The influence of food availability on the breeding effort and reproductive success of Arctic Skuas Stercorarius parasiticus

Territorial attendance, chick growth rate and breeding success of Arctic Skuas Stercorarius parasiticus in Shetland were lowest in the late 1980s when recruitment of Sandeels Am-modytes marinus in the surrounding waters was poor. The relationships between both fisheries-based and avian indices of food availability and annual variation in Arctic Skua chick growth and breeding success between 1976 and 1994 were better described by a threshold effect rather than linear functions. Arctic Skuas conform to the model proposed by Cairns, which predicts the responses of seabirds to changes in prey availability. Skua clutch size, egg volume, hatching success and hatching date were not reliable indices of Sandeel availability. However, annual fluctuations in Arctic Skua breeding numbers may be a useful indicator of changes in prey abundance.