Breeding performance and diet of Great SkuasStercorarius skuaand Parasitic Jaegers (Arctic Skuas)S. parasiticuson the west coast of Scotland

Capsule The Arctic Skua colony on Handa Island is in decline, probably due to pressure from Great Skuas. Aims To assess the factors affecting Great Skua productivity, and the effects of Great Skuas on breeding Arctic Skuas. Methods From 2003 to 2006 population and breeding performance of Great Skuas and Arctic Skuas on Handa Island were intensively monitored, using all-island surveys and regular visits to focal territories from laying through to the post-fledging period. All study chicks were ringed for long-term study. Interspecific interactions were observed and 5116 regurgitated pellets of breeding and non-breeding Great Skuas were collected systematically for dietary analysis. Results Over the study period, numbers of breeding Great Skuas remained steady but there was an overall decline in productivity, with chick loss the most significant proximate factor. However the four-year average of 0.49 chicks fledged per pair was relatively high, with Handa being the most productive monitored colony in Britain in 2004. Great Skuas on Handa have a diverse diet of at least 30 species, including discarded fish and other seabirds. Among the Arctic Skuas, mean productivity was high but so was predation pressure on fledglings from Great Skuas, and population size has halved in four years. Conclusions Handa is a productive Great Skua colony in the context of recent breeding failures in the Northern Isles. A substantial part of their diet is other seabirds and their impact on globally important populations requires further monitoring and analysis.