Adult Red-billed Tropicbird is larger than White-tailed Tropicbird with a slower wingbeat and lacks black patches on upperwing. Immature Red-billed has less distinct barring on the back. Adult Red-tailed Tropicbird lacks barring on the back. Immature Red-tailed has less black on the nape and in the primaries.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.