One comment I would have is that immature and subadult are not very useful terms. Both basically just refer to a bird that is too young to be in adult plumage, but are not specific to any other age class. I think you are using these to indicate 2nd or 3rd calendar year, in which case you would need to be clearer.
I thought that young kites in their first summer (i.e. about 1 year old) moulted into a plumage similar to adults, and that 2nd/3rd year birds were indistinguishable from adults on plumage (possibly still on moult contrast?), so personally I would be cautious about ageing kites beyond separation of juvenile/1st winter(/1st summer?) and adult.
John
