Texas Wildlife Exemption: Bird House Design for Bluebirds, Other Cavity Nesters
For long lasting bird houses, nest boxes should be constructed of a weather-resistant wood such as cedar or cypress. The wood can be painted or stained, but make sure to only treat the outside surface. The entrance hole should be 1.5 inches in diameter. Numerous nest box designs have been used with success.
A good method of assisting fledglings in their climb from the nest to the entrance hole is to roughen up the inside wood surface under the hole with a chisel. The lid or one side of the box should open to facilitate monitoring and cleaning. Three or four 1/4 inch drain holes should be drilled into the box bottom. Ventilation and shade requirements are met by overhanging roofs and holes in the bottom.
Here is the box:
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