Mike, one thing that was noted when watching the Northern House MArtin last week was that nearby Asians could at times seem to also have a fairly large rump patch. John Holmes's recent post (
http://www.hkbws.org.hk/BBS/view ... &extra=page%3D1) shows well an individual with a surprisingly large rump patch (extending to the longest uppertail coverts) which is still clearly Asian. Admittedly, this bird shows dark centres to the longest feathers but these may not be visible on photos such as the first on this thread.
In addition to the square-cut tail, another feature of Asian (just) visible on the first photos is that the black of the face seems to extend down onto the chin - on the recent Northern, this cut through the eye direct to the bill.
I think to get an acceptable record of Northern you need not only the extensive white rump but also the larger size, deeper tail fork, whiter underparts, paler underwing and head pattern. But they probably are being overlooked - I learned a lesson on the recent bird because I had expected a whiter underwing, so I dismissed the bird as Asian. Had I found the bird, the record would have been missed. Fortunately Paul Leader was more persistent with following up on the ID!
