Population increase of Yellow-legged Gulls Larus cachinnans breeding on Berlenga Island (Portugal), 1974-1994

Increases of Herring Larus argentatus and Lesser Black-backed L. fuscus Gull populations have been reported since the beginning of this century (Chabrzyk & Coulson 1976, Cramp & Simmons 1983, Spaans et al. 1987, Migot 1992). The Iberian Yellow-legged Gull Larus cachinnans populations have shown similar increases in recent decades (Bärcena et al. 1984, Barcena et al. 1987, Vicente 1987, Pérez et al. 1994, Mourifio & Abrain 1995, Sol et al. 1995). Within the Nature Reserve Berlenga Island off the Portuguese west coast, the largest breeding colony of Yellow-legged Gulls in Portugal is found. The first known estimate of the size of the colony on Berlenga, c. 1000 breeding pairs in June 1939, was reported by Lockley (1952). According to Vicente (1987) the breeding population remained stable until 1974 ate. 1300 pairs. In 1981 c. 3000 pairs bred on the island and in 1983 this figure had increased to c. 4800 pairs (Teixeira 1983). In this paper the last 20 years of census results, from 1974 to 1994 are evaluated. Based on the raw census data and the available information on breeding success on Berlenga, the proportion of immigrants in the population is estimated. It is shown that immigration must have occurred to explain the population growth and it is suggested that the population of Berlenga maybe part of a metapopulation system that probably includes all gull colonies in Galicia.