Quick reply: voice as heard fits the description for Black-winged in HBW with "three or more high-pitched, descending, downslurred whistles" (though as I too recall from Ba Bao Shan, birds there do sing differently from this) and the dark saturata form from Hainan/Vietnam is said to be smaller.
Since nobody else is here to do so, can I just say that this bird had some feather damage (in the primaries, as seen in the photo) and some slight dishevelment in the upper parts, where, somewhere in the scapulars region or thereabouts. one feather seemed to be standing proud. There also seems to be some damage to an outer tail feather, and the possibility that it was simply missing some rump feathers might explain why it seemed to have a white rump band.
I therefore believe this is a Black-winged Cuckoo-shrike, probably of the form saturata from N Vietnam + Hainan (where maybe a short-distance migrant), with some question marks about how precisely it arrived here.
Mike Turnbull
