The white-tailed tropicbird (Phaethon lepturus) is a tropicbird, smallest of three closely related seabirds of the tropical oceans and smallest member of the order Phaethontiformes. It occurs in the tropical Atlantic, western Pacific and Indian Oceans. It also breeds on some Caribbean islands, and a few pairs have started nesting recently on Little Tobago, joining the red-billed tropicbird colony. In addition to the tropical Atlantic, it nests as far north as Bermuda, where it is locally called a "longtail".
The white-tailed tropicbird breeds on tropical islands laying a single egg directly onto the ground or a cliff ledge. It disperses widely across the oceans when not breeding, and sometimes wanders far. It feeds on fish and squid, caught by surface plunging, but this species is a poor swimmer. The call is a high screamed keee-keee-krrrt-krrt-krrt. Sailors nicknamed the tropicbird the "bosun bird" due to the call's resemblance to a bosun's whistle. 
The adult white-tailed tropicbird is a slender, mainly white bird, 71–80 cm long including the very long central tail feathers, which double its total length. The wingspan is 89–96 cm, and there is a black band on the inner wing There is black through the eye and the bill is orange-yellow to orange red. The bill colour, pure white back and black wing bar distinguish this species from red-billed.
Sexes are similar, although males average longer tailed, but juveniles lack the tail streamers, have a green-yellow bill, and a finely barred back.
The white-tailed tropicbird does not have a yearly breeding cycle; instead breeding frequency depends on the climate and availability of suitable breeding sites. The bird can reproduce 10 months after the last successful breeding, or 5 months after an unsuccessful one.
There are six races:
- P. l. lepturus—Indian Ocean
- P. l. fulvus (golden bosun)—Christmas Island. This form has a golden wash to the white plumage
- P. l. dorotheae—tropical Pacific
- P. l. catesbyi—Bermuda and Caribbean
- P. l. ascensionis—Ascension Island
- P. l. europae—Europa Island, s. Mozambique Channel
- ffrench, Richard (1991). A Guide to the Birds of Trinidad and Tobago (2nd ed.). Comstock Publishing. ISBN 0-8014-9792-2.
- Harrison, Peter (1996). Seabirds of the World. Princeton: Princeton University Press. ISBN 0-691-01551-1.
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