Bluebirds build their nests in nest boxes or in tree cavities that have been abandoned by woodpeckers or other animals. The bird's bill is too weak to excavate a nest cavity for itself.
Also, the nest needs to be far enough off the ground to provide some protection. Because bluebirds are often attacked by other birds for nesting space and harassed by predators, it is useful to set up raised nesting boxes that admit the small bluebirds while keeping out larger birds and pests, such as squirrels or raccoons.
Female bluebirds build the nests themselves. The resulting nest is small, shaped like a cup and lined with feathers, hairs, stems and grass. The female then lays usually four, five or as many as eight light blue eggs and incubates them herself. She also broods the chicks. The male bluebird guards the nest while the female searches for food, but he doesn't incubate the eggs or brood the chicks. However, he helps the female feed the chicks and keep the nest clean.
In the western bluebird, the pair might have helpers who assist them in raising the chicks. These helpers are most likely the older offspring of the pair who couldn't find mates or whose own nests failed.
Sometimes, if their habitats overlap, eastern bluebirds and mountain bluebirds interbreed.