I also hope for others to jump in, so I can learn, but: this Gull looks nearly exactly like the 2cy Yellow-legged Gulls I saw last week in SW-Germany.
-confident, strong build and shape with large, square head profile (but so variable in YLG and its the species where head shape is the most dependent on posture, more than in HG, CG or GBBG. Reason: dont know? Gull experts: please help!)
-extensive washed dark greater coverts that merges into barring there.
-extensive dark centers to back and scapulars
-the right quite uniform brown wing-coverts
Caveat: I have learned from others, that Caspian x Herring Gulls can look similar, but this bird reminded me so good of a YLG, that I couldnt hesitate to comment. And of course hoping for more comments to confirm or correct. I am still in learning phase when it comes to large Gulls and others!
Interesting comments, Alexander. I'm also trying to get to grips with these lwhgs, after successfully ignoring them for many years!
A couple of additional observations: the pale tips to the tertials seem narrow, rather than a broader "thumb-nail", and what I believe are some 3rd generation scapulars do look quite dark grey. I think both features might support your YLG hypothesis.
On the other hand, the bill lacks much of a gonydal angle, and for me it lacks the aggressive expression of this species.
Thanks for your interesting comment! Gut feelings are surely a personal matter, but thanks for confirming the "different impression" of YLG to a HG. For me the Gull with the aggressive expression is a GBBG, while YLG has a slightly different one. We may say the same here, I have written something about my gut feelings of YLG here:
thank you! I would like to hear pro-hybrid-arguements, too. Maybe the short legs (within variation for a YLG from my experience) and someone sorts minor differences in the pattern of scapulars and back for me.
I would like to learn about differences between YLG and CG x HG.