Juvenile/first-winter Kumlien's Gull?
An overall greyish, coarsely marked bird with a dark bill, dark washed primaries and a well defined smooth dark tail band.
Dark wash on the outer primaries, increasing in extent from p5-p10, mainly confined to the outer webs, producing a venetian blind effect.
The bird shows an obvious dark tail band, the retrices appearing to be rather uniformly dark with pale tips.
Second-winter Iceland Gull
More photos on Johnny's site :