Large gulls as predators of passerine landbirds migrating across the southeastern North Sea

Abstract

The predation by large gulls on passerines migrating across the southeastern North Sea was studied on the offshore island Helgoland throughout 1999; 15,307 pellets of Herring Gull Larus argentatus and Great Black-backed Gull L. marinus were examined. The main prey was fish and crustaceans, but during migratory seasons of passerines (March to May, August to November) up to 7% of the pellets contained remnants of passerines (mainly thrushes Turdus spp. and Starlings Sturnus vulgaris, but few species of <50 g body mass). For half-month periods, the number of pellets containing passerines correlated with the number of passerines stopping over on Helgoland for both size categories of <50 g and >50 g. Pellets containing passerines were mainly found during weather conditions favourable for migration, when many birds were aloft, rather than during adverse conditions. Based on the number of large gulls present, an estimated 5,200 passerines might have fallen victim to gulls during the year. Although some passerines may be scavenged rather than captured, gulls must be important potential predators for passerines migrating across the sea.