The Chiffchaffs that occasionally turn up in Hong Kong (this is the 7th record, I think) are Siberian Chiffchaffs Phylloscopus collybita tristis. At the moment, the IOC lists tristis as a subspecies of Common Chiffchaff and this is followed in the Hong Kong Bird Reports.
However, it is possible that some time in the future, tristis will gain specific status. Modern field guides to Asian birds e.g Brazil (2009) and Rasmussen (2005) tend to treat tristis as a separate species.
David
