The Western Bluebird (Sialia mexicana) is a small thrush, about 15 to 18 cm (5.9 to 7.1 in) 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.

They are sometimes confused with other bluebirds, but they can be distinguished without difficulty. The western bluebird has a blue (male) or gray (female) throat, the eastern bluebird has an orange throat, and the mountain bluebird lacks orange color anywhere on its body. It has a stocky build, and a thin straight beak with a fairly short tail.

Its posture consists of perching upright on wire fences and high perches. The western bluebird pounces on the ground when looking for food, such as worms and berries. It also flies to catch aerial prey, like insects, when available. The western bluebird consumes water from nearby streams and commonly used bird baths.