In Oklahoma City, Oklahoma on 29 December 2012 I found this dark-eyed, dark-mantled and yellow-legged gull. I could not come up with an ID for this bird so I sent my photos to Alvaro Jarmillio who stated that he saw Lesser Black-backed here, but either a hybrid, or a taymirensis type bird with which he had no experience. Alvaro sent my photos to Peter Adriaens in the U.K. to get his thoughts. Below are Peter’s comments.

1/02/2013
Hi Alvaro
This is an interesting bird, well worthy of attention!
There are many birds in Japan that look like this.
On the deck, it looks great for a taimyrensis. However, the flight shot reveals a rather more restricted amount of black on P7-8, so it should be an intergrade with Vega Gull, I think.
In true taimyrensis, black on the outer web of P8 nearly reaches the primary coverts, and on P7 it covers more than half of the outer web. Most also have some black on P4. It is a bit of a pity that P9-10 are missing in this bird, as the size of the white mirrors is helpful too. For photos of taimyrensis from the breeding grounds, see Van Dijk et al 2011 (Dutch Birding 33).
I cannot think of a much better option really, even though this is in the middle of the USA. A hybrid LBBG x smiths is probably the most important pitfall, but would not account for the dark iris and the much restricted head streaking.
I include photos of two birds from Japan, fairly similar to the Oklahoma bird. I labeled them taimyrensis but they might be intergrades with Vega Gull (especially the second bird, perched on a pole).
Note that there has also been a good, recent report of a putative taimyrensis from the UK:
http://birdingfrontiers.com/2012/03/23/siberian-gull-in-lincolnshire/
Peter Adriaens

On the same lake in Oklahoma City On January 19, 2014, Joe Grzybowski found and photographed what may be the same bird or a very similar bird. See the link below to Joe’s bird.
https://pbase.com/joe_grzybowski/image/154174739 this photo and the next three

In February 2015 I sent the link to this gallery to Peter Pyle and Nial Moores. They are more inclined to believe that this bird is a hybrid between American Gull species. See their comments below.

2/23/2015
Dear Jim,

Thank you for the mail.

I am fascinated by gulls, but please be aware that there has been minimal research on the breeding grounds of many East Asian taxa; very little good literature is available with which to refine understanding; and there is not the "gull community" here to conduct robust analysis of features that we see, to help test ID criteria in the field or in collections. I think it fair to say, therefore, that no-one yet knows the gulls in this region really well.

With all the usual additional caveats too of trying to ID gulls from a set of images, my opinion is that this does not feel quite right for Taimyr Gull. Structurally it looks a little wrong: the breast is too deep and the primary projection too short on the standing bird. The shade of grey and feel of the bird when seen from the rear is closer to Taimyr - though your comments state that it did not look as dark as this in the field. The upperwing looks paler than I would expect; the black seems too limited in the primaries; and most oddly, there are some white pearls, giving it that Vega look.

Contrary to many living outside of the region, I am not yet aware of any good evidence of extensive hybridisation or intergradation between taimyrensis and vegae. This seems not to be supported by any breeding ground studies (indeed, their breeding ranges should be well-separated); and still seems to be unsupported by observations of birds here (rather, I believe that we are still learning the full range of both of these taxa and of the even more poorly-known birulai). Personally, I would therefore not go down the taimyrensis x vegae route with the bird in your images.

Not knowing what this gull is, and as it is out of range of species you know, I do suspect it