|Putative Azores Gull on Islay, Argyll (Richard Allen).|
On the face of it this gull looks like an Azores Gull, with that 'half hood' of dense streaks pointing to Atlantic island origin. The slightly darker grey upperparts than Yellow-legged Gull is also good for the form atlantis. The combination of newly grown grey inner primaries, grey coverts, solidly dark tail band and mainly yellow bill with darker tip suggests that it is a second-summer bird moulting into third-winter plumage, in which case it is perhaps a bit surprising the legs are still pink - most at this age would probably be at least a dull yellow by now (sometimes they attain this colour in their second winter). But it may just be a slightly retarded bird in this respect.
I don't think any michahellis Yellow-legged Gull would show such a head pattern at this (or probably any) age, which pretty much leaves Herring x Lesser Black-backed Gull as the only other (left-field) option. There was an adult yellow-legged-type large gull in Hyde Park in central London for a couple of winters which resembled Azorean in many respects, though the head was a little more lightly streaked than is typically the case; the upperparts may also have been a fraction too pale.
I sent photos of that bird to Klaus Malling Olsen, who in the end couldn't decide for certain whether it was a genuine Azorean individual or a Herring x Lesser Black-back hybrid. Herring influence might theoretically explain the possibly overly pinkish legs of Richard’s bird, but that wouldn't really count for anything until it was more mature. The shape in this sketch of the bird swimming is quite Herring-like, but this is only a sketch, of course, and not finished artwork.
Taking everything into consideration, the Islay bird seems to be a good candidate (at least) for an Azores Gull. It brought to mind the streaky-headed large gull I had on the Thames late last year which also recalled an atlantis, but which vanished before I could get to grips with it properly (see a record shot of that bird here). Perhaps it will reappear this winter.
You can see more of Richard Allen's work at www.richardallenillustrator.com.