:t:So here I am home.
From Gulls of Europe,Asia and North America by Olsen and Larsson yoour choice is as follows(direct quotes in parentheses):-
I am assuming it is Panjin nexto the North Korea border we are talking about?
Black-tailed Gull Yellow legs and 3 tone bill as adult,so out.
Common/Kamchatka :red pecking spot rules these out.
Mongolian Gull(either full species or race of Caspian):"legs vary from yellow to fleshy,eyes from dark to pale"and worse still"jizz similar to Vega Gull,with which winter range overlaps""compared to Vega slightly longer wings""shape and jizz similar to Vega and Heuglin's".Wings Kodak 5-6
Vega Gull.Kodak 5-8 in wings.
Slaty-backed Gull:darker and jizz of Glaucous-winged,so no way.
Heuglin's Gull:Kodak grey 8-11 and indistinguishable from Lesser-black back.Too dark and yellow-legged .So out."taimyrensis" is considered to be a cross between Heuglin's and Vega.
:brains:So that gives us 3 options(Herring is out incidentally as it is confined to Western Europe.)all pretty similar in jizz and colours,though there is a slight cline on wing colour.Mongolian palest(approaching argenteus at times),Vega in the middle,and "taimyrensis"darkest.
:scribe:To me in your photo wing colour is spot on for argentatus and yellow-legged gull,so the wing colour fits Vega best.I think it is too pale for even the palest "tiamyrensis",judging from photos I can find,they approach LBB far more.The amount of streaking around the head is probably too much fo mongolian,which is similar to Cachinnans in winter.
The eye on Mongolian,though yellow with a red orbital ring and set futher back than in cachinnans is still small.Your bird seems to have a large bright yellow eye.
Vega should show the least primary black of the three,which is consistent with your photo.
I would plump for Vega taking these points into consideration.:smoke:B