Mongolian Gull L.c?mongolicus
9th November 2002 Choshi Chiba Japan
This individual looked finely identical with autumn Mongolian Gulls photographed in Korea. But unfortunately I could not see the P4, though the wings seemed to have rich black. So strictly speaking, the identification would not be 'perfect'. The wings show 3 new primaries when perched. Vega Gulls in similar states was not really rare on the day, but they mostly showed shorter new primaries or some old primaries.
The bill is pretty thick and long, but without a prominent gonydeal angle, and has black markings. The streakings are thin and limited. I don't think Mongolian Gull is complete 'white-headed' gull throughout the year.
The Mongolian Gull behind Vega Gulls. The bill color was obviously duller.