Sub-adult Yellow-legged Gull with Very Dark Eyes

I have seen the occasional Yellow-legged Gull (Larus michahellis) in Britain that appeared to have dark eyes, but the views were often distant, so I may have been mistaken. In Gulls of Europe, Asia and North America it states that in adults the eyes are rarely dark brown or darker. The gull below, that I saw in Portugal, is not quite fully adult, but it clearly has very dark eyes that are not likely to alter significantly as it reaches full adulthood.

Dark Eyed Yellow-legged Gull

It is useful to know that the eyes of 'large white-headed gulls' are variable. It is normal for adult and near-adult Herring and Yellow-legged Gulls to have pale yellowish eyes and Caspian Gulls to have dark brown eyes, but it it worth knowing that there is variation and eye colour is not a feature to rely on when identifying these gulls. 

Dark Eyed Yellow-legged Gull

Yellow-legged Gull of a Similar Age with Typical Pale Eyes