Presumed hybrid
Slaty-backed Gull X Vega Gull 
Larus schistisagus X L. vegae

This 2nd winter gull shows intermediate characteristcs between Slaty-backed Gull and Vega Gull.

The mantle looks obviously paler than that of Black-tailed Gulls, thus it seems too pale for pure Slaty-backed Gull. 

The mantle looked very similar to that of the Vega Gull in the background. But its other features such as head structure and pale iris looked very like Slaty-backed Gull.

The presumed hybrid in the center, with one Slaty-backed Gull on the right, one Vega Gull and  two Black-tailed Gulls in the background.
The primaries look nearly black in the Vega Gull, brownish in the Slaty-backed Gull, intermediate in the presumed hybrid.

The tail pattern was very like Slaty-backed Gull rather than Vega Gull.