The western bluebird was formally described by the English naturalist William Swainson in 1832 and given the binomial name Sialia mexicana.

The adult male is 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.

The western bluebird has been displaced from its natural habitat by the felling of trees; however it has adapted to coniferous forests, farmlands, semi-open terrain, and desert to survive.

Attacks by starlings can be reduced if the nesting box opening is kept to 1.5 in (38 mm) diameter to avoid takeover.

Nest boxes come into effect when the species is limited and dying out due to the following predators: cats, raccoons, opossums, and select birds of prey such as the Cooper's hawk.

Their beaks are too weak and small to dig out their own holes, so they rely on woodpeckers to make their nest sites for them.

They started egg-laying earlier, had higher nesting success and lower predation rates, and fledged more young in boxes than in cavities, but they did not have larger clutches of eggs.

According to genetic studies, 45% of western bluebirds' nests carried young that were not offspring of the male partner.