| Summary: | The effect of the availability of prey on survival and behavior of nestling goshawks (Accipter gentilis) was investigated by directly observing prey delivered to 16 goshawk nests during 1995 and 1996 in northern California. I also investigated the influence of precipitation on prey delivery, identified prey used, and tested the hypothesis that goshawks used prey that maximized the rate at which energy was provided to nestlings. Twenty-eight of 37 nestlings survived to fledge (1.9 ± 0.2 per nest; mean per nest ± 1 SE). I did not detect a relationship (p = 0.85) between nestling survival and the rate energy was delivered at nest sites. However, I detected a relationship (p = 0.01) between the number of nestlings hatched at a nest and the rate at which energy was delivered. Nest sites with a greater number of nestlings required adults to increase the rate of energy acquisition. The demand on adult goshawks to deliver more energy to nestlings may have affected reproductive success at nests with more than two nestlings. Nestling survival was significantly lower (p = 0.03) at nests with more than two nestlings than at nests with two nestlings. Availability of prey may have indirectly affected nestling survival by requiring both adults to hunt to meet the energetic demands of nestlings, which lowered parental care at nests and made nestlings more susceptible to predation or accidental falls. Sibling conflict over prey was quantified at each nest. Aggressive encounters occurred in 6 to 67 % of prey deliveries, depending on the nest. The rate of energy delivery was not related significantly (p = 0.3) to nestling aggression. However, rates of aggression decreased (p 0.01) at nests with greater female attendance. This suggested that the greater availability of prey increased parental care by female goshawks and nest attendance reduced fledgling aggression. Twenty species of prey were identified to at least genus: 11 of the 20 species provided at least 2 % each of the total energy. Goshawks used species of prey (that ...
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