Bluebird House Floorplan
A house suitable for a bluebird house has an oblong shape with a sloping roof and a round entrance hole. Make the floor dimension 4 to 5 inches square and the front panel 10 inches tall. A back wall 1 to 2 inches taller than the front wall allows for an angled roof that will keep the rain out. Create a 1- to 2-inch overhang to keep the hole dry and protected. Most bluebird houses have a hinged roof to allow for seasonal cleaning and nest removal.
Some bluebird houses slope down in the front, creating a house that is wide at the top and narrow at the bottom. This design retains the square, sloping hinged roof, but instead of a floor the bluebirds build their nest in the crevice between the sloping front wall and the straight back panel. Round houses, made from a hollow tree branch or a hollow round piece of wood, have a 4- to 5-inch diameter and a sloping hinged roof.
The Entrance Hole
Bluebird houses have a round hole in the front panel 5 to 8 inches above the floor. Make the hole 1 1/2 inches wide to attract bluebirds. Entrance holes larger than 1 1/2 inches invite competition for nesting space from larger birds and encourage winged and pawed predators. A good birdhouse has the smallest possible hole for the intended kind of bird. Bluebirds do not require a perch outside the entrance; placing one there will only make it easier for predators to get in and encourage unwanted species. Two 1/- to 1/2-inch round vent holes just under the roofline provide fresh air and circulation inside the house.
Wood Selection and Color
Use bluebird houses made from clean, untreated wood. Cedar wood has natural oils that slow decay from water and insects. Pine wood makes a good material for bluebird house construction. Leave the outside unpainted or use a light color. Gray, tan or light blue blend well with the natural surroundings. Don't use dark colors if you paint; a dark birdhouse heats up in the sun and can cook the baby birds in their nest, warns the USDA Natural Resource Conservation Service.