Aspects of the ecology of the black-headed gull (Larus ridibundus) with comparative data on the common gull (L. canus)
Between 1982 and 1984, 893 Black-headed and 117 Common Gulls were trapped outside the breeding season in northeast England, and individually marked with wingtags or colour-rings. About 40% of marked adult Black-headed and Common Gulls returned to the study area in subsequent years, although adult Black-headed Gulls marked at the coast in the 1982-83 season returned in considerably higher proportions. Proportionately fewer second-year and first-year birds returned than adults. These percentages are considerably lower than the estimated annual survival rate for Black-headed Gulls, showing that some of the birds probably spent subsequent winters outside the study area. Some of the birds which did not return to the study area were recovered or seen elsewhere, mainly in eastern parts of Britain: few moved to the west coast. Foreign Black-headed Gulls which overwintered in the British Isles were most numerous compared to British birds in the south and east of the British Isles. A small proportion of Continental Black-headed Gulls remained in Britain during the breeding season: the consequences of these birds joining the British breeding population are discussed. Overwintering Black-headed Gulls in the study area were observed feeding inland on fields and refuse tips, and at the coast. First-years were uncommon compared to adults at the coast, less so on tips, and were relatively most common on fields. Females made up similar proportions of flocks at the coast and on inland fields, but were excluded to some extent from tips. Neither the survival rates of adult and ■ first-year British Black-headed Gulls, nor the weights of adult Black-headed Gulls caught in northeast England, were usually affected by the severity of weather in winter. The migrations and movements of Black-headed Gulls are discussed and compared to those of other species.