I immediately realised it was something 'interesting', and called Martin Cade to come and take a look.
I grabbed some quick footage (which although not great, reproduced best on Vimeo):
https://vimeo.com/123551976
I'll admit to not being particularly good at gulls, and the only thought in my mind to begin with was that it may be an American Herring Gull Larus smithsonianus (based on the tail pattern). In truth, I didn't really know what else to look for, so concentrated on the head, remembering something about differences there with European Herring Gull Larus argentatus. It wasn't until later that some people on Twitter muted the possibility of Thayer's Gull Larus glaucoides thayeri.
Just before Martin appeared, I saw the bird come down and land on the beach amongst other Gulls, but I couldn't find it there having walked closer (the strong wind was very problematic in viewing). The Gulls were later flushed by walkers, and, frustratingly, the bird was not present by the time Martin arrived. It was never seen again, despite much scanning. It was only in view c15mins in total.
That's it for the facts, now for the opinions!
So what was it?
Well, not being able to get detailed notes or photos, we only have my video taken in difficult conditions to go on.
Many have expressed the opinion that it can't be ID'd from the images, but I think it's still worth an attempt at a full analysis, with (better) video grabs and examples of other Gulls to compare to.
So, lets look at the bird a bit at a time.
Tail
With it being almost entirely dark, with just slight paleness towards the outer tail bases, plus a few blotches on the rump (not bars as in American HG), the tail looks very good for a Thayer's Gull. I think any expert seeing this image in isolation would say it belonged to a Thayer's, if they had to make a call.
Wings
Unlike the tail, the wings do not look like a classic Thayer's. Although it has a dark 'venetian blind' type pattern on 5 primaries, the secondaries are pale, like a Kumlien's Gull Larus glaucoides kumlieni. Plus, Kumlien's do sometimes show this pattern on 5 primaries, rather than the usual 4 for that subspecies. Also, the darkness of the primaries is in range for Kumlien's. Most damning of all from a Thayer's point of view, is this pale marking on the 10th primary:
...a feature which is very rare on Thayer's, but usual for Kumlien's. All these combine to suggest that given just the wings in isolation, many experts would call them as likely coming from a dark Kumlien's.
Head
With the idea planted in my head that the bird may be an American Herring Gull, I did pay a bit of attention to the head in the field. To be honest, I can't add much to what you see here, but my impression was of quite a large head and long, sturdy bill. This is worrying when trying to turn the bird into one of the Iceland glaucoides group, as they should look short-billed, with a 'cute' appearance. I feel most gullers would call this head to be from a perfectly normal European Herring Gull. A comparison with an actual Thayer's supports that conclusion:
To sum up, so far we've established we have a Gull which is a 3rd Thayer's, a 3rd Kumlien's, and a 3rd European Herring! So what can it be?
For me, there are 7 possibilities:
- abberant male Thayer's Gull
- dark male Kumlien's Gull
- American Herring x Glaucous-winged Gull hybrid
- American Herring x Thayer's Gull hybrid
- Thayer's x Kumlien's Gull intergrade
- leucistic American Herring Gull
- abberant/leucistic European Herring Gull
I feel It's conceivable that a large male Thayer's could look as long-billed/large headed as this bird, but how much the variation actually stretches to, I suppose no one knows.
Of course the tail is the key Thayer plus point here, as is the dark 'venetian blind' pattern on 5 primaries. Although the p10 spot is a major minus point, it does occur.
For instance, Keith Pritchard drew my attention to this bird, labelled as a 2nd-winter Thayer's, with not only a spot in p10, but also p9 (Erik Bruhnke).
Most convincingly, here is a close-to spitting-image of my bird (taken in better light!), from California (Rich Bonser).
Dark male Kumlien's Gull
As already mentioned, the plus points for the bird being a dark Kumlien's are the pale mark on p10 and the pale secondaries.
Although this is a 1st-winter bird (plus it lacks the pale mark in P10), you could imagine that in it's second winter this dark Kumlien's (complete with all-dark tail) would be a very close approximation of my bird (Geoff Campbell).
Although the fact that my bird has 5 dark primaries does count against Kumlien's, it doesn't rule it out.
American Herring x Glaucous-winged Gull hybrid
This may be a pretty unlikely vagrant to Europe, but in some areas of the US, these are apparently an increasing phenomenon.
The combination of the dark tail of American Herring and pale primaries of Glaucous-winged, on paper at least, would seem an ideal explanation for the mix of features of my bird, plus the anomalies for straight-forward Thayer's. However, having been through loads of photos of this hybrid, there was only one which came close (most were far too dark). Although you'd have to say, on plumage at least, it's not far off! (Steve Mlodinow)
American Herring x Thayer's Gull hybrid
The more hefty proportions of the Chesil freak, combined with the mix of other features may be explained by such a hybrid. However, in truth this is a very unlikely candidate, as although they have been documented (though not photographically, as far as I can find), such hybrids are apparently extremely rare, even where both species occur.
Thayer's x Kumlien's Gull intergrade
This intergrade is apparently fairly frequent in some of the two subspecies range, so it's not unreasonable for one to reach Britain. The mix of features (dark tail + 5 dark primaries = Thayer's, pale secondaries + pale P10 mark = Kumlien's) may make this possibility quite feasible, but unfortunately, I can find very few images of the intergrade, and none of a 2nd-winter.
Leucistic American Herring Gull
The hefty un-Iceland-like head and bill and dark tail may suggest an American Herring Gull, and it's not ridiculous to suspect that a leucistic AHG might show the paler wings of my bird. Images are a tad lacking for this possibility unfortunately, but there is this 1st-winter bird (Steve Howell). Although probably overall too dark, the hint of a 'venetian blind' pattern on the primaries of this bird look promising.
Along with the unlikeliness of a leucistic AHG occurring in Europe, the lack of barring on the rump of my bird, is a key point against this possibility (but doesn't rule it out).
Abberant and leucistic European Herring Gull
As birders, we all know how incredibly variable Herring Gulls are, but is this a step too far?
Well, the two main features which count against EHG certainly both occur (in separate birds), of that there is no doubt.
Although a 1st-winter, you could imagine that in its 2nd-winter, with its leucistic-looking wings, this bird could look remarkably like the Chesil Gull (Martin Cade) - yes, it's from Portland, but the wrong age for being my bird, sadly!
This bird of course lacks the dark tail, but that undoubtedly occurs in EHG too. For instance, see this bird (Chris Gibbins). The rump pattern is similar to the Chesil bird too.
So, could both these features occur in the same bird i.e. a leucistic dark-tailed Herring? Well, I think yes they could.
There are other features of this bird which point strongly towards Herring and away from an Iceland type. The head and bill shape of course has already been covered above - more Herring than Iceland. Then there's this feature I've noticed:
There appears to be quite a large gap between the primary coverts and the start of the dark in the primaries, a feature which seems to be unusual in Kumlien's and Thayer's (and therefore, a plus point for EHG), although confusingly, in other videograbs this doesn't appear to be quite so pronounced! Of significance? I'm not sure!
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After all that then, this is purely my opinion on the likelihood of each possibility, based on my research. Appearance and likelihood to occur in Europe both considered:
10% - Abberant male Thayer's
15% - Dark male Kumlien's
6% - American Herring x Glaucous-winged hybrid
> 0.1% - American Herring x Thayer's hybrid
6% - Thayer's x Kumlien's intergrade
3% - Leucistic American Herring Gull
60% - Abberant and leucistic European Herring Gull
Well, whatever it was, I have to say this whole experience has been hugely educational!
P.S. Just to throw a final curve-ball into the mix take a look at this bird (scroll half-way down)! It's exactly the right age, right head-shape, but paler. Could a gull show slightly darker wings and tail as it got older I wonder?!
http://jameshanlonbirder.blogspot.co.uk/2013/12/more-northern-delights-and-troublesome.html
Thanks to Brett Spencer and Rich Bonser for their ID advice.
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