Michael,
There are several records of Northern Goshawk in S. India, south to Kerala. Whether these are wintering birds originating in Siberia or from Himalayan populations is uncertain; Siberian birds are much more migratory than, for example, European breeders and seem the most likely source.
However, if the bird in the image is a Crested Goshawk (structurally it certainly looks like one and there is some barring visible on the flanks) I wonder how many of the S. Indian Northern Goshawk records might have been mistakenly identified on the basis of extensive underpart streaking, something which isn't fully documented for Crested Gos. It would be interesting to know whether N. Gos has ever been collected in S. India or whether all records are sight records.
Satrow,
As far as I'm aware moult and plumage development in Crested Gos. is the same as that in all the other accipiters, i.e. only two plumages exist: juvenile and adult, with adult plumage attained with the complete replacement of juvenile feathers at an age of about 8+ months. The only exception to this rule that i know of concerns Northern gos which, as I mentioned above, sometimes retains a couple of juvenile flight feathers (always secondaries?)after the first complete moult and can thus be confidently aged as '1st adult' or somesuch. It is not surprising perhaps that this large accipiter sometimes shows a plumage development more in keeping with some larger raptors. I wasn't aware that Americans had developed an ageing system for raptors using the term 'immature' and would be interested to know more details of how this works. In Europe the standard system is to refer to plumage generations present for larger species (aquila sp etc) and calendar year terminology is used for species with more consistent patterns of development (falcons and harriers, for example).
Spud