On the 3rd November 2011 whilst scanning through the thousands of gulls on Richmond Bank, Cheshire with Pete Berry, I came across an immediately arresting and somewhat familiar gull which stopped me in my tracks, accompanied by a small expletive. Having seen and studied many Caspian Gulls in Eastern Europe and the Middle East annually (and sometimes twice annually) for the past ten years now, I had no doubt I was once again watching this species. It was a nice adult and I watched eagerly for that moment it would perhaps flash the underside of P10. This didn't take long but when it did I was hit by a mixture of puzzlement and frustration. I had never personally experienced anything other than the classic P10 underside in all the adults I had ever seen but this one was no classic, instead exhibiting a pattern typical of Thayer's Gull Larus (glaucoides) thayeri. Whilst everything else about the bird was spot on I was duly thrown.
That evening Pete and I searched through the available literature but it was only the very most recent publications, plus the excellent website of Chris Gibbins which clearly seemed to suggest that such a thayeri pattern of P10 in Caspian Gulls is well within their range of variation.
Below: Here this individual portrays all the pro-Caspian features one would expect. A small, rather 'skinny' headed jizz with a comparatively obvious ventral bulge and high bosomed appearance, dark eye (iris not black but actually dark brown on very close examination), darker grey mantle than argenteus Herring Gull (akin to the mantle hue of argentatus) , already a fairly clean white head and a very straight edged bill, noticeably rather greenish-yellow for the basal three quarters at least. The legs were a very pale yellow, often appearing with a hint of fleshy colouration. The left leg appeared to have something wrapped around it just above the foot but it could not be discerned if it was a foreign object of some kind or a growth but the bird had the occasional peck at it so perhaps it was the former.
Below: Eventually the bird dropped the far wing during preening which allowed the underside of P10 to be scrutinised. It was something I had never experienced on this species before and I was suitably perplexed. The thayeri type pattern is plainly evident here.
This pattern is apparently perfectly acceptable for pure Caspian Gulls however and it is estimated that some 1-5% display this thayeri pattern. In order to rule out any argentatus Herring Gull parentage though, the colour/grey hue of the tongue on P10 needs to be paler than the upperparts and ideally whitish (Chris Gibbins pers. comm.).
Below: A closer view reveals that P10 is actually not fully grown yet and is approximately a couple of centimetres or so short of its fully grown length. It has a complete, long white tip but instead of having the typical solid black separating the tip from the tongue, here is completely broken and all that remains is somewhat restricted black on the outer web (extending only a fairly short way up the feather) and nothing but perhaps a tiny black speck on the inner web. Therefore the long white tip merges with the white tongue, giving rise to the thayeri pattern. Note here especially, that the tongue is clearly whitish.
Below: The upperwing in this image exhibits a solid black subterminal band on P5 and one of the white moons can just be made out on P7.
Gibbins, C., Small, B., Sweeney, J. Identification of Caspian Gull. Part 1: typical birds. British Birds 103: 3, 142-183
Thanks to Pete Berry for assisting in searching out literature and especially to Chris Gibbins for his very instructive comments on this subject.
Ian McKerchar, November 2011