I'm surprised there haven't been more comments about this bird, but maybe others have been like me with the photos. I've come back to this post a few times and I am still not entirely sure about the species. But I think it is worth trying to determine the ID of this bird, so here goes...
It is clearly a female Aythya diving duck, of which there are a few species in HK. Common Pochard can be most easily ruled out, on a number of factors.
The relatively square head initially seems to suggest Tufted Duck, but the dark eye should rule that species out. I also think the bill is too dark. I wonder whether the bird has been diving, which sometimes affects the apparent head shape. Greater Scaup also doesn't fit in terms of eye colour and bill pattern (and the head is too dull for a typical Scaup anyway).
That leaves Baer's Pochard and Ferruginous Duck. These two species can be fairly similar and I don't have a huge amount of experience with either. For me the dark flanks would favour Ferruginous over Baer's. I also would expect Baer's to typically look deeper-billed (especially at the base), and I think the bill pattern fits Ferruginous better than Baer's - not as dark as Baer's, with a black tip similar to Tufted. And the head pattern also seems better for Ferruginous - I would expect Baer's to be darker-headed and less likely to show the paler pattern seen on this bird.
So overall I think that this bird is a female Ferruginous Duck. I would welcome any other comments though, especially from anyone with more experience with these species.
It's a very good record (especially on a fish pond!), congratulations.
