THE INFLUENCE OF TERRITORY DENSITY ON THE VULNERABILITY OF ARCTIC SKUAS STERCORARIUS-PARASITICUS TO PREDATION

Foula (Shetland) holds one of the largest and densest Arctic skua Ster corarius parasiticus colonies in the UK and the largest colony of grea t skuas Catharacta skua in the world. The distribution of Arctic skua territories on Foula changed markedly between 1976 and 1994, with a di sproportionately 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 hedging success. However, h atching success and short-term post-fledging survival were considerabl y better in high-density areas where Arctic skuas seem able to mount a stronger defence against great skua predation. This was not without s ome associated costs, since a negative correlation existed between chi ck growth and the number of near neighbours in 2 out of 3 years. A sim ple model of local fluctuations in numbers during the early mid-1980s suggested that spatial variation in the mortality or emigration rate o f established adults may have been responsible for the changes in dist ribution, probably as a result of pressure from great skuas. In additi on, low density areas with poorer overall productivity seem to have at tracted few new Arctic skua recruits, similar density-dependent proces ses may have been responsible for long-term declines and changes in di stribution of Arctic skuas at other colonies which have been attribute d circumstantially to predation by great skuas. This study, therefore, provides key data on the influence of breeding density on vulnerabili ty of Arctic skuas to predation. This is particularly relevant in the light of recent concern that the impact of great skua predation on oth er seabirds, including Arctic skuas, is unacceptably high in Shetland, and that some degree of population management should be considered. ( C) 1998 Elsevier Science Ltd. All rights reserved.