Inter-colony variation in diet and reproductive performance of great skuas Catharacta skua

Sustained population growth of great skuas Catharacta skua during the current century has been attributed largely to a high abundance of sandeels, principally Ammodytes marinus, and a plentiful supply of discards from whitefish trawlers in the vicinity of breeding colonies. A new colony of great skuas was established at St Kilda, Outer Hebrides, in 1963 and has since grown rapidly despite an apparent lack of sandeels in the waters surrounding the archipelago and the presence of only a small whitefish fishery. This paper presents an analysis of diet and breeding ecology of great skuas at Hirta, St Kilda, over 3 seasons, 1994 to 1996, with comparative data from Foula, Shetland, the largest and one of the oldest of the British colonies. At St Kilda, breeding adults, nonbreeders and chicks all fed extensively upon other seabirds (44 to 65% of pellets over the 3 yr), and to a lesser extent on goose barnacles Lepas sp. (18 to 30% of pellets) and fish (16 to 30% of pellets), with sandeel virtually absent from the diet (0.2 to 0.3% of pellets). In contrast, at Foula only 4 to 12% of pellets were of other seabirds, with sandeels and discarded fish together constituting the vast majority (4 to 36% and 55 to 88% of pellets, respectively). Despite the low availability of sandeels and discards at St Kilda, egg volumes, chick body condition and annual productivity were at least as high as al Foula, and territorial attendance of breeding adults was much higher, indicating considerably lower foraging effort during chick-rearing. Great skuas at St Kilda are therefore able to make effective use of other seabirds as a food resource during the breeding season, in sharp contrast to Foula where adult mortality increased and chick growth rates and breeding success plummeted during the marked decline in sandeel abundance in the late 1980s.