The taxonomic status of the white-headed gulls in general is still up in the air, with the east asian birds especially so. Vega has recently been split from Herring Gull, either as a monotypic species, Vega Gull (L. vegae), or as East Siberian Gull (L. vegae vegae with Mongolian Gull as L. v. mongolicus). The most current research indicates that they are both valid species, which is how I treated them.

An excellent discussion of this, and the ID of these birds, by Nial Moores, can be found on the WBKEnglish website at http://www.wbkenglish.com/lwhgid.asp.

The Black-tailed Gull is in 2nd-summer plumage.