Hybrid Glaucous Gull x Herring(Vega)Gull?
Choshi, Chiba, Japan, April 7th 2001.

This individual looks like almost Glaucous Gull.
But the dark grey markings on primaries will be sign of hybrid.
The general structure, bill size, and wing projection 
show obvious Glaucous-features unlike kumlieni.

Note the kumlieni -like wing-pattern.                 Another hybrid gull on the same day.
The structure is very different from Iceland Gull (image below )
 Iceland Gull March 3rd. 2001