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4th-winter and Adult Yellow-legged Gull Larus michahellis
4th-winter Yellow-legged Gull, 10th December, 1999.

This bird was identified by many as Caspian, but the bird and bill is too bulky.

Compare the pattern of the longest primary: a small white tip and large black area on the primaries is much more like Yellow-legged, although the base of the inner web is markedly white - usually grey.
Adult Yellow-legged Gull, 11th December, 1999
Adult Yellow-legged Gull, 21st February, 2000