Effects of prey delivery on the fledging success of the northern goshawk

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 contributed at least 2 % of the total energy) similarly (p = 0.99) between nests. The Douglas squirrel (Tamiasciurus douglasi) and golden mantled ground squirrel (Spermophilus lateralis) accounted for 57 percent of total energy provided to nests. These two species of prey may be important for successful nesting in these northeastern California habitats. Goshawks used 3-6 species of prey (that contributed at least 2 % of the total energy) at each nest. Eleven nests used only species of prey that were available within a 0.8 km circle around the nest. However, four nests used species of prey that were found 2.4 km from the nest. Foraging costs at distances up to 2.4 km from the nest were balanced by a significant increase (p = 0.04) in edible biomass (67 %) delivered to nestlings. The use of prey near the nest and selection of larger prey as foraging distance increased allowed goshawks to maximize the rate at which energy was delivered. Large foraging areas around nests were required by nesting goshawks. The ability of goshawks to successfully fledge nestlings ultimately depended on their ability to supply food to the nestlings. Management decisions should incorporate the habitats used by goshawks to forage that were beyond the area immediately surrounding the nest. Providing habitat with adequate prey or foraging, particularly in the 0.8 km surrounding the nest, is necessary for adult goshawks to maximize the rate of energy delivery to nestlings.