Note that it is absent from nearly all urban Kowloon, and only occurs in areas where there is recently reclaimed land. While this bird may well be one that occurred naturally in Sham Shui Po and was preying on sick ex-captive birds, it may well itself have been captive, such is its rarity in Kowloon.
Either way, although it is a 'migratory' bird, the evidence is still very strongly in favour of its being linked to the cage bird trade in some way.
Geoff