My first response on seeing Yann's excellent pictures was 'on no, it's that horrible age class'. The nearly- adult age in both Thayer's and Kumlien''s, I have found in the past to be, in my opinion, probably one of the most difficult. Some forth winter Kumlien's can have loads of dark on the primaries, and in my view, mimic or get very close to patterns in adult Thayer's. I can came to the conclusion a while ago that this age class might at times be unidentifiable, even though the adult subsequent adult plumage of such birds might be relatively straight forward. So already my bias with the Icelandic bird would be that it's difficult to be certain of the identification, simply because it has dark on the primary coverts, even though the rest of the pattern appears to be good for Thayer's, and outside of the range for Kumlien's.
My second observation of the photographs is the bill. Some small female type adult Thayer's can obviously be at the weaker billed of things, but nevertheless my recollection of hundreds of Thayer's Gulls, is that there is always a 'weightiness' about the bill that is just so lacking on this Icelandic bird that I'd have to be strongly persuaded that such a weak looking bill was ok. for Thayers.
Having commented on this interesting gull in Iceland, I decided to check the pictures again as I've not had much time, and immediately saw something that gave me more insight into the birds potential identification. The general assumption seems to have been that the wing tip pattern is like that of an adult, even though there are dark markings on the primary coverts indicating a fourth or fifth winter type plumage. This is presumably based on the pattern of the primaries as depicted on the under wing.
The pattern on the upper wing is harder to be sure of because of the particular fore-shortened angle in which it appears in the photograph. What I could see as I looked at this with a little more concentration, is that there is a longitudinal brownish mark on P5 (presumeably on the outer web along the shaft), besides the small dark subterminal mark, and the dark patterning on P7 (although it's very hard to be sure), appears to reach almost as far as the primary coverts. If my perception is correct then this is not right for either adult Thayer's or Kumlien's Gull. That's a complicated way of saying that I think from what I can see in the photograph, that whole of the primary pattern of this bird is that of an immature pattern, and not an adult pattern. I have certainly seen such an immature pattern as this on near adult/ sub-adult Kumlien's Gulls. Essentially all young Thayer's and Kumlien's at some stage (around third or fourth winter), can show long dark fingers in five or six primaries, as I suspect this bird does.
A similar problem can be encountered in michahelis Yellow-legged Gulls, which can look just like full adult birds, except for some dark marks on the primary coverts and the overall primary pattern is confusingly the same as armenicus or barabensis and even though it looks like an adult primary pattern, it's an immature pattern and the bird develops a normal michahelis wing pattern when fully adult.
If we change our paradigm and call it an immature gull primary pattern, then we're much closer to calling the bird a sub-adult Kumlien's Gull.
I gather there may be more photographs of the bird forthcoming, which hopefully will display the primary pattern more accurately, and prove me wrong or otherwise! I would be interested to hear what others think.
Best wishes,
Martin Garner.
