The Mountain Bluebird is most likely to be confused with other bluebirds. Male Mountain Bluebirds lack any reddish coloration on their underparts unlike Eastern and Western Bluebirds. Females are more difficult to separate. Eastern Bluebirds have a brownish throat and white belly while Mountain Bluebirds have gray throats and bellies. Western Bluebirds are browner on the breast than Mountain Bluebirds and have thicker bills. Male Mountain Bluebirds might be confused with other all blue birds like Indigo Buntings and Blue Grosbeaks but these birds have much thicker, conical bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.