Two Black-headed gulls on Midsummer Eve evening in Øresund.
This is how the Royal Society for the Protection of Birds defines the Black-headed gull:
Not really a black-headed bird, more chocolate-brown - in fact, for much of the year, it has a white head. It is most definitely not a 'seagull' and is found commonly almost anywhere inland. Black-headed gulls are sociable, quarrelsome, noisy birds, usually seen in small groups or flocks, often gathering into larger parties where there is plenty of food, or when they are roosting.