As Tung mentioned, the features on Northern House Martin include a larger white rump (extending further onto the uppertail coverts), cleaner white underparts and a grey underwing. It is also larger than Asian with a deeper tail fork. In contrast, Asian often looks tiny (smaller than Pale Martin) with a square-cut tail, strong greyish wash to the underparts, narrower white rump (often with dark steaking if seen well) and blackish underwing coverts.
The recent photos taken by John Holmes at Mai Po show Asian House Martin very well.
http://www.hkbws.org.hk/BBS/redi ... o=lastpost#lastpost
Note that some field guides show Northern to have a white underwing - this only applies to the european subspecies. The eastern has a darker grey underwing, but this should not be as dark (blackish) as on Asian.
On this basis, I agree that the first photo posted by xiaocao shows a Northern House Martin (for the reasons given by Tung). The second photo looks like Asian House Martin to me.
On the basis of range, Northern House Martin should be expected to occur occasionally in South China, and has been predicted in the past as a likely addition to the Hong Kong list.
