Tuesday 6 October 2020

And in Gull Class Today...

This post is in response to a query regarding yesterday's Caspian Gull. Basically, how did I identify it? So I'll try and go through the process, and hopefully there will be some useful tips for anyone who's interested.

If I've done the sums right, yesterday's Casp is my 17th ever. Following a 2nd-winter on the West Bexington Mere back in January, it is my second in Dorset; all the others were on the Axe Estuary, Seaton. I personally found 13 of them, or 14 if you count one which I later learned had been seen a day or two earlier. I actively look for Caspian Gulls, regularly picking through crowds of large gulls for the tell-tale field marks which identify them. I found my first in 2009, but I'd been searching for a couple of years prior to that, so we're talking roughly 13 years in total. In other words, I've found about one a year on average. Mind you, last winter I found four, but at this stage I'm not sure what to put that down to. Are they becoming less scarce in the southwest? Or was I just exceptionally jammy? Or perhaps there are sometimes 'good years' for them? Who knows? Time will tell I guess...

Meanwhile, think of them as a very scarce bird and you can't go wrong. Search for them, by all means, but don't view failure to find one as some big deal. They are genuinely scarce down here, bordering on rare.

So, when it comes to my seemingly self-appointed role as Caspian Gull ID Guru, what are my qualifications? Please reread paragraph two. Yep, very scanty. I am no guru. How it works is this:

  1. Read up a bit
  2. Go out and look at gulls
  3. Repeat steps 1 and 2 ad infinitum, especially step 2

And that's it. I could increase my field experience of the species by regularly travelling to some of the country's Casp hotspots, but I'm not prepared to do that, hence only 17 birds. Mind you, that's not a bad tally for local birding in Devon and Dorset. So anyway, down to business...

How come I got so excited, so quickly, on seeing yesterday's bird?

It was with Great Black-backed Gulls, including several juveniles, and I simply began to check all the juvs to make sure they were actually GBBGs. The first one I looked at clearly wasn't! How did I know? Its scapulars were too pale. And even through bins alone, at considerable range, I could see it had two pale wing bars and a line of plain, dark-based greater coverts, a really good pointer for 1st-winter Casp and a combo that I have yet to see on a Herring Gull. So here's a photo of the 1st-winter Caspian Gull with a Herring Gull and GBBG of the same age. Have a look, and pick out those obvious differences.

Front to back, we have 1st-winter Casp, HG and GBBG

All this talk of coverts and scapulars might be a bit confusing, so here's a photo with assorted useful bits labelled...

In the next photo the bird is mid-preen, so its wing is untucked and relaxed, with the feather tracts really obvious. Incidentally, note pale areas beginning to come through on the bill. Typical of Casp.

Have another look at those greater coverts. Now remind yourself what a typical Herring Gull looks like (top photo again). Chalk. Cheese. Also check out the blackish tertials, and see how the pale markings are restricted to neat tips only, without any pale notches further down the edge of the feathers. Again, compare with HG. Note also the relatively long tibia. When it walks about there is a slightly comical 'on stilts' effect, especially in a strong wind!

One more pic with arrows...

When juv Caspian Gulls moult into 1st-winter plumage, one of the most obvious changes is seen in the scapular feathers, and the same is true of Herring and Yellow-legged Gulls. The plain brown juvenile feathers are replaced by patterned grey ones. October 5th is my earliest date for a Casp, and you can see that yesterday's bird still has a few juv lower scaps, a feature shared by my only other October bird of this age. A typical 2nd-generation feather will be pale (or pale-ish) grey, often with a dark diamond at the base, a thin dark line down the centre, and a fine anchor-shaped mark near the tip. A few like that are visible in the photo. Herring Gull (and Yellow-legged Gull) scaps are typically much more heavily marked, appearing darker overall.

Unlike the scapulars, almost all the wing feathers are retained throughout the winter. In other words, they are juvenile feathers, which gradually become more and more worn in appearance. Yesterday's bird appears to have moulted an inner greater covert, and replaced it with a grey, 2nd-generation feather. I'm not sure that I've ever seen that before.

One more preening photo. Note the light streaking on the crown. That will soon wear away I reckon, leaving us with the classic white-headed vision of loveliness for which I am constantly on the lookout during the winter months.

See that grey inner greater covert? I resisted pointing at it with a big yellow arrow.

And here is the only flight shot worth a look. Pale underwing = good. Very contrasty tail with neat black band = good. Blur and missing parts of bird = bad.

In a nutshell, with this bird the closed-wing pattern nailed it. It has many other excellent pro-Caspian features (several mentioned above) but this individual has such a classic set of wing coverts that identification was almost instant. Not all Casps are quite as straightforward.

As I implied at the beginning, words on a page (like the above) are all very well, but there is no substitute for looking at gulls in the field. As often as possible. Relate what you see with what you've read, and gradually it falls into place. It's one of those things where you get out what you put in.

All the best with it!

Disclaimer: there may be errors in the above. My wine glass is empty.


  1. Gav, those repeat ID's of Caspian Gull of yours appear to be having an effect. I mean, when even I can remember the what's and why's of the topography, that's progress.

    That said, when I saw the video of the Gull walking about, I reckoned that was enough for a description. "It appeared to be on stilts".

    An ID of YLG would be handy also. But let us all absorb the Casp. And I'll think about the one I couldn't fathom out stood on the ice at Stockers. I knew it was something - but what?

    1. Thanks Ric, and good point about the video, which certainly shows the 'on stilts' effect! This bird probably has the plainest set of coverts I've seen on a 1st-winter Caspian Gull. Most have shown at least some chequered markings on the inner greater coverts. Your Stockers Lake bird could well have been a Casp, but that nagging uncertainty will have to be laid to rest by your finding one this winter! 😉