Nesting failure in Finnish Northern Goshawks Accipiter gentilis: incidence and cause

Habitat composition is an important factor influencing nesting failure probability in birds. However, although various habitat effects such as fragmentation and edge density are known to have clear negative effects on the breeding success of passerines, the role that habitat composition plays in shaping nesting failure patterns among other avian groups is less well known. We studied nesting failure in a large forest raptor, the Northern Goshawk Accipiter gentilis, during the period 1999-2003 in Finland. Illegal human persecution was found to constitute a major cause of failure among nests where cause could be determined accurately (comprising c. 60% of all failed nesting attempts). Egg predation by corvids and nestling predation by Eagle Owls Bubo bubo were also common. However, the exact cause of nesting failure could not be inferred in every case. Overall, the degree of initial parental investment (clutch size and egg volume) was significantly smaller in nests that failed than in nesting attempts that were successful. This did not apply to nests that were destroyed by humans, in which investment was at a level equal to nests that were successful. Although the probability that a nesting attempt would fail was also related to small-scale nest concealment, nesting failure probability was not associated with main prey density or several measures describing territorial habitat composition at larger scales. Small initial parental investment, not habitat composition, is thus the major correlate of nesting failure in Goshawks.