Available ringing data from the Atlantic coasts of Northern Spain (Galicia, Asturias, Cantabria and Basque Contry) of Yellow-legged Gulls ringed as chicks at breeding colonies are analyzed and compared with available recoveries of birds ringed in Gerona, a province holding most of the breeding population in the Spanish Mediterranean. The Atlantic populations appear to be nonmigratory, with the majority of recoveries occurring within 100 km of the ringing colony. This fact contrasts sharply with the characteristic postbreeding movements of Mediterranean birds, and resembles the dispersal behaviour of the Herring Gull (Larus argentatus) -a closely related species- on European Atlantic coasts. Spanish Atlantic gulls show significant differences in age-dependent dispersal distances, with inmature birds moving apparently further from breeding colonies than adults. Such age-related differences were not found in the Mediterranean population. Dispersal distances also show a clear seasonal trend, with a reduction in the range of distances from May to September for Atlantic gulls and from March to June for Mediterranean birds. The populations in the Spanish Atlantic seem to be quite isolated from other populations of Larus cachinnans. In Galicia there are some indications of a distinctive wintering area for inmature birds in the coastal northern half of Portugal.
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