Colorado birds that will use nest boxes include bluebirds, robins, chickadees, nuthatches, titmice, brown creepers, prothonotary warblers, wrens, tree, violet-green and barn swallows, phoebes, purple martins, flycatchers, woodpeckers, owls, ospreys and some hawks. Many birds prefer the basic single-family dwelling consisting of a floor, four sides, a roof with sufficient pitch to shed water and an entrance hole. Robins, barn swallows, phoebes, ospreys, red-tailed hawks and great horned owls require a nesting shelf rather than a totally enclosed box. A nesting shelf is a platform with three sides, an open front and a roof. Unlike other birds, purple martins nest in groups. Their houses should have at least four large rooms.
Wood makes the best birdhouses because of its outstanding characteristics: it breathes, is durable and has good insulating qualities. Bald-cypress and red cedar are more durable than pine and exterior-grade plywood. Painting the outside of a pine or plywood birdhouse with tan, gray or dull green water-based, exterior latex paint increases its durability. The light, dull colors reflect heat and are less obvious to predators. White paint is best for purple martin houses. Natural gourds make good birdhouses that when hung, sway in the wind making them less attractive to house sparrows and starlings. Houses made from a mixture of concrete and sawdust offer more protection because predators cannot chew their way into them.
When choosing a birdhouse in which to nest, birds basically consider two things: their safety and box size. According to the U.S. Fish and Wildlife Service, small birds, such as chickadees, titmice, nuthatches, bluebirds, wrens and some woodpeckers, need birdhouses with 4- to 5-inch square floors, heights of 8 to 12 inches, and 1-1/8 to 1-1/2 inch entrance holes located 6 to 10 inches above the box floor. Generally, the larger the bird, the larger the nest box and entrance required. Barn owl houses measure approximately 10-by-18-by-18 inches and have 6-inch entrance holes. Nesting shelf dimensions also depend on bird size. Robin platform floors measure 7-by-8 inches, while those for ospreys measure 48 inches square.
A birdhouse's location is just as important as its design and construction. Birds have different preferences regarding habitat. Some birds, such as wrens, chickadees and robins, are not particularly picky. Bluebirds prefer open meadows with houses attached to tree stumps or wooden fence posts. Purple martins require approximately 40 feet of unobstructed fly space around their houses. A house's distance from the ground is also a primary consideration. Data provided by the Fish and Wildlife Service indicates that smaller birds usually accept houses placed between 3 and 6 feet from the ground. In contrast, woodpeckers and owls prefer houses located on tree trunks at heights ranging from 10 to 20 feet.