This bird had me reaching for the books and it took me quite a while until I was able to age it.
It’s a Yellow-legged Gull Larus michahellis, but because of my lack of experience with sub-adults of this species, I couldn’t determine whether it was a 2nd- or 3rd-calendar year individual.
What confused me was the mixture of brown 2nd-calendar year-type feathers and the grey adult-type ones. I initially thought it had too much brown (especially in the tertials and greater coverts) to be a 3rd-calendar year gull but at the same time too much grey (inner primaries) and too much yellow in the bill and legs to be a second-calendar year bird.
When looking for references, I was struggling to find photos on the Internet of similar looking birds. My main source gull-research.org unfortunately lacks photos of Yellow-legged Gulls showing open wings (time for a new project guys? ;-)) What I did found out though is that there are a lot of similarities between 3rd-calendar year birds and (advanced) looking 2nd-calendar years (2cy example 1, 2cy example 2, 2cy example 3).
I finally settled on this being a 3rd-calendar year Yellow-legged Gull mainly based on the adult-looking inner primaries with white tips.
So all in all a very educational bird for me. I was especially surprised to see black markings as far down as inner primary P3.