Adult winter Thayer's Gull Larus thayeri
Choshi, Chiba, Japan, December 2nd 2000.

Thayer's Gull (left) with Vega Gulls, and black-tailed Gulls
This individual has been observed here since 1987.
Note the small bill, rounded head, slightly paler mantle.

Its texture of head streaking looks intermediate
between Vega Gull and Glaucous-Winged Gull.