Monday, 27 February 2012

A(nother) mystery gull

By Keith Vinicombe

The odd gull (second from right). Note the slate-grey upperparts, heavy dark neck streaking, the heavy bill and dark-centred tertials (despite the adult-like plumage).

I didn’t make a New Year’s resolution, but if I had it would have been to give up looking at gulls. Or to be more precise, big gulls. This is because I’ve come to the reluctant conclusion that they’re more trouble than they’re worth, as the following example will illustrate.

I came across a really odd gull on 23 January – a first for Britain no less. Well, that was my fantasy. To get my retaliation in first, it was probably just another hybrid. It was on Herriott’s Pool at Chew Valley Lake, Somerset, where hundreds of gulls come in to bathe every afternoon prior to joining the 60,000 strong roost. To all intents and purposes, it was a Lesser Black-backed Gull, except that its upperparts were very obviously a shade or two paler than the accompanying British graellsii. I managed to take a few digiscoped photos, some of which are reproduced here.

It was in fact a very distinctive bird: dark leaden grey above with a very thick and obvious white tertial crescent. What was particularly odd was that it retained very distinctive neat brown bases to the tertials, the only signs of immaturity on what was otherwise a fully adult bird. Structurally, it was very similar to a Lesser Black-back, being rather tapered in shape, but it had quite big, almost ‘woolly’ head and its bill looked consistently longer and heavier, with a stronger gonydeal angle.

The mystery gull (right). Note the thick, blunt bill with a strong gonydeal angle, the heavy and extensive head streaking, the dark upperparts and the dark-centred tertials.

The obvious answer to such an ‘intermediate’-looking bird is that it is a hybrid: probably a Herring x Lesser Black-back, or maybe even a Lesser Black-back x Yellow-legged Gull. But, if it was either of those combinations, why did it have heavy and extensive head streaking? It had a profusely and finely streaked head, with the streaks become blacker and much more lined on the sides of the neck. The streaking then spread more diffusely across the upper breast to form quite a definite hood. Having already moulted into summer plumage, virtually all our local argenteus Herring Gulls are white headed and, as the above photos show, our local Lesser Black-backs are rapidly heading the same way (and agentatus Herrings are rare around here).

In flight it showed yellow legs, pale grey under primaries and secondaries (paler than graellsii) and a very thick white trailing edge to the secondaries – thicker than Lesser Black-back– contrasting with the leaden upperwings. It also looked stockier bodied in flight.

Note the pale grey under primaries and secondaries, the very thick white trailing edge to the secondaries and the heavy neck streaking.

It reminded me of photos that I had seen of Heuglin’s Gull – in particular the more eastern taimyrensis, which may be a hybrid form between heuglini and the eastern Siberian Vega Gull L vegae. When I got home, I had a look at the Gulls in Oman website which has some excellent winter images of taimyrensis. The Chew bird looked superficially similar to some of those images, but looking at the books was a different kettle of fish – they are so bogged down in detail that you can’t see the wood for the trees.

One ray of sunshine was Hadoram Shirihai’s account of heuglini and taimyrensis in the Macmillan Birders’ Guide to European and Middle Eastern Birds. Specific points he mentioned included: the moult (Heuglin’s moults very late, with adult’s primaries often still growing into late winter); the single isolated mirror on the outer primaries (confined to P10); the well-streaked head and neck; and traces of immaturity on otherwise adult-like birds – the last three points all fitted the Chew bird!

It had a single isolated white mirror to P10, something apparently shown by Heuglin’s Gull. Note also the fresh primaries.

But I accept that the bird was probably a hybrid, just like all the others. But the problem is, even if you do get an excellent candidate, such as the Rainham Slaty-backed Gull, how do you prove to the satisfaction of a national records committee that it’s 100 per cent genetically pure? The answer is that, without a DNA sample, you can’t – and even mitochondrial DNA can’t always provide a definitive answer, not least because it only follows the female lineage.

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