Tuesday, 4 May 2010

Off-season gulling

An unseasonal mystery gull at Rainham - comments welcome.

Having downloaded my images from the last two trips to Rainham, I have been looking again at Friday’s oddball gull. OK, it’s not gulling season, and you may not like gulls anyway, but hear me out …

I hadn’t planned to do the tip, but as I drove past the entrance I noticed that refuse was being dumped and bulldozed within easy scanning distance. So I stopped to check quickly the 100+ large gulls in the fray and on the adjacent spoil heaps, hoping for a Yellow-legged for the day list. What I got was something rather different.

The bird is clearly a large gull, either adult or almost so – there are remnant dark markings on the bill tip in these poor images (taken at long range with just a 300 mm lens and 1.4x converter). It is big, obviously more so even than Herring Gull, and in one of the shots it can be seen driving a Herring Gull away.

The tone of the upperparts is an ashy grey, darker than Yellow-legged Gull but not quite as dark as in adult graellsii Lesser Black-backed Gull, which was present for direct comparison. Note that in the open wing shot, the mystery gull appears slightly paler than when perched and than it did in the field. It was always possible to see clear contrast between the black wing-tips and the ashy-grey upperwings. The outer primaries themselves seem worn, but even allowing for this there is very little white visible, apparently ruling out what might be the most likely candidate, nominate argentatus (or ‘Scandinavian’) Herring Gull, which often looks appreciably bigger than argenteus but which, though darker than that subspecies, is also slightly lighter in mantle colour than this bird seemed. There is a small white mirror on P10, and the black of the wing-tip appears to extend to P6 at least.

One curious feature of the upperwing is the rather broad white trailing edge to the secondaries, obviously visible even at range. It is less visible on the inner primaries, though because these are more spread in the open-winged shot that may be something of an illusion. The legs were pinkish.

One possibility could be a hybrid, perhaps Great Black-backed x Herring Gull – this rare combination might account for the upperpart colour and obvious white edge to the secondaries, but does the bird show enough white in the outermost primaries, bearing in mind the former’s usually extensively white P10 and white mirror on P9? The only other wing-tip/upperpart colour combinations that approach this seem to be those of Slaty-backed Gull and occidentalis Western Gull, possibilities too outrageous to take seriously … and anyway, structurally can they even be considered?

Back on planet earth, I will look for this bird again, with the 500 mm lens next time and up close. But experience says a repeat showing is probably unlikely, especially in spring with large gulls now shipping out of the Thames en masse. In the meantime, all comments as to its likely identity/parentage are welcome.

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