Thanks a lot Ken, Steve, Phil and Steve!
The Armenian Gull is an interesting idea, however I can not image how it can reach Moscow from its limited range on the Middle East...
As far as I understand the terminology, the plumage parts are as follows on the attached pic.
So the tertials are quite light, whereas the greater coverts and scapulars are with dark center and white fringe... Or I am wrong?
Hi Pasha, I agree with what you have labelled as the tertials (very dark centres, broad pale fringes) and that most are second generation. However, at least one (possibly 2?) inner tertial appears to have been replaced for a third generation (grey) feather (directly below the left hand end of your label 'greater').
I had no idea when I mentioned Armenian Gull that they weren't really a viable option in your region, so I'm sorry if I misled you. I broke my own 'rule' of trying not to comment unless I have direct field experience of a species or race.
The superficial resemblance of your bird to either Ring-billed or Armenian Gulls should be eliminated (on plumage and geographical likelihood respectively) so I would conclude that your bird is most likely a 2nd calendar year European Herring Gull, unless someone else with more experience in the region can offer another suitable possibility. If not a Herring Gull then it's certainly a 2cy (or older) large gull, but my lack of familiarity with what's possible in your region limits my knowledge of what else might be likely?
2cy Herring (or even 3cy) can certainly look like your bird-the range of variation is huge, as it is in most large gulls.
Both Common Gull and Ring-billed Gulls would only show brown tertials in their first year (and then, not so dark as in your bird, nor with such bold white fringes). Both should have plain grey second generation tertials by the time they reach their second winter. Likewise, the wing coverts (mainly greaters visible in your pics) are of a type consistent with a 2cy+ large gull species, and definitely don't match either 1cy or 2cy Common or Ring-billed.