Snake Guard Idea for your Bluebird House
I sadly remember the day I saw a snake rob the eggs in our bluebird house when we lived in Ohio. I had attempted to grease the pole to deter snakes from climbing the pole to no avail.
Here are a couple of links to info from the Virginia Bluebird Society and the North Carolina Extension Service that may give you some ideas to protect the nests in your Bluebird and Purple Martin Houses.
A recent chat with my brother in Virginia led to the idea of using plastic zip ties to place wooden carpet strips on the pole with the nail side out. He sent me these photos of how he set this up. I am not in favor of doing harm to the good snakes in nature, many of which are also the ones responsible for cleaning out the bluebird nests. So hopefully this works to deter them while not causing injury so they will feed on mice and voles rather than bird eggs. To provide extra protection for the pole feeders I suggest you sprinkle Ortho Snake B Gon Repellent on the ground around your nesting boxes.

Carpet Strips to help deter snakes from climbing the pole to the nests in your bird boxes. (click photo to enlarge)