Mongolian GullLarus (?) mongolicus
Mar. 4th 2013 Choshi Chiba Japan
The combination of white head-underparts and structural jizz stood out among hundreds of Vega and Slaty-backed Gulls. There are no prominent smudges on the head-underparts other than thin, sharp streaking on the hindneck.
Note the narrow tail band.
2nd winter Vega Gull and Slaty-backed Gull (background) on March 11th for comparison.