Identification of immature tropicbirds is tricky, but my thoughts are as follows:
1. Not Red-billed due to the lack of black nape collar.
2. Not Red-tailed as this species has rather coarsely, darkly and densely barred upperparts extending onto the crown and nape. This is obvious on the only immature Red-tailed photo currently on the Oriental Bird Club images website:
http://orientalbirdimages.org/search.php?action=searchresult&Bird_ID=1074&Bird_Image_ID=5265&Bird_Family_ID=&p=7
Further, while sizeable, the bird today did not appear to be as large and bulky as Red-tailed is supposed to be.
3. This leaves White-tailed, which has the least white in the wing of the three species in immature plumage, based on the illustration in Rasmussen and Anderton (Birds of South Asia). Peter's photos show well just how little (I did not see this in the field as it was flying away from us). Further, the barring in this species is described in the same guide as 'fine, widely spaced', which fits the bird today. The bill is stated to be 'dusky-yellow'; although Red-tailed may have this at some time in its progression to adult plumage, it seems to be standard for White-tailed.
I believe on range White-tailed is the most likely.
Geoff
