Man-made nesting boxes have supported the comeback of the Eastern Bluebird. These boxes need to be installed with predator guards to prevent predation by black snakes and raccoons. Chickadees, Tufted-Tit Mouse, House Wrens, and Brown-headed Nuthatches also use these boxes. Inspection and maintenance of the boxes is important to prevent nesting by House Sparrows, who will kill the target species and take over the box.
This photo is from the Ohio Bird Conservation Initiative website.