Adult males of these two species are easy to separate - JPF is rather dark overall on the upperparts (as shown on fkm's excellent photos), whereas APF is chestnut-brown above.
The confusion lies in the ID of 1st year birds and females. There seems to be a certain amount of disagreement in separating the species in these plumages. Unfortunately the subspecies passing through HK seem to be more similar than in other parts of the species range.
In my opinion, the photos posted by fkm show a 1st winter APF, with entirely chestnut upperparts. Aging as a 1st winter is based on the contrast within the greater coverts - some have been moulted, while others are retained.
Ho-fai's (trapped) bird is less obvious. I would consider this to be JPF based upon the maroon colours on the mantle - this is clearly different from fkm's bird. The tail and tertials are more chestnut than I would usually expect for JPF, however.
I find the contrast between throat and breast (and between breast and belly) to be difficult to judge and rather subjective. This feature seems to be strongly affected by light conditions. Gloss on the crown will also be affected by light, and probably also by the age of the bird.
Other features may be useful in separation of the two, and may be less subjective. Looking at photos, there may be some difference in the patterning of the tertials/coverts, and the undertail coverts may be slightly chestnut on APF (compared to pure white on JPF). It would be great if some of the HK photographers could get more good photos of these species in HK to help resolve this ID problem. The photos linked by fkm are very good, and are useful in ID.
I would be interested to know how the MP bird was aged/sexed as an adult female - I have little experience of handling these species and do not know which features are reliable for aging. I would have depended on bill colour (pale, plain) and lack of eye-ring as suggestive of a 1st-winter bird, but as Ho-fai suggests these features may not be reliable at this time of year.
