CVL birdingCVL birding

THE CASPIAN GULL AT CHEW
Keith Vinicombe


I thought it would be helpful if I discussed the identification of the controversial Caspian Gull that has been at Chew for the last three months (June to September 2004 and still present in early October). I will try to do so without getting too bogged down in the plumage detail.

I first saw the bird on Herriott's Pool on the evening of 29th June. I was pretty sure that it was a first-summer Caspian Gull but it was not until the following evening that I had a good look at the underwing. Within a few days, some excellent shots of it appeared on Paul Burrows's website (photo 1) but shortly afterwards news filtered through the grapevine that its identity was being questioned. It was apparently being dismissed as 'a gull with a deformed bill' although no-one seemed able to suggest which species of gull it actually was!

It transpired that it had been there about a week before I saw it. Over the next three months, I watched it for about 11 hours. The following is a summary of the main points and an explanation of why I remain convinced of its identity.

The bill in June
When it first appeared, it had a noticeably long, slim, pointed bill. This was pale horn-yellow with a narrow and diffuse dark ring. Beyond the ring was a long pale yellow tip, with the upper mandible decurved, producing a noticeably pointed bill tip. It was this long tip that prompted people to say that the bill was deformed (see photo 1). I personally thought that the tip looked long simply because the band was narrow and was positioned quite a long way back, therefore accentuating the long tip.

It was also said that the bill colour was wrong, first-year Caspian having a predominantly black bill. This latter point was easily dealt with: the bill colour was 'wrong' for any first-summer gull but, since Caspian is the one that acquires more adult-like bill colours soonest, the advanced bill colour actually supported the identification as Caspian.

Caspian Gull  Larus cachinnans
Photo 1. Early July (Paul Burrows).

Structure
It was slightly but noticeably bigger than a Lesser Black-backed Gull and markedly smaller and slighter than any Yellow-legged Gulls that were present. It had rather a scrawny body, a long neck and a small head. In addition, it often showed a flat, sloping forehead, particularly when active. At other times, the head could look small and rounded, strongly suggesting that of a Common Gull. It looked 'high-breasted' and it often showed the effect of having a 'bulging crop', rather like a raptor that had just eaten its prey. When preening, it sometimes revealed a 'ruff' of rather loose feathering around the upper mantle. Also, it often showed a prominent bulge behind its legs in the ventral area. The legs were long and, when running, it had a strange 'lolloping' gait. When feeding on the water, its shape resembled a giant Slender-billed Gull. Although its structure was classic for Caspian, this was best appreciated when the bird was active: either feeding or preening.

However, when resting or otherwise at ease, its shape was much less impressive. The head would then look more rounded, the long neck disappeared as the head was hunched into the body, it would look fuller breasted and the legs would look shorter due to the fluffed-up belly feathering. Unfortunately, many of the photos were taken from the Bernard King Hide where the gull would loaf around during the afternoons, so many did not capture the bird in classic pose and simply did not do it justice. This simple fact led to some serious misconceptions by people who hadn't actually seen it.

On many occasions, the gull was seen with Yellow-legged Gulls, often of the same age. The structural differences were then very obvious: in fact the two species were like chalk and cheese. The Yellow-leggeds were larger, much bulkier and showed a heavy, square head with a much sharper angle where the crown turned into the nape. In addition, the Yellow-leggeds had thick and heavy bills with a much blunter tip. The Caspian was readily separated on this last feature alone. Again the photographs often did the bird a disservice as, unless the bill was exactly side-on at 90║ to the camera, it would look foreshortened, appearing shorter and blunter than it really was. Also, there was a narrow sliver of black on the lower mandible, right at the gonydeal angle. In some of the photos this small area of black merged with the dark background to give the illusion that the gull had a strongly down-curved bill tip!

In June, when it still had its old primaries, it was very long-winged, even compared with Yellow-legged Gull. On one occasion, Andy Davis likened it to a Cory's Shearwater as it glided towards us on its long, bowed wings. Like Yellow-legged Gull, it in fact showed a markedly arched skua-like inner wing shape when flying directly towards the observer. When head-on, it also showed prominent buff 'headlights' at the base of the primaries, again a feature shared by second-winter Yellow-legged Gull.

Caspian Gull  Larus cachinnans
Photo 2. Late August (Rob Turner). It still has the long pointed bill tip.

Caspian Gull  Larus cachinnans
Photo 3. Late August (Rob Turner). Note the white inner wedges on all but the two outer primaries.

Wing-coverts
When it first appeared, it was stated that the wing coverts were too strongly barred for Caspian, but this was because all the lesser coverts were missing, due to moult. When the lesser coverts re-grew, not only was their pattern spot-on for second-winter Caspian but they hid most of the 'offending' barring on the underlying median coverts.

Head pattern
Another source of confusion was the head pattern. This in fact was very distinctive: the crown and nape were finely streaked with pale grey and there was a more prominent fan-shaped patch of pale grey streaking below and behind the eye. This streaking isolated an obvious white eye-ring, the upper ring being the more obvious. This eye-ringed effect recalled that of first-winter Ring-billed Gull.

There is a widespread misconception that Caspian Gull has a white head all year round. But like Yellow-legged and Great Black-backed Gulls, Caspian Gull does have a winter plumage in which it shows head streaking, it's just that 'winter plumage' is a couple of months earlier than in Herring Gull. By early- to mid-winter (when most are seen in Britain) Caspian Gulls will have started to lose their head streaking and they will have gained their white-headed summer plumage. I saw around 1,000 Caspian Gulls in Ukraine in September 2002 and the head pattern of the Chew bird corresponded exactly with those birds. Interestingly, on 26th September 2004, I actually located a winter plumaged adult Caspian Gull on Herriott's Pool purely by its head streaking.

Other plumage features
I won't go into its plumage in too much detail as there are no real diagnostic individual differences from Yellow-legged Gull, but a number of features pointed towards Caspian: (1) very heavy nape streaking/mottling; (2) the grey of the mantle was slightly paler and slightly less blue than Yellow-legged; (3) it showed dark blotching within the grey of the upper mantle/leading scapulars; (4) retained first-winter scapular feathers had fine anchor-shaped marks; (5) it had predominantly white underwings (see photo 5 by Gary Thoburn), although note that second-winter Yellow-legged can have a similar underwing pattern - having said that, even when it was first seen in June it had a predominantly white underwing (first-summer Yellow-legged should show much browner underwing-coverts); (6) the inner webs of all the primaries except P9 and P10 were whitish, producing the characteristic 'Venetian Blind effect' when spread (see photo 3, by Rob Turner) - as a consequence of this, when viewed from below in flight the inner under-primaries 'flashed' white as the bird flapped; (7) in second-winter plumage the black tail band was of medium depth and quite sharply defined (apart from the central feathers).

Caspian Gull  Larus cachinnans
Photo 4. Early September (Gary Thoburn).

Nostril
It has a long, thin slit for a nostril, a feature associated with Caspian Gull.

Gape line
It had a long gape line, another feature associated with Caspian.

Behaviour
There were three distinctive feeding traits: (1) when feeding in its 'Slender-billed Gull posture', it would frequently up-end like a duck to pick food from the bottom of the lake (this up-ending is something that I saw frequently in Ukraine when Caspian Gulls were strung out along the shoreline of the Black Sea); (2) it would sometimes plunge-dive for food, either vertically from a height of about two feet, or from a low angle like a shearwater (whilst Yellow-legged Gulls will plunge-dive vertically for food, I have never seen them do the shallow 'shearwater-plunge' shown by our bird); (3) when it had food, it would sit on the water with its wings spread - it once held this posture for about five minutes (this behaviour is mentioned in many of the articles on the species).

Call
On 26th September, I heard its 'long call'. This was nothing like the deep, resonant call of Yellow-legged Gull. It was in fact a husky sound but it was markedly higher-pitched than Yellow-legged.

The final twist: the bill in September
In late August the bill still showed its 'deformed tip' (see photo 2, by Rob Turner). But by early September, some strange things started to happen to the bill! Firstly, it started to gain more black and the bill band extended in size and transformed into quite a large subterminal black blob. It also acquired some black towards the base. Presumably, the reduction in the black in its first summer had been hormonal - associated with greater sexual activity - and the black increased again in autumn as its sex hormones subsided. But what was most odd was that the bill appeared slightly shorter and blunter than it had been previously. I initially assumed that this was an impression caused by the increasing area of black near the tip (it is well known that a large area of black at the tip of a gull's bill will give the impression of a heavier, blunter bill tip). But an excellent photograph by Gary Thoburn revealed what I was starting to suspect: it had actually shed the so-called 'deformity' at the tip of its upper mandible! The long, pointed tip to the upper mandible had gone and, what's more, if you look carefully at Gary's photo (photo 5) you can actually see a ridge running across the upper mandible, from side-to-side, just in front of the black band, where the outer 'sheath' of the bill had snapped off. Whether this outer 'sheath' really was a deformity or was simply due to damage, I don't know.

The ironic thing was, whereas some people used the original long pointed bill shape as a reason for 'rubbishing' the bird, based on Gary's photo some were now saying that the bill was too short and blunt for Caspian! In actual fact, in exact profile the bill shape and length were now spot-on.

Subsequent plumage developments
After Gary's photos were taken, it acquired a narrow band of grey median coverts. Also, new grey feathers were growing over the barred inner greater coverts, reducing the conspicuousness of these feathers. By late September the bird looked almost identical to the December second-winter shown on page 26 of the September 2004 issue of Birdwatch.

Analysing the photographs
One thing that this bird taught me is how difficult it can be to analyse birds from photographs. Having watched the gull for long periods of time, there is no doubt that field experience was essential to gain a proper impression of its true shape and structure. Another problem with the photographs was that some were simply too contrasting, exaggerating both the head streaking and the underpart streaking. Even Gary Thoburn's excellent September shots show the underpart streaking as being more prominent than it really was. I also felt that human psychology played a part, some people being negative from the outset and looking for reasons to doubt the record. I even felt that the some of the doubters were moving the goal posts once some of the original doubts were assuaged either by new photographic evidence or by changes to the bird itself. For example, in some of the more relaxed postures, people said that the bird's head was too big and chunky for Caspian, yet when commenting on Gary Thoburn's recent side-on shot, it was said that the bird's head was too small and rounded!

Caspian Gull  Larus cachinnans
Photo 5. Early September (Gary Thoburn). This photo shows the structure well. Note that
the 'deformed' bill tip has been lost. It is possible to make out a slight ridge across the top
of the bill tip where the outer 'sheath' snapped off. (But note that sharpening and resizing
has accentuated this in the photo - RMA)


In classic posture, it really was a softer, gentler, more delicate bird than Yellow-legged Gull and it really did recall a giant Common Gull (or even a giant Ring-billed Gull, given its white eye-ring). It was as different from Yellow-legged Gull as Black-headed is from Med Gull! It was reassuring that the adult seen well on 26 September was identical in shape and structure.

To deal with one final 'doubt': predictably, it has been suggested that the bird could be a hybrid. In my opinion, there is no evidence to suggest this and, to quote from the BOURC: 'it will never, in practice, be possible to eliminate the possibility of the genetic influence of some other taxon in any individual andů the only pragmatic solution is to accept records of cachinnans provided that an individual displays no anomalous characters' (BOURC Press Release, 5th December 2003).

Finally, Andy Davis went to Bulgaria in September and had a good look at the Caspian Gulls. He was able to confirm that our bird matched some of the second-winters that he saw there.

Keith Vinicombe
6th October 2004

Update:
In hindsight, this bird still looks pretty convincing for Caspian Gull but both Rudy Offereins and Brian Small, who were consulted about its ID, were not convinced about it. On thing they both seemed unsure of was the amount of barring on the wing coverts, as well as the streaking on the head. Since 2004, we have obtained more information about the northward spread of Caspian Gull across Eastern Europe and it seems that there is a hybrid zone across Poland where the species is interbreeding with argentatus Herring Gulls. It may well be that our bird, although ostensibly a Caspian Gull, was not a pure individual. One interesting point is that on Birdguides there is a close-up photograph of the head of the returning Ukranian-ringed Caspian Gull at Southwold, Suffolk, on 2nd August 2004, and that bird also seems to have a line across the top of the bill where the outer sheath over the tip of the bill appears to have snapped off. Could this be normal for Caspian and indeed other gulls?

  home      top of page