Using time-lapse video monitoring to study prey selection by breeding Goshawks Accipiter gentilis in Central Norway

Abstract

Two nests of breeding goshawks were monitored by means of time-lapse video monitoring in boreal coniferous forests in Central Norway in 1996. One was located in a large-scale forestry area, the other in a mixed agricultural/forestry area. Altogether 311 prey items were registered during 915 hours of recordings and 75 remain items were collected at or near the nests. About 70% of the prey were identified to family level or higher from video, but species identification was more difficult. In contrast, ca. 85% of remain items were identified to species level. The video method proved superior in detecting unfeathered young and small prey, and it was fairly reliable and involved little disturbance. Thrush-sized birds (fledglings/juveniles) were the dominating prey group. Line transects showed a high abundance of thrush-sized passerines, but low densities of larger prey. At the first site the proportions between prey groups found by video corresponded to those from line transects. At the second site there was a significant preference for thrush-sized birds, and the overall prey density there was twice that of the first area. Significantly larger prey were brought to the nests in the last part of the breeding season when both parents hunted, compared to the early part when the male was hunting alone. Video monitoring is a valuable tool for studying the food brought to the nest by raptors, as it is able to provide better quantitative information on prey selection and the differences between the sexes regarding hunting effort and prey choice.