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This odd-looking 2cy large gull was seen at Poolsbrook Country Park, Staveley, Derbyshire, England on 6th December 2018. It has the size of a large Herring Gull and structure of a GBBGu but the mantle comprises clear grey adult-type feathers, (similar in shade or slightly paler than the presumed 'argentatus' Herring gulls adjacent); the tail band is solid blackish. I think that a hybrid origin is probable but there seems little in the literature about a Great Black-back / Herring pairing.
Any comments welcome, especially regarding other possible pairings e.g. GBB / Glaucous or GBB / Yellow-legged, as mentioned elsewhere in this forum.
Kind regards to all,
Richard Lowe (Derbyshire, U.K.)