Any of three North American bird species (songbird genus Sialia) of the chat-thrush group. The eastern bluebird (S. sialis), which is 5.5 in. (14 cm) long, and the western bluebird (S. mexicana) are red-breasted forms found east and west of the Rockies, respectively. The mountain bluebird (S. currucoides), also found in the West, is all blue. Bluebirds arrive from the south in earliest spring. They live in open country and woodlands and nest in holes in trees or in fence posts and bird boxes.