The parents continue to feed the first brood almost until the second brood hatches. In the southwestern United States western bluebird flocks are nomadic in winter, traversing large areas in search of berries and water. Highest densities of western bluebirds were observed in plots that had been irregularly cut in strips. Severely thinned plots had lower western bluebird densities than irregular strip cut plots, but lowest western bluebird densities occurred in untreated plots. The combination of lower temperatures and more precipitation during the winter and early spring is important in determining the survival of permanent residents including western bluebird.
Western bluebirds are more restricted in nest box selection than mountain bluebirds.
Western bluebirds are primarily insectivorous but also consume fruit from late summer to early spring. Most of the plant foods eaten by western bluebirds are fruits. Populations in the southwest have probably declined due to forest closure as a result of fire exclusion. Grazing reduces herb density and may benefit mountain bluebird at the expense of western bluebird. It may also be responsible for the near extinction of mainland populations west of the Cascade Range in Oregon and Washington, and reduced numbers on nearby islands.