Looks like Northern to me, especially from first shot.
While there has clearly been a significant influx into South China with the exceptional weather in the last seven days or so, I think that with greater observer awareness, and a lot more people looking and taking photographs, it may turn out that this species is of rare, but regular occurrence in HK - I think the very first House Martin I saw in HK, at Tai Long Au on c 28 October 1987 was in fact Northern, as it too had a huge gleaming white rump, but it just shot through.
In fact, Brazil in Birds of East Asia says Northern occurs regularly in South China in winter, or something similar; does anyone know the reference he bases that statement on? Hopefully not on a misreading of Cheng who lumps Northern and Asian, though the Jabouille record of lagopoda for Guangzhou Bay may refer to Northern.
Mike Turnbull
