Hi Marek,
Where do I start?Given the darkness of the photograph,it's difficult to see much detail on the bird.For example,is the iris actually blackish or not?
Nevertheless,I shall offer a few comments:
I have to say that,for a brief moment,the bird gave a 'Caspian' impression,due to the dark-eyed appearance,but the bill is too short and stubby for a typical
cachinnans,and also too brightly coloured for all but adult summer(if even that!).Agree with Andrew that the wing-tips are too black,but subadult Caspian could show this.Still,I think that we can rule that out(here's where you say that it was one!)
It
could be a Herring Gull,perhaps,but some features seem odd for a typical bird of any race.The structure especially seems off for
argentatus in particular,and probably most
argenteus.
We then come down to the potentially tricky trio of
michahellis Yellow-legged,an unusually pale
graellsii LBB and a Herring x LBB hybrid.While the pale head,head shape and apparent bill shape/colour(not to mention perceived mantle shade,but this can't really be evaluated here) all would suggest the first mentioned,can the other two options be conclusively ruled out based on this photograph?
'Pending' awaiting further information....
Harry H