Foraging behaviour of non-breeding Pomarine Skuas Stercorarius pomarinus in the North Sea in summer

From late May to mid September 2000, the unusual event of a summer assemblage of up to 16 Pomarine Skuas occurred on the island of Helgoland (German Bight, North Sea). Most of the birds were immatures in second and third calendar-year, which moulted primaries, tail feathers and wing coverts. The Pomarine Skuas usually foraged by kleptoparasitism of Black-legged Kittiwakes Rissa tridactyla carrying food to the breeding colony. Success of attacks was higher with Black-legged Kittiwakes compared to other victims (e.g. Sandwich Tern Sterna sandvicensis and Arctic/Common Tern S. paradisaea/hirundo), placing Pomarine Skuas between Arctic Skuas Stercorarius parasiticus (preferably hunting terns) and Great Skuas Catharacta skua (unable to kleptoparasitise terns and attacking Blacklegged Kittiwakes less often than Arctic and Pomarine Skuas do). When chasing Blacklegged Kittiwakes, age of Pomarine Skuas and size of the hunting group (sometimes including Herring Gulls Larus argentatus) did not affect the success rate, but with larger group size the success per group member decreased. The unusual occurrence of a group of moulting Pomarine Skuas suggests that the sea around Helgoland holding many seabirds during the breeding season is usually under-exploited by skuas.