The Black-headed Gull does not have a black head! In winter it has a white head with a dark patch at the "ear". It also has a red beak tipped with black and red legs.
In summer breeding plumage the gull has a dark chocolate brown face, including crown and chin but not nape.
In transition it would appear to have two rows of dark grey headphones, impersonating a Bar-headed Goose.
Like most gulls, they gather in flocks and can be noisy and boisterous over food.
Black-headed Gulls dive-bombing Cormorants to steal their fish
Stealing food from Moorhen
Young Black-headed Gull almost in adult plumage; beak and legs are still orange rather than red.
The ghost of the Tower (of London)