How to Construct a Nest Box
Here are the plans used by Troop 91 of Asheville, NC, a group of Boy Scouts that constructed 22 nuthatch nest boxes in August 2014. Make sure the opening for the bird is 1" to 1.125" in diameter. This is what makes a nuthatch nest box nuthatch-specific. Bird-Friendly Communities of Audubon NC suggests to use an excluder. An excluder is a metal component that fits over nest box openings and shrinks them to a smaller size.
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Also, a time-lapse video of the construction of a next box at the project.