Back in Gull Scool

As long suffering NQS readers know, I do like a Caspian Gull. Like any other of the so-called large white-headed gulls, Casp is a variable beast. However, they hardly come more classic-looking than the lovely example with which Cogden blessed me a few weeks ago. Life would be a lot easier if they all looked like that. But they don't. In my limited experience, yer classic Casp is very much in the minority. Most of the 1cy (first calendar-year) and 1st-winter birds I've seen have warranted fewer descriptive superlatives than that one received. Which brings me to the flooded field behind Rise Restaurant at West Bay, around 09:00 on 30th September.

Picking up the story where the previous post left it, this is what I was looking at...

One of the first photos I took, full frame at 2000mm zoom. It's the bird in the middle, in profile.

Three minutes later it flew left, out of the flock, landing alone. It then peered around for about 90 seconds, then lifted off and departed. It had been present for around five minutes in total. Apart from my initial brief scope view before it joined the flock, I had concentrated on getting photos. Here are the best of them...

This bird reminded me very much of an ugly-duckling Axe Estuary Casp from December 2019. That one had been ringed as a Caspian Gull in a mixed gull colony in eastern Germany which does produce hybrids, but for a number of reasons I was happy enough to call it a Caspian Gull. Those reasons were...

• A Caspian Gull ID paper published in British Birds (December 2011: BB 104 pp.702-742) proposed a scoring system that could be used to separate Caspian Gulls from look-alike hybrids, and the Axe bird comfortably passed that test.
• A few London-based gull enthusiasts gave it the thumbs-up.
• To my eye it had 'the look', that subjective, indefinable something that puts me in a happy place. In other words I felt quite comfortable calling it a Caspian Gull.

Here is the bird in question...

German-ringed Casp with 1st-winter Herring Gull 18/12/2019

Hopefully it is obvious why the West Bay bird reminded me so much of this one.

The mucky underparts on the Axe bird definitely caused reservations at the time, for me and for others, but there would have been none at all had it been cleaner below. A few weeks later, another Casp appeared on the Axe. Lovely white head, much cleaner underparts, but the chunky bill (with obvious gonys) and rather dark, heavily-marked scapulars were less than ideal really. Still, BirdGuides liked it, and published a photo in their monthly summary. As I said, not all Casps are classics...

The West Bay gull (above) compared with a couple of vintage Axe Casps. Incidentally, both those birds scored almost exactly the same according to the BB paper parameters: 19 and 18 respectively, where 18 is the mean for Caspian Gull and 21 is the safe upper limit; 22 or higher could be a hybrid.

So, here is a pic of the West Bay gull with annotations galore...

Also, just to illustrate how different this bird is to standard Herring Gulls of the same age...

Gull folk always like to see the underwing of a 1cy Casp, and 'the paler, the better' is basically what we're looking for. So here is the West Bay bird in comparison with the two Axe Casps...

The 7/1/20 Casp clearly - and unsurprisingly - has the palest underwing. Nice.

However, we shouldn't get too hung up on the underwing thing. The collage below is from the BB paper...

Page 734 of British Birds vol. 104. We need to stop worrying about dirty underwings.

One final thing. According to the aforementioned ID paper by Gibbins et al, another useful metric in Casp ID is bill shape, specifically the ratio of length to depth. A Caspian Gull will ideally have a nice, slender bill anyway, without a prominent gonys, but actually calculating the length:depth ratio by the method outlined in the paper will give you a nice bit of data to play with too. So I did.

The following photo is probably the best profile shot. I cropped it massively so that the head nearly filled a sheet of A4, then printed it out and measured the bill. The length:depth ratio was a touch over 2.5.

Probably the best profile shot.

So there we go, even the bill shape falls comfortably within range for Casp. In fact it is very close to the mean, and outside the standard deviation of both Herring Gull and hybrids.

Conclusion

The only thing I haven't done is run this bird through the trait scoring process that I used on the Axe birds. Mainly that was because my photos probably are not good enough to comprehensively cover all the features that need close examination. But also because I had already made up my mind about this bird.

Without doubt it has a ton of pro-Casp features. However, I think the photos of it alone out on the grass finally killed it for me. In the 13 years since the publication of that ID paper Caspian Gull has expanded well into Western Europe, breeding in mixed colonies among Herring and Yellow-legged Gulls. None of these so-called species is particularly good at telling which is which, and the resultant genetic cocktails are legion. The West Bay bird may be a pure Casp, but I highly doubt it. Perhaps it is some subtle head-shape thing, but the bird simply doesn't have 'the look'. So I'm not counting it as one.

Personally I reckon a lot of birds that are published as Caspian Gulls are not quite the full ticket, and that probably includes a percentage of mine. But who cares? Not me. When there are stonkers like that recent Cogden bird out there, gulls will always be worth checking.