Baltic Gull Larus (fuscus) fuscus
This is the eastern form of Lesser Black-backed Gull, treated by some authorities as a distinct species. Identification is very difficult due to the variation within the intermedius form of Lesser Black-backed Gull and some authorities maintain that certain identification of fuscus in a vagrant context is not possible. Currently British records committees are not accepting any records of fuscus unless they can be proven to originate from the range of fuscus (i.e. carrying a ring). Nevertheless a few of the keenest gull-watchers in Britain are regularly finding birds that are so distinctive that they are convinced that they are looking at fuscus.
I was with two of those when they picked out the following bird which on account of its size and structure as well as its plumage (including head pattern) they believe was one such bird.
apparent Baltic Gull Larus (fuscus) fuscus (with Lesser Black-backed Gulls), Cley, 8th September 2015