Never - Never use a wooden Post. Your sure to lose Nestlings !
Let’s look at the Bluebird’s worst nightmare the Rat snake. All Rat snakes are powerful constrictors and can be quite long. These snakes are designed to suffocate their prey with strong, unyielding constrictions. They bite then coil around their victim and each time the prey exhales, the coil is tightened. Eventually the prey is suffocated and then swallowed, usually head first. Though their name implies "rat" hunter, these snakes are formidable hunters of both cavity and non-cavity nesting birds. Rat snakes raid numerous bluebird nest boxes each year. These snakes often hunt at night and this significantly reduces the chances that a bluebird landlord would see one crawling near his/her nest box. Unfortunately the vast majority of bluebird nest boxes and poles offer no protection from these predators. Many landlords do not believe that these snakes are living in well-maintained yards, or that they can climb metal poles. All bluebird landlords need to beware of the "belly crawlers" and how these formidable predators find and raid nest boxes. They don’t need sight nor do they need sound. (Snakes cannot hear anyways) They don’t need great speed. They don’t need a "hunter’s moon" in the dead of night and they can find their way in total darkness. The rat snake has a "secret" biological weapon that would rival the most sophisticated radar tracking system known to mankind. All they need is a few molecules of warm-blooded "scent" and that will be enough. Their prey can be as little as five feet off the ground, or 30 feet high-up in a tree in an open bird’s nest. The concentration of available prey fills the air with its "scent" that disburses widely into nearby thickets, overgrown lawns, and trees - places where a rat snake may be crawling or hiding and "sniffing" the air. Once these snakes detect the "scent" of a possible meal, the hunt begins and their sensors guide them to their victim(s). They are relentless in their silent pursuit and follow the "scent" like a wolf on the trail of a deer. If you have experienced losing your nestlings and or the female bluebird and the nest was undisturbed, consider the Rat Snake as the predator. I found a metal movable stove pipe guard mounted on the pole (larger the better) works fine. Or a movable 8 to 10 inch PVC pipe over the pole works well also. Keep in mind that when it comes to rat snakes climbing, nothing is 100% effective. If you did not have a snake guard on your pole this year, be sure to start planning to add one for the next bluebird season. It will only be a "matter of time" before a "belly crawler" finds your nest box. Think about that rat snake entering your bluebird’s nest box during the dead of night. If you do find a rat snake in your nest box or on the box, please do not harm it. Do not harm any snakes because they are very helpful for the environment. The small ones eat harmful bugs and insects. The larger ones eat rats, mice, gophers, and animals that destroy crops. Farmers and gardeners know how helpful most snakes are and are happy to have them around. All snakes should be kept safe from harm. It’s the landlord’s responsibility to safe guard his/her nest box by using predator guards.