There are lots of gulls on Knutsford’s Moor at the moment, almost all black-headed gulls, as on the featured image. Then there is a single larger gull, which stands out from the rest, does its own thing, and is treated with some suspicion by them. It is not one of the gang.

This is a lesser black-backed gull, featured in the following images, standing on the ice. The excellent information given by the RSPB shows that this particular bird is in its second winter

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Click to see as slideshow.

The lesser black-backed is typically half as big again as the black-headed variety, but significantly smaller again than the great black-backed gull. See also Cornell Lab info.