Nocturnal Foraging by Chatham-Island Skuas

Study of the cooperative breeding of southern great skuas (Stercorarius skua lonnbergi) on the Chatham Islands has identified differences in food availability among territories. Skuas here feed substantially on breeding petrels. Transmitters were placed on 10 birds of pairs and cooperatively breeding groups to monitor their movements on dark nights. Birds on territories containing petrel burrows remained within the territory, and fed on petrels caught on the ground. The territory of one pair on a rocky point lacked breeding petrels. The male was tracked at night away from the territory to an inland site. No birds left the island at night or followed flying petrels. No differences were found in the nocturnal behaviour of the individual birds of cooperative groups. The evidence obtained that skuas may feed both within and outside territories has implications for theories of territoriality and cooperative breeding based on resource defence.