The taxonomy of stilts is pretty complicated, and there is still no consensus on how many species are involved in the Black-winged Stilt complex (some authorities argue just one species, others as many as five). This is complicated by the fact that each taxon is very variable, especially in terms of head pattern.
Black-necked Stilt
Himantopus (himantopus) mexicanus is an American species (see
http://www.birdforum.net/opus/Black-necked_Stilt). It has extensive black eextending from the back onto the crown and surrounding the eye (with a small white spot near the eye). This bird clearly does not fit this ID.
White-headed Stilt
Himantopus (himantopus) leucocephalus occurs in Australia and Indonesia. It has apparently turned up in Japan as a vagrant, which would suggest it is plausible in Hong Kong. It typically has a white head and neck with a black nape, similar to this bird.
The compication is that Black-winged Stilt has a very variable head pattern (look at the variability in a small numbers of birds here:
http://orientalbirdimages.org/se ... _ID=&pagesize=1), and can show a similar pattern to the typical White-headed with a white head/neck and black nape (I have heard this is commoner in south-east Asia than elsewhere, but I don't know if there is definite evidence for this). One key difference seems to be that the black feathers on White-headed are actually longer than the surrounding white feathers, creating a slightly 'fluffy' appearance - compare
http://orientalbirdimages.org/se ... Family_ID=&p=8.
The bird in question does not seem to have these longer nape feathers, and therefore seems to me to fit better for Black-winged Stilt. I have seen individuals like this in HK before and similar confusion about the possibility of White-headed Stilt has arisen in the past.
