Thanks to Alfred, Jason, Tsu Shi and Tun Pin for your comments.
When faced with a potential rarity, the first step is always to eliminate the 'default' common species.
In the case of the small calidrid, the default species is Curlew Sandpiper. So, is it one, and if not, why not?
Ang picked it out at first due to the shorter-than-usual bill. This in itself is not enough to rule out Curlew Sandpiper, as some males can show shortish bills, but it does at least put us on the alert. Are there any other features which are 'odd' for Curlew Sandpiper?
Well, the overall body shape is rather 'dumpy' for a start. Curlew Sands tend to look elegant and slender, a combination of longish legs and a quite attenuated 'back end'. This bird has a short primary projection (very little black visible beyond the tertials), and the wingtip appears to fall just beyond the tail tip. Again, this is suggestive rather than diagnostic of Dunlin . Most Curlew Sandpipers show a longer primary projection beyond the tertials and the wingtip tends to fall well beyond the tail, but this isn't always the case (as, for example, with this bird).
Unfortunatetly the bird is wading in deep water, which doesn't help us to asses the leg length easily. However, the visible tibia (between the body feathers and the leg joint) is very short, and in the shot I posted yesterday, Ang has managed to capture one leg raised momentarily above the water, revealing a very short tarsus. This is the first solid indication that this cannot be a Curlew Sandpiper.
Going back to the bill and head, the bill curvature is not quite even - not as kinked as a Broad-billed Sandpiper - but not as smooth a curve as is typically the case with Curlew Sandpiper, and this is also a feature of many Dunlin. The face pattern is rather bland, with the supercilium virtually disappearing behind the eye. Curlew Sandpiper should show a much more prominent supercilium behind the eye, whereas non-breeding plumaged Dunlin is typically bland-faced. Finally, Dunlin tends to show a high forehead and rather domed crown (see the pictures here to see what I mean).
So we can give a number of reasons why it isn't a Curlew Sandpiper and why it IS a Dunlin:
1. Very short tibia and tarsus
2. Dumpy body and short primary projection
3. Bland face and indistinct supercilium behind the eye
4. Slightly kinked bill
5. High forehead and domed headshape
Aging this bird is not easy from the picture we have, but the lesser coverts (visible just below the scapulars) seem to have diffuse grey-brown edges and the wing coverts as a whole do not appear in neat rows. Both of these features suggest an adult. This is confirmed by the presence of black flecks on the ventral area, which must be the last vestiges of breeding plumage.
There are lots of races of Dunlin worldwide - up to ten depending on which taxonomy is followed. There's a helpful map showing the breeding ranges of the various races here, from which I conclude that five: actites, kistchinski, sakhalina, articola and pacifica, could potentially occur. Which one this is I wouldn't like to speculate - perhaps someone with more experience of Asian Dunlins can help.
Dunlin is a genuine rarity in Malaysia, with probably fewer than 12 records in the whole country. So congratulations to Ang on a sharp piece of spotting!
Another picture of the Dunlin (below) with a first winter Curlew Sandpiper (above) taken by Ang on the same day, which illustrates the structural and plumage differences between the two species very nicely.