The temporal and geographical distribution of Continental Black-headed GullsLarus ridibundusin the British Isles

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Abstract
The number of Black-headed Gulls in the British Isles increases considerably after the breeding season as Black-headed Gulls from Continental Europe arrive to overwinter. Appreciable numbers arrive in July and the build-up continues until November for adults and second-years and until December for first-years. First-years and second-years from nearby countries tend to arrive in the British Isles earlier than those from distant countries, but there is no relationship between distance and arrival time in adults. Continental birds of all ages remain in the British Isles until March. Adult and second-year Continental Blackheaded Gulls leave the British Isles abruptly in late March and early April; those breeding in Denmark and Germany leave in late March, those breeding elsewhere in Europe in early April. First-years stay later, with 36% of the overwintering population still present in May and 17% in June. Substantial numbers of adults (6%) and second-years (9%) also remain in May and June, and may breed in the British Isles. Compared with British Black-headed Gulls, Continental birds form a higher proportion of the wintering birds in the south and east of the British Isles. We estimate that 71% of all Black-headed Gulls in England and Wales during the winter are of Continental origin.