A juvenile is a bird in its first plumage. An adult is a bird with a plumage which is not changing in subsequent moults (at least not much). Useful definitions for immature and subadult plumages are that an immature is a non-adult bird and a subadult is a bird which is neither juvenile nor adult (there are others - for example an immature bird is still not capable of reproducing).
Young Black Kites in autumn are very patterned. the Underparts are strongly streaked pale buff and the covert have pale tips. The whole plumage is uniformly fresh. They are quite different looking from adults.
By the first spring, the birds have become worn and also rather bleached and are more difficult to tell from adults because much of the juvenile patterning has been lost. The plumage is still uniform, but not fresh any more. Additional characters
helping in ageing are the secondaries which are barred almost to the tip (adults have a wide black terminal band) and darkish eyes (more reddish in adults).
During the first spring the young birds start to moult and the moult is completed in autumn (in autumn you can some times identify three age classes: juveniles plus second year birds and adults, if you can see which feathers are not yet moulted and tell if they are juvenile or adult). After the moult is complete the plumage is adult-like, but still not fully mature. Exact ageing is probably not possible. This is based on ageing Black kites of the European nominate (sub)species, lineatus is somewhat
different.
The type locality of lineatus is "China" so Hong Kong birds very probably should be called by that name independent of how the different (sub)species of Black Kites are split or lumped.
