My Blue Bird houses are wooden non-treated wood. (I use Cypress wood). They need to be mounted on a metal poll with a rat/snake shield around it (the pole) and placed in an area with at least 14 feet of clearance, no limbs, shrubs and etc around it. The hole (door) size is critical and if I remember correctly should be 1 1/2 inch in diameter, it looks too small but isn't.
Now that I am ready to leave my home of the past thirty two years, I have discovered a perfectly safe way to mount bird feeders or bird houses.
Using the standard 1/4 inch diameter steel Shepherds Crook, I bore an 1/4" vertical hole about six inches deep in the top of my fence's 4x4 posts. Since the Shepherds crook is around six feet tall already, it becomes elevated that additional height above the top of the fence. The feeder or house can then be suspended from the crook. To refill a feeder, or clean a house, all you have to do is lift the Shepherd's crook from its snug hole, then replace it the same way. No screws, brackets or other devices are required.