The dimensions of the birdhouse depend on the size of the bird. Small birds such as chickadees, titmice and nuthatches can comfortably live in a space measuring 4 by 4 by 8 inches. Large birds such as owls need larger birdhouses. For example a barn owl needs a space of 22 by 22 by 18 inches.
The size and placement of the entrance depends on the bird's size and habits. Tiny birds such as bluebirds, Carolina wrens and hairy woodpeckers like 1-1/2-inch entrances. Robins, barn swallows and phoebes prefer having at least one side of the birdhouse completely open.
Birds like having their nests at a certain height. Purple martins prefer birdhouses erected on a pole that is 10 to 20 feet high. Eastern bluebirds like a height of 4 to 8 feet and a distance of about 10 feet from a bush or a perch. Barn owls like having a nest in an open building near the roof.