Sunday, 17 September 2017

A Tale of Two Sand Martins?

When the waders start to get boring, there are of course the hirundines to look through. Typically, this means the one or two Sand Martins Riparia riparia or Pale Martins Riparia diluta that turn up annually in amongst the Grey-throated Sand Martins Riparia chinensis at either Tu Cheng or San Gu, though precisely what they are (perhaps both species) remains unclear. There were three adults to go at on Thursday afternoon, two tidy-looking ones and one that looked a bit scruffy. The scruffy one only landed for a few seconds, leaving me with only one tidy-looking one to play with.

There isn't much that can be said really about this Sand Martin, other than 'it looks like a lot like one'; the caveat here of course being that so do the fohkiensis Pale Martins that pass through Hong Kong (and should be breeding across the water in Fujian). It can at least be aged as adult due to the lack of broad pale fringes and a moult limit in the inner primaries. Although difficult to see in the photos, it does have tarsal feathering, and the shape and extent of this might be of some use. Each leg has two tight clumps of long 'spur-like' feathers on it, and these 'spurs' are located immediately above the hind toe. There is no feathering higher up on the tarsi than this which (given how well-developed the two clumps of 'spurs' are) one would expect on an adult Pale Martin. From what reading I have been able to access, this is actually the kind of distribution of tarsal feathering I would be expecting to see on a Sand Martin, hence that is what I presume this first bird to be. It (or another near-identical individual) was also present in the same spot at San Gu on Friday.

Once more there isn't much that can be said about this bird, but I think it could be described as rather dumpy/stocky. It's difficult to say precisely whereabouts the primaries fall, but I would estimate that it is about level with the tail tip. In addition to lots of dark steely-greys in the plumage, there are also plenty of dark tans, all of which suggest Sand Martin. The scruffy bird also returned on Friday and hung around for longer, and this bird proved to be much more interesting indeed!

There are so many differences between this individual and the other two that it's hard to know where to start. First of all, it is obviously paler and greyer than the other birds above, with a much stronger contrast between the greyish back and wing and far less between the face and throat. It also differs in structure, being slenderer and lacking the 'chunky' look of the Sand Martins (especially at the neck and shoulders); indeed, it looks variably 'scrawny' or 'emaciated'. It is even smaller and slenderer than Grey-throated Sand Martin, with a much smaller bill (and seems to have a thinner/less deep lower mandible than the Sand Martins, giving a finer bill). It also has a face pattern more akin to Grey-throated Sand Martin given the absence of a white half-collar on the neck, a quick way of picking out 'side on' Sand Martins in amongst flocks of Grey-throated Sand Martins. The throat patch is therefore much smaller than on Sand Martin, and the border between it and the ear coverts more diffuse. It also adopted a different posture to any of the other (hundreds of) martins around it, permanently crouched and almost lying flat on the ground.

Although various factors might have resulted in a Sand Martin coming to have such an appearance (it could be a runt, actually be emaciated, be individual variation, etc.), what I find interesting about this bird is that, although it might not be one, it is the first individual to turn up here that actually looks like a (textbook) Pale Martin! Most texts I have found on the subject deal exclusively with Pale Martins in western parts of its range (desert areas), where such birds are reportedly small, pale, and greyish, not at all unexpected given the habitat. However, small, pale, and greyish seems to be a very good description of this bird! As there are no texts describing eastern race fohkiensis and any of the morphological differences it might have from ijimae Sand Martin, it's hard to be certain even what the 'control group' of Sand Martins are (i.e. the first bird(s) above), but from the shape of the tarsal feathering, they are likely Sand Martins. Unfortunately, I have no images sharp enough to determine the shape and extent of tarsal feathering in this individual, so a question mark continues to hang over its identity. However, I can't help but feel that it probably is a Pale Martin, and perhaps even one from a population other than fohkiensis.

My customary default at San Gu at this time of year is to go and find some Little Stints Calidris minuta. There are literally dozens around at the moment (ranging all the way from orange-rufous to almost sepia-toned), and they come in all shapes and sizes!

It's all very repetitive stuff year in year out, but at least I did manage some sharper shots of one or two birds over the weekend (which makes for a pleasant change from recent form). The martins only land on the road when the wind is very strong, and with these now easing after the passing of a nearby typhoon, it's unlikely I'll manage any better shots of these birds than those I already have above! Above photos taken at San Gu, Tainan County 14-17/9/17.

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