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. Birds of North America.