Friday, 18 November 2011

Roosting Caspian Gulls

By Keith Vinicombe

On a recent trip to East Anglia, one of our target species was Caspian Gull, and a report of six at Minsmere persuaded us to pay a late afternoon visit to the RSPB’s flagship reserve.

A lot has been written about Caspian Gulls in recent years, but this has tended to polarise birders into two camps: those who fully embrace the subject and those who steer well clear of it – many people are simply put off by all the detail.

Among all the talk of black bars on P5 and white tongues on P10, one thing that tends to get overlooked is the species’ jizz. Or to put it another way: how would you notice a Caspian Gull in the first place? Our experiences at Minsmere may help to clarify this. On our arrival, a quick scan through the gulls soon revealed a strong candidate. It was sitting down, front on, with its neck hunched into its body, as shown in the digiscoped photo below.
The adult male Caspian Gull is the bird at the front sitting down. Photo by Keith Vinicombe.
It stood out from the nearby Herring Gulls due to its pure white head, its beady black eye, its long down-turned gape line and its rather long, slim, washed-out, yellowy bill. It was a large, bulky bird, with a deep and well-rounded breast, but it also had a gently rounded head and, unlike the other large gulls, a rather benign, gentle character.

Front on, the combination of its woolly white head and beady eyes reminded me of an albatross, something that has struck me on previous occasions. The colour of the upperparts varied, looking similar to the accompanying Herring Gulls when front on, but a darker, flatter shade of grey when side on, more similar to Yellow-legged Gull.

After a while it became active and its entire shape was transformed. The head appeared small and rather rounded (although, surprisingly, it could also look very square), the bill seemed long, slim and tapered (lacking a strong gonydeal angle) and the neck was very long too. One of the most distinctive things about the extended neck was that it seemed to have an extra curve, producing a prominent bulge at the front, which gave the impression of a full crop (see the photo below).

When active, the male showed the classic Caspian gull shape, with its small rounded head, beady black eye and long, slim, tapered, washed-out bill. Note the prominent bulge in its long neck, resembling a full crop. Photo by Keith Vinicombe.
Its facial expression was completely different from the Herring Gulls, with its black eye and a narrow white upper eyering – a combination that imparted a soft, almost dopey look. The bill was a washed-out, dull yellow with small black marks behind the red spot. When walking around, the legs looked long and spindly, and they were a rather insipid greyish-pink.

After a while, a second bird glided in and sat down next to it. It was pretty much identical, except that it was distinctly smaller in every respect. The birds seemed comfortable in each other’s company and it was clear that they were a male and a female, perhaps even an established pair.

Much of the excellent work on Caspian Gulls has been carried out on rubbish tips, where the birds may be very active in their search for food. Consequently, what tends to be emphasised in the literature is their long, slim, tapered bill, the small pear-shaped head and the long, thin neck. However, their structure often looks quite different at nocturnal roosts, where the birds are relaxed and ‘ready for bed’. Instead, the head looks woolly and rather rounded and they appear short necked when the head is sunk into the shoulders. Also, because the relaxed feathers makes the head look larger, the bill does not stand out as being that different from other large gulls, in either its length or its structure.

For birders searching for Caspian Gulls in evening roosts, this relaxed, rounded pose is the normal default posture. At Minsmere, this was particularly true of the female, which often looked surprisingly Herring Gull-like in its structure and overall appearance, as shown below. In fact, at rest, the female reminded me of a giant Common Gull, with that same gentle expression, a high, rounded crown and a full, rounded breast. Also, it was not as long billed as the male.
The female Caspian Gull (middle, next to a Lesser Black-back) was distinctly smaller than the male and, as this photo shows, could like rather Herring Gull-like. Photo by Keith Vinicombe.

When relaxed, the female had a rounded body (with a full breast) and a very rounded head. The bill was shorter than the male's and did not look particularly long. Photo by Keith Vinicombe.
This is the same female, side on. In this photo, you can make out the darker grey upperparts, the dark eye and the slim, tapered bill. It also shows the flat crown and low forehaead, which are more typical of active Caspian Gulls. Photo by Keith Vinicombe.
Having soaked up the birds’ characters, we did, of course, look at the important primary pattern. Adult Caspian Gulls have much less black in the open wing than both Herring and Yellow-legged Gulls, with the black is, to all intents and purposes, confined to the rim.

At rest, the important feature to look for is the pattern of the underside of the outer primary, which is readily visible on the ‘opposite’ underwing, especially when the bird is preening its primaries. Caspian Gulls have a large white mirror at their wing-tip but there is also a long lobe of white on the inner web of the outer primary and this is separated from the white mirror by a relatively short area of black. This white-black-white pattern is very distinctive.

Three final points are worthy of clarification:
• Although the above account emphasises the black eye, when carefully scrutinised in good light, both these Caspian Gulls showed pale eyes, albeit a dull ivory colour. This is in fact perfectly normal in good, close range views – I once checked hundreds in the Ukraine!
• Although winter Caspian Gulls are often said to be white headed, they do in fact acquire grey head streaking just like other large gulls. It’s just that this streaking is so fine, so pale and so delicate, that it is often not visible except on very good views. Believe it or not, both the birds illustrated above had this streaking, but it normally manifested itself as a light dusting of grey that was not immediately apparent.
• It is also worth remembering that there is complete overlap in the bill measurements of Caspian, Herring, Yellow-legged and Lesser Black-backed Gulls.

Just before we left Minsmere, a third Caspian Gull dropped in, completing an excellent couple of hours of educational gull-watching!

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