Ecology and behaviour of great skuas breeding on Foula (Shetland)

Aspects of the ecology and behaviour of Great Skuas breeding on Foula were investigated by studying individually marked birds. Pre-incubation behaviour of Great Skuas was described and predictions that arise from theories of territorial intrasions and copulation behaviour in birds were tested. Partnership composition and behaviour associated with mate change were studied. In particular, an attempt was made to test which of two alternative theories provides a better explanation for the occurrence of divorces in species where mate change is associated with a (short-term) measurable cost. Investigations were made to evaluate the effect of hatching date on different life- history stages in Great Skuas, including chick growth, survival to fledging, post-fledging survival and reproductive performance. The implications for the optimal timing of breeding in adults were analysed. Clutch characteristics, including total volume and within clutch asymmetry, were analysed in relation to parental age and breeding performance. An experiment was designed to infer foraging ability from territorial attendance of adult skuas, and detect small individual differences in those patterns. Results were analysed in relation to adult age and body size. Mathematical models were built to allow simulations of the effects of age on the study of individual repeatability in a breeding parameter: laying date. The consistency of individual laying dates over long time intervals was assessed. Results were discussed in relation to the concept of a permanent individual quality. Studies of the behaviour and breeding performance of individual skuas were analysed in relation to sex and body size. Results were discussed in relation to specific predictions made by several theories that explain the evolution of reversed sexual size dimorphism in the Stercorariidae and other birds with a raptorial lifestyle