Starlings and house sparrows are cavity nesting birds imported from Europe during the late 1800s. Since they are more common near buildings, keep bluebird boxes at least 100 yards away. Some research indicates that light-colored cavities are less attractive to these intruders. Snakes are adept at climbing most short poles to rob nests. Use the same tricks that foil 'coons and cats, and leave nest boxes open at the end of the season to keep an otherwise helpful snake from taking up residence.
Blowflies can be deadly parasites of young bluebirds.
These fly larvae build up through the nesting season and attack later broods most heavily. Lawrence Zeleny, noted bluebird expert, recommends pyrethrin as the safest insecticide to use in the nest box, and it can be placed beneath the nest. It may also repel wasps and ants. Remember, bluebirds return to their natal area and to successful nesting grounds, so you may develop quite a crowd of pretty neighbors. You might be interested in the materials, plans and information that the North American Bluebird Society puts out, also.