The influence of territory density on the vulnerability of Arctic skuas Stercorarius parasiticus to predation Original Research Article

Foula (Shetland) holds one of the largest and densest Arctic skua Stercorarius parasiticus colonies in the UK and the largest colony of great skuas Catharacta skua in the world. The distribution of Arctic skua territories on Foula changed markedly between 1976 and 1994, with a disproportionately large decline occurring in most of the northern half of the island. Examination of breeding data collected in recent years (1992–1994) found no effect of Arctic skua breeding density on laying date, breeding synchrony, clutch volume or fledging success. However, hatching success and short-term post-fledging survival were considerably better in high-density areas where Arctic skuas seem able to mount a stronger defence against great skua predation. This was not without some associated costs, since a negative correlation existed between chick growth and the number of near neighbours in 2 out of 3 years. A simple model of local fluctuations in numbers during the early mid-1980s suggested that spatial variation in the mortality or emigration rate of established adults may have been responsible for the changes in distribution, probably as a result of pressure from great skuas. In addition, low density areas with poorer overall productivity seem to have attracted few new Arctic skua recruits. Similar density-dependent processes may have been responsible for long-term declines and changes in distribution of Arctic skuas at other colonies which have been attributed circumstantially to predation by great skuas. This study, therefore, provides key data on the influence of breeding density on vulnerability of Arctic skuas to predation. This is particularly relevant in the light of recent concern that the impact of great skua predation on other seabirds, including Arctic skuas, is unacceptably high in Shetland, and that some degree of population management should be considered.