Larus vegae
Jan.17th 2012 Choshi Chiba Japan

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Very fresh individual. Almost juvenile plumage, except a few replaced scapulars.

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Somewhat advanced compared to the previous one.

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Browner individual.

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Light-built individual, female?

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Whitish-headed individual with large amount of white in tertials. Mongolian Gulls tend to have sparsely mottled belly with whitish ground color, or longer wings/legs compared to this bird.

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The darkish greater coverts might be influence of  'taimyrensis' gene?

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A flock of Vega gull with one Glaucous Gull on the right. 
Most birds in this photo seem to be within the range of normal Vega Gull, except one whitish-headed bird (I didn't notice it in the field) behind the Glaucous Gull looks like a candidate of  Mongolian Gull.

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Comparison of 3 forms in typical individuals
All photos were taken in Choshi, Japan. Note that Vega Gull looks intermediate between the two forms above in coloration. Therefore, some problematic individuals can be found between both Vega-American Herring and Vega-Mongolian.