Interesting! From what I can see in photos and there aren't too many good ones--the gular stripe on Japanese Sparrowhawks appears less distinct than Besra or Crested Goshawk. This birds seems to have a have a quite pronounced gular stripe.
Also, from what I can see for the Avifauna of Hong Kong, Japanese Sparrowhawks seem to start showing up in October. There does not appear to be any summer records of this species so it would be interesting if this bird could be IDed as a Japanese Sparrowhawk.
