The Western Bluebird is most likely to be confused with other bluebirds. Male Western Bluebirds have blue throats while male Easterns have orange-red ones. Male Mountain Bluebirds lack any reddish coloration on their underparts. Females are more difficult to separate. Western Bluebirds have gray throats and gray bellies while Eastern Bluebirds have a brownish throat and white belly. Western Bluebirds are browner on the breast than Mountain Bluebirds and have thicker bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.