Optimum Hole Sizes (Inches) for Common Birdhouse Nesting Species
- Chickadees, bluebirds, nuthatches, prothonotary warblers, swallows, titmice, wrens, downy woodpeckers and hairy woodpeckers prefer a hole close to 1 1/8 inches - 1 1/4"
- Great-crested flycatchers prefer a 1 3/4-inch hole.
- House finches, red-bellied woodpeckers and red-headed woodpeckers need a 2-inch hole.
- Northern flickers prefer 2 1/2 inch hole.
- Purple Martins prefer a 2 1/4-inch hole.
Research Papers on House Wrens and no need to clean nestboxes.