Male Ruby-throated Hummingbirds have deeper red throats, black masks, are somewhat smaller, and lack the Broad-tailed Hummingbird's distinctive wing whistle. Females are larger and bigger-tailed than the small Calliope Hummingbird, but are similar to Rufous and Allen's Hummingbirds. Broad-tailed is best identified by its larger size, large, broad tail, and restricted amounts of rufous in the tail, more blue-green back, whiter undertail coverts, as well as its call note.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.