The Cassin's Kingbird is similar to several other species of kingbirds and can best be identified by its dark breast band. Tropical and Couch's Kingbirds have forked tails and darker cheeks. Western Kingbird has white outer tail feathers. Thick-billed Kingbird usually has whiter underparts and has a darker head. Brown-crested, Ash-throated and Great Crested Flycatchers are somewhat similar but have darker, browner crowns and brown and rusty tails.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.