The long, pointed wings and long tail mark this bird as being a falcon. It is most likely to be confused with other falcons. Merlin is much smaller, has a less-distinct mustache mark, and a more strongly barred tail. Prairie Falcon has a black underwing patch and lacks the strong moustache mark. Gyrfalcon is larger with broader wings and tail and lacks the bold moustache mark. When perched, other falcons' wings do not reach the end of the tail as they do in the Peregrine.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.