The Cave Swallow can be told from all swallows except the Cliff Swallow by its pale rump. Cave Swallow has a darker forehead and a paler throat than the Cliff Swallow. The Cave Swallow's tail is more squared at the tip than most other swallows.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.