Peter Adriaens (Belgium)
Ies Meulmeester (Netherlands)
Larus cachinnans hybrid X007 1CY, October 05 2016, Katwijk, the Netherlands. Picture: Mars Muusse.
Yellow X007 and metal DEH Hiddensee EA-202407 ringed as pullus on June 07 2016 in a mixed colony of Laußig, eastern Duitsland (51,34N 12,38E). Male.
Probably both argentatus (for sure) and cachinnans involved.
Voice was like cachinnans (although not a full match)
but many traits are in line with argentatus. See also HERE (in Dutch).