There has been a leucistic herring gull breeding at Drinkim on Caldey Island for at least five years and this year we ringed a chick (see the Teifi Ringing Group website for a photo) which was clearly also going to turn out as a leucistic bird.
The only caveat to this is that the bill of the bird on Caldey is, I am fairly sure, a standard yellow type which is what you might expect whereas the Teifi bird has this stonking pink or red bill.
Could it be a 4th year Glaucous - there is a picture of a similar bird in the Poyser gull book.
