April 1st 2001 Uramoto Itoigawa Nigata Japan.Photo by Sumio Washizawa.
This strange gull is similar to this individual that was seen in Choshi in the past.
Normal hybrid gulls  in Japan that seem to be Glaucous Xvegae
look similar to Glaucous Gull except for its primary-pattern.
But this individual shows darker mantle shade that looks similar to vegae.
If this were not vegaeXGlaucous, I would suggest possibilities of Slaty-backedXGlaucous or
taimyrensisXGlaucous though knowing parentage is not easy in this case.  BeEh򐟗Y
Its wing pattern is somewhat similar to kumlieni, 
but the outer primaries look more uniform dark gray except for white tips in flight.
And its general structure (longer legs and stout bill ) 
and mantle shade seem enough to rule out  kumlieni.
 The primaries and secondaries make obvious dark grey band  unlike kumlieni in its underwing