It's a good time to warn all bird lovers about tree tubes. These are used to protect saplings from deer. Usually consisting of a plastic tube about 4' high held up by plastic ties and wooden stakes, these tubes are attractive nuisances for bluebirds: the male bluebird wants to explore all possible nesting cavities, so he will go into the tube and fall to the bottom and not be able to get out.The post goes on the describe two possible solutions: exclude birds from the top of the tube with a physical barrier, or lift or open the bottom of the tube to provide an escape hatch:
The tree tube manufacturers sell (or include) woven plastic tops, or "socks" to go over the tops of the tubes. These will effectively prevent male bluebirds from going into the tubes. If not, you can use some means to create a small exit slot or hole at the bottom of the tube, such as pulling the stake out of the ground 1.5 inches.I imagine that panty-hose or other stretchy fabric across the top of the tube would also serve well as a barrier.
Nuthatch cites an original source for this, but doesn't provide a link.