Territorial intrusions and copulation behaviour in the great skua, Catharacta skua

Birds of many species intrude on to territories in order to obtain extra-pair copulations, and frequent within-pair copulations are thought to be a response to cuckoldry where mate guarding is not possible. Great skuas are colonial birds in which females are left alone in the breeding territory while males forage for the pair. Opportunities for cuckoldry are therefore numerous, and it could be predicted that sperm competition should be intense in this species. We tested several hypotheses that attempt to explain territorial intrusions by female great skuas. Few intrusions coincided with the main fertile period and extra-pair copulations were almost never solicited, and therefore the sperm competition hypothesis was rejected. Only 0.9% of the copulations observed (N=339) were extra-pair. Thus opportunity for cuckoldry is a very poor predictor of the intensity of sperm competition, in spite of the relevance given to this factor in the literature. Of three extra-pair copulations observed, two involved unpaired territorial males. This suggests that genetic benefits were not the aim of unfaithful females. All three were preceded by courtship feeding, while only 26% of within-pair copulations followed successful food begging. This statistically significant difference constitutes evidence for a trade of copulations for food in a monogamous bird. Evidence is presented supporting hypotheses that females intrude on to territories in order to induce males to give away some food, and to become familiarized with potential partners for future breeding seasons. (C) 1997 The Association for the Study of Animal Behaviour.