Since 2014, during the summer months, I have been searching through the first-summer Lesser Black-backed Gulls at Shawell looking for fresh looking black primary feathers. I struck lucky in July 2015 when I spotted a very good contender for a first-summer Baltic Gull at the landfill site. It is still residing with the BBRC and it may well be there for a while, as it would be the first or one of the first non-ringed Baltic Gulls accepted for Britain as far as I know.
The other LBBG sub-species moult their primaries in late autumn and there have been no proven records of either L. f. graellsii or L. f.intermedius that have moulted all their primaries, especially the longest ones, during the summer months. Studies using colour rings have failed to find any second calendar-year graellsii or intermedius having moulted all their primaries by July. Whereas L. f. fuscus generally moult their primaries whilst they are on their wintering grounds and so reappear in northern Europe with a set of new primaries in many cases.
The criteria for separating first-summer L. f. fuscus (Baltic Gull) from the other sub-species during June, July and possibly August is as follows: (1) all retrices (tail feathers), secondaries and at least eight primaries are second generation (2) the upperparts are plain dark brown, mixed with dark grey to blackish-grey adult-like feathers. In some birds, the dark brown scapulars may have acquired paler fringes due to wear, while in others a faint pat- tern on the (greater) coverts may be visible. Birds that show scapulars and/or wing-coverts with ob- vious markings (cf plate 389) or unusually pale grey adult-type feathers (cf plate 390), however, should not be considered. Full paper HERE.
Heughlin's Gull doesn't yet have a full criteria to help identify extralimital examples.
I am awaiting on comments from Western Palearctic Gulls Group on Facebook. I have discussed it Mars Muusse one of the others of 'Field identification criteria for second calendar-year Baltic Gull' and he was impressed by this gull, but felt that the upperparts were a problem. Mars felt it wise to put it on hold until such time that more is understood about heughlini. There is a possibility that it could be a hybrid between Baltic and Lesser Black-backed Gull, but if so would the hybrid moult in line with Baltic Gull/advanced Heughlin's?