The Caspian Gull used to be treated as a subspecies of the Yellow-legged Gull but it is now treated as a full species.
This species can be found in eastern Europe, the Middle East, north-west Africa and central Asia.
It is resident in much of the Black Sea and Caspian Sea. 
Small numbers are sometimes seen in western Europe. 

Structurally, Caspian Gull has a long and slender body (with a high breast, clearly shown on the 4th photo), a usually long, parallel shaped bill without a clear gonysangle, a sloping forehead and strikingly long, thin legs.
Yellow-legged Gull is more heavily built with a more squarish head, a deeper breast and has an on average heavier bill with a clearer gonysangle than even Herring Gull.
The eyes are small and often dark, the legs vary from pale pink to a pale yellowish colour. 

First 4 photos: Enisala area, Tulcea County, Romania, 23 May 2018
Last photo: Mila 23 area, Danube Delta, Romania, 24 May 2018