Adult winter Thayer's GullLarus thayeri
  Choshi Chiba Japan, January 14th 2002
This has slightly large bill compared with this Thayer's but the legs are the shortest among the old-timers of Thayer's Gull. And the head streaking looks very uniform just like Glaucous-winged Gull.
 What short legs! 
And compare its head size 
with the Vega Gull on the left.

Seen from close up, the primaries look grayish black unlike Vega Gulls. The legs often look deeper rose-pink.