1st. winter Mongolian Gull L. c? mongolicus
7th November 2001 Sakawagaw-river Odawara Kanagawa Japan

This is a very worn, whitish individual suspected to be a southern-born bird. Note the extremely whitish breast and belly, and pale gray new scapulars with V-markings. Such advanced birds seem to be observed in Sosan (west coast of Korea) regularly, but are relatively uncommon in east Japan. 

Its state of feathers is similar to Slaty-Backed Gulls (right) rather than Vega Gull.
But, its long-winged structure and pale new scapulars differ from Slaty-Backed Gulls.

Vega Gull on the same date
Mostly juvenile plumage, more uniform brownish appearance.