Well done all the photographers - and the photos are going to be very important because this is a very difficult bird.
I see why others have gone for White-tailed and they might be right, but...
I am not convinced that any of the text descriptions in field guides are very accurate as there is very little published primary data out there (actually surprisingly little for such widespread species).
Instead of looking at field guides I have done a net search for photos. This wasn't very productive but was enough to convince me that bill colour of non-adults is (a) variable, (b) little understood except in Red-billed (c) blackest in fledglings and becoming paler later. The key point was that the photos which I looked at provided limited support for the comments on bill colour in Harrison.
I did also find one web site which discussed id from Don Roberson who is a Californian seabird expert http://montereybay.com/creagus/tropicbd-id.html
This gave me the best match for our bird - not the first four in the paper but the Red-tailed photo which comes after the specimen photos. Note the black leading edge to outer 3 or 4 primaries.
Also note how extensive black is on the primaries in (all?) White-tailed photos, admittedly mostly adults. Now, this could increase with age but I find this counterintuitive: in most largely white seabirds (actually most white waterbirds) the amount of black decreases with age [there is a reason for this, white plumage wears more quickly but is more conspicuous and hence is relatively more advantageous for adult birds]. White-tailed Tropicbird could show a different pattern but unlikely I think.
To sum up I think that bill colour on our bird was within the range for both Red and White-tailed (but not Red-billed) and extent of black on primaries matches at least one Red-tailed photo (albeit more than I would have expected from published texts) but is not extensive enough for White-tailed. I haven't found any other plumage features which help - overall extent of black fringes to body feathers is going to be dependent on age and wear for example. Bill structure might help - I think it is heavy (which supports Red-tailed) but not obviously so.
At the moment I wouldn't like to call this one definitively and I think that we might need skin examination but my opinion for now is (still) Red-tailed.
Great bird!
Mike
