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