As John mentions, he had previously shown this photo to me, and I agreed that it was a Besra. In this case, John has been lucky enough to see some of the useful plumage features. Useful features visible on this photo are the gular stripe, heavily marked underparts and plain grey cheek. These effectively eliminate all other Hong Kong accipiters except Crested Goshawk, which in plumage is similar to Besra. Unlike Crested Gos, the underwing coverts are fairly heavily marked and the undertail coverts are not particularly prominent. For a useful comparison, see Thomas's recent photos of a Crested Goshawk here:
http://www.hkbws.org.hk/BBS/redi ... o=lastpost#lastpost
Of course, it is often not possible to get such good details of accipiter plumage. In this case I tend to rely on a combination of size, structure and habitat.
Crested Gos is large (females can look very large) with broad, paddle-shaped wings "pinched in" at the base and with bulging secondaries (again, compare Thomas's photos). The tail tip is rounded, which can be useful on perched birds. It is generally found in forest such as Tai Po Kau.
Besra is medium size. The wings are fairly short and broad but without the bulging secondaries and narrow base of Crested Gos. The tail is moderately long and square-tipped. It seems to be associated with smaller woodlands on the edge of open country, and seems to be the commonest accipiter at Mai Po.
Japanese Sparrowhawk is small (males can look tiny). The wings are relatively longer and more pointed than Besra. The tail is often noticeably short and square-tipped. The gular stripe is weaker or absent and the underparts are more finely barred. Habitat is similar to Besra, but Japanese occurs mostly on passage with some birds in winter.
Eurasian Sparrowhawk is moderately large (between Besra and Crested Gos). The wings and tail are both relatively long and narrow. It lacks the gular stripe, is finely barred below and has a well-marked supercilium. This species is fairly uncommon on passage and in winter, mostly in fairly open country.
Chinese Sparrowhawk is distinct, with fairly long, pointed wings and therefore sometimes looking more falcon-like. The black wingtips and plain underparts are useful plumage features. It often occurs in flocks and is widespread during passage, especially in spring (and especially this year!)
Of course, accipiters are all tricky to ID and it takes practice and field experience. I do not claim to be as expert as others in HK, and often leave birds unidentified. No doubt others can elaborate on or correct my various comments above.
