In south-central Washington western bluebirds select and defend nest boxes soon after arrival but mountain bluebirds continue to prospect. In Montana pairs using nest boxes on fenceposts or utility poles mostly stayed along the fence or utility line and its immediate vicinity. Mountain bluebird hatchlings are altricial, blind, and without feathers. Natal down appears on the second day.
Eyes begin to open by the fourth or fifth day.
Mountain bluebird nestlings are fed by both parents but usually only brooded by the female. In Montana large flocks of mountain bluebirds gather from October to November. In western Montana mountain bluebirds use whitebark pine snags for nest sites at high elevations. Preferred sites are open or semi-open. In Wyoming mountain bluebirds used an abandoned American dipper nest on the underside of a bridge.