A sliding scale of Caspian Gulls

Where do you draw the line between one species and another? That is so often the burning question when it comes to large white-headed gulls.

Not only does each species tend to show great intraspecific variation in its physical characteristics, but species also routinely hybridise, further clouding the issue and eroding the recognised parameters between defined taxa. Perhaps the most recognisable example of a so-called 'hybrid swarm' among Palearctic gull-watchers is the Iceland-Kumlien's-Thayer's 'spectrum' breeding across Greenland and Arctic Canada. Of course, these are now apparently to be considered as one and the same species, but it was nonetheless a fine example of how a man-made scale, assessed on the basis of quantifying several physical features, was – rightly or wrongly – used to attach a label to any gull in question within the complex.

To some degree, the same 'problem' is rife in Central Europe. In parts of eastern Germany and Poland, and indeed to a lesser extent elsewhere, Caspian Gull has, on its westward march, come into routine contact with European Herring Gull, which itself has spread inland in recent decades. These two species seem to have no qualms about breeding with each other, as the numerous mixed colonies testify, and hybridisation between the two is very well documented through intensive studies by dedicated researchers, including long-standing ringing schemes.

Whether it's a case of increasing observer awareness or a genuine upturn in the frequency and number of hybrid pairs – or, more likely, a mixture of the two – birds exhibiting mixed characteristics seem to be on the increase in Britain. These birds show any number or combination of features of both parents, both in terms of plumage and structure. Perhaps I'm too critical with my Casps these days, but it seems to me that I'm seeing more and more of these 'unimpressive' birds – albeit a longer-term assessment of well-photographed individuals is probably sensible before leaping to surefire conclusions.

This was illustrated pretty well by the weekend's Thames gulling. On Saturday, I had a rather nice-looking first-winter Casp at Princess Alice Way, Thamesmead. It transpired that this bird, with its particularly distinctive look to the face, had been seen by Jamie and Dante at Crayford back on 31 December, but not since (Dante recognised it immediately). OK, no moulted coverts and perhaps a slightly mucky underwing, but plumage otherwise absolutely spot on for a textbook first-winter Casp.

2cy Caspian Gull, Thamesmead, 9 February 2019

Sunday, however, was enlivened by a couple of troublesome birds, one of which I have subsequently described more than once as a "horror show". It is perhaps no surprise that both bear German rings, to the point that Rich actually said of the first (X21A), "I bet that has a yellow ring on it" before it revealed its legs. Sure enough, it did. In terms of plumage, though, X21A – seen at Erith Pier – was the better of the two. Apart from its well-marked greater coverts, it otherwise looked fine, but structurally lacked Casp qualities in many poses. It was quite vocal, and sounded like a typical Caspian Gull.

X21A at Erith Pier – look at that big, dopey eye and the rather feeble bill, as well as the boldly marked greater coverts, rather extensive inner primary window, mucky underwing, tail pattern etc ...

X21A again, but looking a bit better here.

However, if you thought X21A was bad ... then allow me introduce you to X90A. From the same colony as the above, this confusing mess of a Larus again sounds like a classic Caspian Gull, with the hoarse call often announced as this bird throws its weight arounds. And, at many angles, looks good for a cachinnans, with its rangy structure and various plumage features looking fine. But just take a look at those greater coverts, the extensive inner primary window, the well-marked tertials, the Herring-like scapulars, the comparatively uniform (and extensively pale-fringed) upperwing, the streaked head, the tail pattern ... suddenly you realise things aren't exactly looking great for a pure Casp. And, when you see it on the deck, it's enough to make you have nightmares.

X90A – actually looking alright here, in the grand scheme of things. Just don't stare too closely at those greater coverts ...

X90A (left) alongside a typical Herring Gull, thus exacerbating its cachinnans-like feel.

X90A has been seen a Thamesmead twice and was also noted off Erith Pier in January, so it seems to have taken a liking to the Thames ​– perhaps we'll get the chance to see it develop in the coming months and years, as it'd be fascinating to see what it ends up looking like as a mature bird.

Sightings from the weekend otherwise included the usual mix of Yellow-legged Gulls, including this cracking adult against a typical Thames background at King Henry's Wharf, Woolwich. You don't see too many adults in winter, so always nice.

Adult Yellow-legged Gull, King Henry's Wharf, 10 February 2019