According to research, bluebird houses and bluebird nest boxes should have
certain features. First and foremost, bluebird houses should have a specific-sized entry hole to allow access for bluebirds only.
If the hole is too big, other birds will co-op your bluebird houses.
NABS recommends the following for Eastern Bluebirds:
a hole size of 1-1/2 inch for round holes; or 1 3/8 x 2 1/4 inch for vertical oval holes; or 1-1/8 inch opening for horizontal slot entrances. For Western and Mountain Bluebirds, NABS recommends looking for 1-9/16 inch round openings. And should you live in an area where you have more than one type of bluebird, NABS recommends seeking bluebird houses with 1-9/16 inch round
Bluebird houses should also have the proper venting to allow for
air movement through the house. Some bluebird houses achieve this with a small gap between the roof and the upper part of the side walls. Other bluebird houses achieve this
through small notches in the corners of the floor. Look for bluebird houses built with
redwood, cypress or cedar. If you prefer painted bluebird houses, look for
cool colors. It's important that during nesting season, your bluebird fledglings
not be too cold or too warm.
Many bluebird houses feature wooden or metal predator guards around the entry hole to discourage animals such as squirrels from enlarging the entry hole. Bluebirds houses do not feature perches, since bluebirds are perfectly capable of perching on the side of the house, using either the hole or the wooden side for secure footing.
Another feature to look for in bluebird houses are designs that can be opened for cleaning, easy access and monitoring. Bluebird houses should either open from the sides, top or bottom. Bluebird houses with sides or roofs that open should be secured to prevent other predators such as raccoons from assaulting the bluebird houses.