His partner [top of head just visible if you look closely] is nesting inside a chimney.
Up pops the female's head and the male squawks to warn off a rival who has come too close to his nest.
Luckily, the chimneys around here aren't in use
Edited to add-
It turns out that these gulls are actually yellow legged gulls Larus michahellis - a Mediterranean sub-species of the herring gull that is now considered to be a full species of its own -also known as the yellow-legged gull or Goéland leucophée in french