Thank you Lou!
I was suspecting it plumage-wise, but once again the structure falls out of the range of the Caspians I am used to confidently name Caspian Gull
This means that this winter I will have a lot of fun trying to make my cachinnans box bigger (today already 2 Caspians and an Herring)!
Here are some examples of the easier 1st winter from last year (4 different birds, both ringed ones are from Poland):
ciao
Michele