Mongolian Gull Larus [vegae/cachinnans] mongolicus
December 20th 2011 Choshi Chiba Japan

This adult Mongolian Gull has relatively extensive head streaking. But it is thinner than in Vega Gulls and the range is restricted to mainly hindneck. And its frontneck and breast are almost pure white unlike most Vega Gulls. The slenderer overall structure formed by longer pale pink legs, longish neck and relatively straight bill with black spots stood out among Vega Gulls, and somewhat recalled Caspian Gull.  

The Mongolian Gull (left) with 2 Vega Gulls.
The mantle shade was a little paler than most Vega Gulls, and was presumedly intermediate between Vega and American Herring Gull. Note that primaries of the 2 Vega Gulls are still  growing (P8 is the longest), though many of them already have finished their molt in this time of year. Unfortunately postures and angles of each birds are different in this photo, but the structural difference between them was more obvious in the field. 

It is difficult to find out this individual from thousands of Vega Gulls when it is in this posture, nevertheless somehow it looked a little strange compared to my personal typical image of Vega Gull.

It's too bad this individual suddenly jumped off and disappeared behind the seawall, so I could not take any good flight shots. But this shot luckily caught a black mark on the inner web of P4. This feature is common in Mongolian Gull, and is uncommon in Vega Gull.