An important factor in bluebird propagation is loss to predators. Cats have been known to crouch beneath nesting boxes and pounce upon adult birds when they prepare to enter the box. Snakes and Raccoons and other predators climb posts to get at eggs or chicks. One way of reducing these losses is to raise the height of the box. Bluebirds will occupy boxes 15 feet or more above the ground. Unfortunately, boxes more than 5 feet high are difficult to monitor and maintain. Go telescopic, I prefer two pieces of conduit pipe 1.5inches inside a 2in. both section 5 to 6 feet in length. Place a pin into desired height.
( Never, Never place a nest box on a Tree, or a Fence Post because you are SURE to loose eggs and chicks. 1'' to1 ½ inch pipe 5 to 6 feet high with A baffle install on the pole to prevent predators from climbing works great! )
Like all other birds during nesting season, bluebirds are territorial. I recommend nest boxes to be placed 150 yards apart. Remember location - location - location is the most important factor to remember when where should I place my nest box.