Wednesday, 26 May 2010

Oh my godwit!

This Black-tailed Godwit at Rainham is strongly suggestive of nominate limosa rather than the expected islandica subspecies. Note the washed-out tone of the chestnut head and breast, the fact that it doesn't extend down over the breast, and the plain upperparts.

I've been out in the field quite a bit since last weekend's post, but with little to show for it. The same applies to this morning's jaunt to Rainham, then Grays, then back to Rainham and finally the Warley area. Of the few minor sightings of interest, which have included more Common Buzzards, Hobbies, Cuckoos and Yellowhammers in suitable breeding areas (but still no Turtle Doves), possibly a wader is of most interest.

Shorebirds have often been conspicuous by their absence at Rainham this spring, especially on the reserve itself, despite plentiful pools and patches of standing water (albeit rather overgrown ones). So finding a late Greenshank on the reserve was nice, as was a Black-tailed Godwit - the first I've seen here for a few weeks. This latter bird also stood out from those typically seen on spring passage in Britain - which are usually richly coloured islandica individuals - with its subdued breeding plumage, the chestnut being washed out in tone and also restricted largely to the head and neck. The upperparts were also relatively plain.

As almost all passage and wintering Black-tailed Godwits in Britain are considered to be islandica, and the British breeding population of nominate limosa is only 50-60 pairs, just how unusual is this sighting (assuming it indeed relates to limosa)? The Birds of Essex (Wood 2007) states: "There is no direct ringing evidence linking Essex with limosa. Indeed, there is no evidence of limosa from continental breeding populations occurring in Britain on passage (Migration Atlas); an Essex specimen, one of many ascribed to limosa in the British Museum of Natural History (Vernon 1963), was presumably from the small British population of this race."

After a quick search the information on nominate limosa occurring in the London Area seems even more vague, if not non-existent. So this sighting is potentially significant - assuming, of course, that it does relate to limosa? The most useful recent reference I can find is Vinicombe 2005 (Birdwatch 154: 18-20), in which it is stated: "... a summer-plumaged islandica in spring may be separated from limosa if it shows all of the following features: (1) a shorter bill and legs (the bill averages about 1 cm shorter); (2) at least 70 per cent of its body feathers are in summer plumage; (3) the orange plumage is deeper and more 'saturated' and rufous in tone; and (4) the rufous on the underparts extends further down onto the flanks and is more extensively barred with black on the belly, flanks and undertail -coverts." On points 2, 3 and 4 the images of the Rainham bird certainly seem to suggest limosa and not islandica; point 1 is perhaps impossible to be sure of in the field, given variation and differences between males and females of both taxa.

And what does first-summer islandica look like? Vinicombe comments that "first-summer Black-tailed Godwits retain greater numbers of non-breeding feathers, breeding plumage being restricted to the head, scapulars, mantle and the sides of the breast". This is clearly a potential pitfall for the Rainham bird, though I've read elsewhere that even first-summer islandicas will likely show some variegated patterning on the upperparts.

I feel on balance that the Rainham bird has good credentials for limosa, but all comments welcome.

Also on the reserve today were this brief Greenshank and two Yellow-legged Gulls (either side of the adult graellsii Lesser Black-backed Gull), neither of which appears to be the bird hanging around Aveley Pools over the last two weeks.


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