The juvenile Sooty Tern is similarly dark but has a pale belly, a forked tail, and lacks the pale cap. The Black Noddy is very similar but is smaller, has a more slender bill, a more prominent white cap, and is blacker.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.