Abstract
In order to observe the process of acquiring new breeding territories by Northern Goshawks Accipiter gentilis, we followed individual movements by using radio tracking. One adult and three juvenile goshawks were captured and fitted with transmitters in northwestern Saitama Prefecture (Honjo hillside), Honshu. Changes in home range size, and competition among the four males were observed through to first breeding. Two of the juveniles established breeding territories when one year old, the third established a territory as a two-year-old. All three established breeding territories in the area they had inhabited prior to breeding. The home ranges of the juveniles were 2-12 times larger than that of the adult, and overlapped with each other before they first bred. At the begining of their first breeding season, the sizes of their territories, and the extent to which they overlapped, decreased. Furthermore, their behavior towards the adult male also changed. Before the young birds began breeding they avoided the area around the adult's nest, and the adult male had the advantage in inter-male competition. The adult made territorial display flights signaling the start of his breeding season, whereas the juveniles did not display while they were single. After maturing, each of the three young birds also displayed over their territories. Our results show that young male goshawks colonize new areas during their first winter and establish breeding territories within their home range until they mature. We believe this to be the first report to reveal the process of breeding territory formation in the Northern Goshawk by tracking individual movements and observing male behavior.
