Interactions between parents and non-residential intruders at a breeding colony of herring gulls larus argentatus

We observed 10 breeding pairs within a subcolony of Herring Gulls to ascertain the frequency of intrusion by non-resident gulls, the reasonsfor these intrusions, and the nature of the responses to them by the territory holders. Most observa-tions were made during the post-hatch phase in order to determine how intrusion might affect chick rearing strategies. Intrusion was frequent and increased over the season. However, it was independent of the number and sex of the parents on the territory and of the precise age of the chicks. Food theft, chick theft and testing for territory occupation were the most probable reasonsfor intrusion. Success in any of these was infrequent. Aside from a slight tendency for males to respond more frequently there were no sex differences in either the rapidity or intensity of response by parents when alone on the territory. This may be explained by the fact that intruders did not contest any attempts by residents to expel them and leads to the conclusion that, under the conditions present in this study, intrusion ’was not sufficiently serious a problem to require a compromise between chick defence and an overall parental strategy focussed on optimal provisioning of food.