Monday, 4 March 2013

Unsolved mystery

Unidentified gull, Rainham, Greater London, 1 March 2013. Note the mid-grey upperpart tone, red orbital ring,
pink feet and pattern of black in the wing-tip extending to P5.
After the practical side of gulling, today it was time for the theory. A week ago I had arranged to visit the Natural History Museum collection at Tring, mainly to look at skins of Vega Gull Larus (smithsonianus) vegae. By extraordinary coincidence, however, three days beforehand I found an adult-type large gull at my regular larid patch of Rainham, Greater London, which showed several features suggestive of that taxon. Common sense dictates it is far more likely to be something other than a species never before recorded in Europe. But what?

The same gull showing the underwing pattern, with its show-through grey 'shadow' a good indication of the visibly darker grey upperwing than argenteus European Herring Gull (ruled out by wing-tip pattern and orbital ring colour).
In short, the answer is I don't yet know. Vega Gull should be possible to prove if seen well and documented with decent photos. But there are many unknowns to factor into the equation – especially hybrids. A key problem to me could be the far more likely option of European Herring L argentatus x Lesser Black-backed Gull L fuscus, and eliminating such a combination would surely be essential in ever confirming an adult Vega Gull in Europe.

The only shot I was able to grab of the bird on the ground, immediately on discovery and just as it was about to take off. In this over-exposed image the legs look paler and less strongly pink than they did in life. The white tertial crescent looks relatively broad here.
As can be seen from these photos, the Rainham gull showed a combination of darker grey upperparts, black on the six outer primaries, mirrors on P9 and P10, pinkish legs and red orbital ring, all potentially invoking thoughts of vegae. But are some (or even all) of these characters also theoretically possible in a hybrid or back-cross? It's difficult to say for sure as many reported European Herring x LBBG hybrids are merely identified as such on plumage traits, rather than being of known parentage. On photos of typical Vega Gulls, the legs appear more strongly pink (light raspberry perhaps). According to some of the literature and what I noted on skins at Tring, if there is a mirror on P9, it is more usually either on the inner web or spread across both webs, whereas on the Rainham bird it is on the outer web (at least on the right wing). And for Vega, perhaps the trailing edge of the secondaries should show a broader white margin? I'm not yet clear on that.

This shot gives a slightly clearer view of what's going on with the white mirrors on P9 and P10. Note also the faint dark mark near the bill tip, possibly indicating residual immaturity.
Intriguingly, however, the bird hasn’t finished growing its outermost primary – something that any likely parent species should have long done by the beginning of March. As has already been pointed out to me by one gull expert, the fact that it is still growing P10 at this time of year could be an indication of origin in the far north, as gulls from Arctic areas moult later than those further south. Vega Gull certainly has a later moult than European Herring Gull, and the Rainham bird's primaries with broad, well-preserved white tips again fit. The literature suggests that white sub-terminal markings in the mid primaries (P5-P7) might often form a so-called 'string of pearls', rather like Slaty-backed Gull L schistisagus, but on examining this feature on 21 skins from eastern Asia labelled as vegae at Tring, almost 40 per cent actually showed very narrow white crescents or even fainter borders very close to those of the Rainham bird (see below). On the downside, however, perhaps P8 in the Rainham bird is too extensively blackish in these photos compared to Vega.

Adult Vega Gull wing-tip lacking the white sub-terminal spots - the so-called 'string of pearls' - on P5-P7, instead having the faintest white borders separating the grey tongues from black tips. Almost 40 per cent of 21 vegae skins in the Tring collection had a pattern broadly similar to this. Photo: Dominic Mitchell © Natural History Museum.
Adult Vega Gull wing-tip showing much more obvious white tongue-tips on P5-P7. This was the commoner of the two vegae patterns in the Tring collection, though several skins were intermediate in this respect.
Photo: Dominic Mitchell © Natural History Museum.
What I'd like to establish next is if hybrids ever show such a similar pattern of white in the mid-primaries, and also the extent and pattern of black on P5-P7 (again more variable than I expected on the vegae skins at Tring). To that end, I'm very keen to obtain photos of European Herring x LBBG hybrids, ideally showing the open wing-tip pattern, and especially if their parentage is known. Please leave a comment here or contact me directly if you can help; equally, if you can provide informed comment from experience on this bird that one larid aficionado has already said "could well be a Vega", I'd like to hear.

* Thanks to those who have already commented privately and in person, and to Hein van Grouw at the Natural History Museum, Tring.

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