Initially, pairs aggressively defend a large area around their nest site, but as the breeding season progresses, the size of the defended area decreases and becomes more localized around the nest. Mountain Bluebirds defend territories against others of their own species, or conspecifics, as well as against other bluebird species in areas where their breeding ranges overlap. Mountain Bluebirds are basically monogamous. But, as with other bluebirds, both males and females sneak copulations with individuals that are not their mates. For this reason, males guard their mates, from the time a pair forms until the female lays her eggs.
Some pairs mate for more than one breeding season.
As in many species, the fact that many pairs mate with each other year after year probably reflects a male and female fidelity to a particular breeding site rather than to one another. Like other bluebirds, Mountain Bluebirds compete with House Sparrows and European Starlings for nest sites. Only the female builds the nest, and it takes her anywhere from a few days to over a week to complete. Working most diligently in the morning, she constructs the nest of grass, weed stems, pine needles, twigs, rootlets, bark, and, sometimes, wool, hair, or feathers. Males are very attentive to their females during the nest-building period and spend most of their energy guarding their mates.