Bluebirds require a large, open area with low vegetation and a few scattered trees in order to perch and forage for insects, their dietary staple. Rural areas, meadows, orchards, parks, cemeteries, and the edges of suburbs make good sites.
Keep the box comfortable and free of predators by following a building plan designed specifically for bluebirds. The entrance should be 1-1/2 inches across for Eastern bluebirds and 1-9/16 for Western and Mountain bluebirds.
Use at least 3/4-inch-thick lumber for proper insulation. Western red cedar is naturally rot-resistant. Pine or plywood requires weatherproofing with linseed oil. Do not coat or paint the inside of the box.
Place the box at least 5 feet from the ground on a waxed or greased metal post to keep cats, raccoons, and snakes at bay. Trees, shrubs, and fences provide perches, but the box should be placed 25-100 feet away to deter climbing predators. If you have more than one bluebird box, space them 100 yards apart.
Open the box once a week from April through August to check on the well-being of the nest, eggs, and fledglings and to remove any unwanted tenants, such as starlings, sparrows, and blowfly larvae. As soon as the first young leave, remove old nests and debris, so the female can rebrood. Clean again in winter.
Note: Don't monitor the box once the nestlings are 12 days old or older. Opening the box at this time may cause premature bolting from the nest.