The western bluebird is a small thrush, about 15 to 18 cm in length. Adult males are bright blue on top and on the throat with an orange breast and sides, a brownish patch on back, and a gray belly and undertail coverts. Adult females have a duller blue body, wings, and tail than the male, a gray throat, a dull orange breast, and a gray belly and undertail coverts. Immature western bluebirds have duller colors than the adults, they also have spots on their chest and back. These color patterns help play a part in the mating ritual, when males compete for breeding rights to females.