This page shows comparisons of 1st winter Caspian and Mongolian Gull. Both have white head to underparts with blackish bill, and slenderer overall structure than Vega Gull (-extremer in Caspian) . The most useful feature for separating the 2 forms at rest seems to be wing coverts coloration.


A male-type, large Caspian Gull in Oman with Heuglin's Gulls. Note the white head to underparts and dark wing coverts.

A male-type Mongolian Gull in Japan for comparison. Note the whitish wing coverts. No bird in Oman 2005 looked like this.

Caspian Gull (same bird as the top) with 2 Heuglin's Gulls.
Note the dark brown wing coverts.

Note the whitish wing coverts.

A female-type Caspian Gull in Oman.
Note the combination of pale gray scapulars and dark wing coverts.

A female-type Mongolian Gull in Japan. Note the whitish wing coverts with rough markings.

Note the dark wings.

Mongolian Gull in Japan. Note the pale wing coverts and 'window' of inner primaries.

Caspian Gull (left) with Heuglin's Gull (right).


The white head and underparts strongly contrasted with the dark wings.

Note the whitish wing coverts.

Note the dark wing coverts and slender overall structural impression.

Same bird as the second image from the top.