May is definitely not the best month of the year to try and learn your gulls! I am certainty not saying you shouldn’t go out gulling in may of course, but you have picked the most difficult period in terms of identification, in a group famous for the identification problem it pauses.
As to your bird: many, many 2cy YLG look just like this at this time of the year and the plain scapulars are no longer a good feature. See for instance this recent post by Rémi Jugieux on Facebook’s Western palearctic gulls:
Log in to Facebook Notice that 2cy YLG missing plain grey scapulars are actually a minority.
There is such a wide array of variations in large gulls that it’s difficult to pinpoint a really diagnostic character that would separate YLG from Caspian (although, like Lou said, tail pattern is a very stable feature in first cycle birds that should be checked on any contentious bird), especially in worn plumage as here. But typically, 2cy Caspian in may would appear even plainer/uniform on the mantle than your bird and would have a « silvery pale » look to it. When present, the black internal marks to the scapulars would mostly be « longitudinal » streaks running along the shaft and not transversal as in your bird. See:
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Détail de l'observation - www.faune-iledefrance.org
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Détail de l'observation - www.faune-iledefrance.org
Also, most Caspian would still show the typical white band form by the tips of the greater coverts. See these for comparison:
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Détail de l'observation - www.faune-iledefrance.org
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Détail de l'observation - www.faune-iledefrance.org
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Détail de l'observation - www.faune-iledefrance.org
The bold black and white barring to some median and lesser coverts is on average a better fit for YLG than Caspian.
And while the legs of your bird are indeed long and would fit just fine on a Caspian, the bill structure looks much better for a YLG. The head shape/profile does not give me particularly strong Caspian vibes either I must say. We can’t gauge the neck length/thick on your images but I thought I would mention this for the sake of completeness and be alos because structure is such an important tool when the plumage is bleached…
In essence, I would pass this bird as one of the numerous Caspian-lookalike YLG that are so common in spring.
Of course there are some generalizations in what I wrote and not everything is bulletproof or 100% reliable, but I think that will give you a better idea of what to look for in your quest for finding 2cy Caspian gulls in spring.
