The Yellow-legged Gull has only around 2007/2008 been recognised as a separate species, having previously been considered to be a race of Herring Gull.
Adults have darker grey backs and wings than Herring Gulls, but are paler than Lesser Black-backed Gulls.
They have more black in the wing tips than Herring Gulls and smaller white 'mirrors'.
The legs are bright yellow, there is a red ring around the eye and the bill is yellow with a large red spot.

There are two subspecies of the Yellow-legged Gull:
1. Mediterranean Yellow-legged Gull (L. m. michahellis, found in western and southern Europe, northwest Africa, Mediterranean); and
2. Atlantic Yellow-legged Gull (L. m. atlantis, found in Madeira, Canary Islands, Azores).

Birds breeding on the Atlantic coasts of Morocco, Portugal and Galicia (and spreading north from there) are usually also included here, but are sometimes considered to be a third subspecies: L. m. lusitanius.
Atlantic Yellow-legged Gulls usually have darker wings and back by comparison with Mediterranean Yellow-legged Gulls, creating a more pronounced contrast to the white parts.

During a boat trip on the North Sea, I found this 3rd year Mediterranean Yellow-legged Gull in a large group of Herring Gulls, Lesser Black-backed Gulls and Black-headed Gulls.
North Sea, Vrouwenpolder, The Netherlands, 9 October 2016