Hi Bittern!
Common Green Magpie
Cissa chinensis and Indochinese Green Magpie
Cissa hypoleuca are similar species with their main ranges in Southeast Asia. In China they both extend as far east as Guangxi (according to Mackinnon). Both are resident species within their range, so a sighting in Hong Kong would presumably be of a released/escaped bird, although I don't know how common they are in the cagebird trade. The bird at Lau Shui Heung was feeding low down in forested undergrowth and was difficult to observe - rather like in the wild, in fact!
David
www.hknaturewalks.com
