Nesting birds, such as bluebirds, chickadees, wrens and swallows, are particular about the size of the opening on the front of a birdhouse. Other birds, such as robins and barn swallows, prefer an open-sided home and purple martins look for entryways with a special crescent shape. Before you build a birdhouse, decide which types of wild birds you'd like to attract, so you can make the hole to spec.
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3-inch Holes and Larger
Building a home for barn owls requires a 6-inch entrance hole. Screech owls and kestrels are smaller and only require a 3-inch entrance opening. The pileated woodpecker prefers a rectangular opening that is 3 inches wide by 4 inches high. If you are building a haven for wood ducks, make the front entrance hole 4 inches in diameter.
Holes Between 2 and 3 Inches
The red-headed woodpecker, the crested flycatcher and the house finch prefer birdhouses with a 2-inch diameter entrance hole. The common and Northern flicker and the red-bellied woodpecker all prefer birdhouses with a 2 1/2-inch diameter entrance hole.
1 1/2-inch to 2-inch Holes
Make 1 1/2-inch diameter birdhouse entrance holes for yellow-bellied sapsuckers, violet-green and tree swallows and hairy woodpeckers. Bluebirds will accept entrance holes that range from 1 1/2-inches to 1 9/16-inches in diameter. House sparrows look for homes with 1 3/16- to 2-inch openings.
1-inch to 1 1/2-inch Entrance Holes
Many wild birds prefer entrance holes that range between 1 and 1 1/2 inches in diameter. The chickadee prefers 1 1/8-inch diameter openings, the titmouse, Bewick's wren, and the downy woodpecker look for homes with 1 1/4-inch diameter openings. Nuthatches prefer 1 1/4-inch to 1 3/8-inch diameter openings.
Robins, doves, hawks, barn swallows, osprey and phoebes prefer open-sided nesting shelves or platforms to enclosed birdhouses. Of these, the osprey requires the largest platform at 48 inches square, followed by the hawk at 24 inches square. Robins and doves prefer a nesting shelf that is at least 7 inches by 8 inches; barn swallows and phoebes need a nesting space that is at least 6 inches square.