Effects of supplemental food on parental-care strategies and juvenile survival of Northern Goshawks

Using food supplementation, we tested whether food limits juvenile survival in a population of Northern Goshawks (Accipiter gentilis) in northeastern Utah. The influence of additional food on female nest attendance also tvas investigated because those strategics may influence predation mortality rat es of juveniles. We provided supplemental food near 13 nests from close to hatching until close to independence during the 1996 and 1997 breeding seas ons. Thirteen additional nests served as controls and received no supplemen tal food. We compared the following variables at treatment and control nest s: (1) adult female mass, (2) nestling mass and size, (3) female nest atten dance, and (4) juvenile survival. Following supplemental feeding, adult fem ales from treatment nests were heavier than their control counterparts, and remained closer to the nest during the latter part of the nestling period and throughout the postpledging period. Nestlings from supplemented nests w ere significantly heavier than those from unsupplemented nests, but results for size measurements were equivocal. Survival rates for treatment nestlin gs were significantly higher than controls in 1997, but not in 1996. Those results support the hypothesis that food does not limit avian reproductive success on an annual basis. Most deaths in 1997 were the result of starvati on or sibling competition. That observation, and the fact that fed nestling s were heavier, is consistent with the idea that treatment nestlings were i n improved nutritional condition. Overall patterns of mass and nest-attenda nce for adult female goshawks supports the hypothesis that female condition and behavior are adjusted in response to food supplies. However, it is les s clear what role the females' presence in the nest stand plays in mediatin g juvenile deaths, because we did not document predation as a primary morta lity factor during the two years of this study. The apparent flexibility in female nest attendance behavior suggests that such plasticity may be an ad aptation to lower the risk of predation.