Dear Anding
It's a Chinese pond heron one of the commonest birds at Mai Po and around Deep Bay.
You're strongly to buy the Birds of HK and South China by Viney, Phillips and Lam, Chinese or English version and join outings
to Mai Po as soon as you can. The trip leaders and fellow birdwatchers will be happy to tell you what you ask. There are lots and lots
for you to learn.
Cheers
S L Tai