly higher than that of juveniles (9.4%). Our estimate of adult survivorship is comparable with those published for other accipiters, but our estimate for juveniles is lower. Age differences in survivorship may be attributed to risk taking or inexperience in juveniles and/or difficulties in dealing with transmitter attachments. Two types of mortality (predation and collisions) were observed in the study. Whereas predation by owls was a major source of mortality in rural habitat, no predation was observed in the urban habitat. Our results suggest that predation by owls may have important implications for the behavioral interactions between accipiters and their prey.

SURVIVAL AND CAUSES OF MORTALITY IN WINTERING SHARP-SHINNED HAWKS AND COOPER'S HAWKS