The hole in a bluebird house should be about 1 9/16 inches in diameter. The size depends on the bluebird. Eastern bluebird houses should have a hole between 1 1/2 inches and 1 9/16 inches. Western or mountain bluebird houses are either 1 9/16 inches or 1 3/16 inches.
The size of the hole in a bluebird house prevents unwanted species from getting inside. Woodpeckers and squirrels may enlarge the hole in the birdhouse over time. If this happens, simply replace the front panel of the birdhouse, ensuring that the new hole is the correct size. Small to medium birdhouses without perches are the best for dissuading unwanted predatory birds.