[Tropicbirds] Immature Tropicbird sp. 鸏

The identification of this juvenile tropicbird is interesting, so I tried to ask the opinion of Dr. Nic Dunlop because he is familiar with both Red-tailed and White-tailed Tropicbirds. Dr. Dunlop is an animal ecologist with a long term research interest in tropical seabirds and had conducted research on Tropicbirds.
His reply is : The juvenile White-tailed Tropicbird has a continuous black area on the outer primaries (seen also in the adults). This is just a few light grey bars in the Red-tailed.

Here are 2 photos of  Red-tailed Tropicbirds on nests taken by Dr. Dunlop