This is the wonderful "Mrs Gould’s sunbird" (Aethopyga gouldiae). I will be writing more about them over the next couple of days...with some more pics; but this in-flight shot is one of my faves, with tricky lighting, a fast moving and very small subject.
As mentioned, you are almost guaranteed to see one on the coral trees of Tai Po Kau right now...but hurry, as its only a winter visitor!
I would highly recommend taking a birding guide with you....as they can show you sooooo much more, and its a GREAT time to visit with visiting birds galore ....sunbirds, minivets, leaf birds, and a whole bunch more.
See my recommendations on https://www.wildcreatureshongkong.info/wildlife-tours