Black-headed Gulls are very common in the Dutch delta in all months of the year.

The large colonies are a key element in the coastal ecosystem. Sandwich Terns f.i. 

will only breed when there is a Black-headed Gull colony around and if so they will

settle in dense clusters amidst the gulls leaving the defence to the poor gulls which

will suffer great losses to marauding Herring and Lesser Black-backed Gulls.

Mediterranean Gulls, Barnacle Geese, Red-breasted Mergansers, Tufted Ducks and

Gadwalls very much depend on the protection of the Black-headeds and even Avocets

and Common Terns who can perfectly fend for themselves like the Black-headeds to

serve on the front line.