|This nest box is perfect for wrens. Photo by J.J. Meyer|
Not all birdhouses are nesting boxes. Cute and whimsical appeals only to humans, so most colorful birdhouses are nothing more than garden ornaments.
For a house to be functional for birds it should have no outside perches, which invite predators. It should also have ventilation and drainage holes; an extended, sloped roof; thick walls made of untreated, unpainted wood; rough or grooved interior walls for the birds to climb out and easy access for cleaning and monitoring.
Avoid using metal for the roof or sides, which can increase the temperature and kill the birds inside.
The entrance should be properly sized for the birds you are trying to attract. Bluebirds require a
1 1/2-inch hole, and wrens a 1 1/4-inch hole to keep uninvited guests out.