Here are some other tips regarding bluebird boxes:

Place eastern bluebird nest boxes in open grassy areas that are accompanied by widely scattered trees and shrubs. Open areas are preferred, as placement of boxes in or near areas thick with brush and woody vegetation likely will result in habitation of many boxes by house wrens. Boxes should be placed at least 100 yards apart to accommodate individual territories established by nesting pairs during the breeding season. Boxes should be placed well away from buildings because of the high concentration of house sparrows generally found near human habitations.