I try to identify this bird but i'm not sur is it's a Caspian Gull or not. I think yes but I'm not sure.
Date of picture : march 7 2021
Localisation : France - Mortagne-du-Nord on Escaut river near belgian border.
welcome to birdforum. There are some renowned experts in Gull-identification around here, and I am sure, you get a confident, expert id soon, but:
With a small, dark eye (although its a 4 cy, most adults lack dark markings in tertials and bill), and the right bill-shape, your bird looks good for a Caspian Gull for me, too.
But: it seems that your bird has a narrow black band cutting the long white tip on P10 (not sure, if this is indeed the case or an illusion) and shortish legs for a Caspian Gull.
But I hope, one of those mentioned people will soon jump in, and say, if this is a pure (=acceptable) Caspian Gull or a Caspian x Herring Gull hybrid, so we both can learn!