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My impression is that the majority, perhaps the great majority, of birders in Britain are now comfortable with the ID of this bird as a Slaty-backed Gull. This was especially the case aftr its reappearance later in the winter at Pitsea tip, when a new series of closer photos - plus shots of skins showing variation in mantle tone in 'Birding World' - seemed to settle the main bone of contention.
Several Rarities Committee members have also seen the bird and appear to support it, but of course it remains to be seen whether it will be accepted by both the Rarities Committee and the British Ornithologists' Union's Records Committee as a British first.
Naturally, I hope it will be, but either way the fact that the bird appears to show no problematic feature for Slaty-backed Gull, and nothing clearly indicative of a hybrid origin, should be good enough.