A birdhouse, often called a nest box, is a human-made substitute for the natural tree cavities that certain bird species require for reproduction. These structures offer a sheltered, enclosed space where birds can build a nest, lay eggs, and raise their young safe from predators and weather extremes. The construction of these artificial homes is significant because urbanization and modern forestry reduce the availability of natural nesting sites, such as dead trees and snags. Providing a nest box directly supports local bird populations whose survival depends on finding these secure, pre-existing hollows. The birds that readily use these enclosures are known collectively as cavity nesters.
Understanding Cavity Nesters
The birds that use artificial nest boxes are known as secondary cavity nesters; they rely on existing hollows and do not excavate their own nesting sites. In nature, these birds utilize cavities created by primary excavators, such as woodpeckers, or natural hollows formed by decay or broken branches. Artificial birdhouses mimic these natural shelters, providing the deep, protected space necessary for successful breeding.
Eastern Bluebirds are among the most recognized species that readily occupy nest boxes, especially those placed in open, grassy areas with scattered trees. The Tufted Titmouse and Black-capped Chickadee, small songbirds that prefer wooded areas, also frequently use birdhouses that match their size requirements. These birds seek out cavities to protect their nests from climbing predators and adverse weather conditions.
Other species that frequently use nest boxes include the House Wren, known for its territorial behavior and tendency to fill multiple cavities with nesting material, and the Tree Swallow, which selects boxes in open fields near water. The primary necessity for this type of nesting is protection; the small entrance hole deters larger predators like raccoons and snakes from accessing the nestlings. Habitat loss has intensified the competition for natural cavities, making human-supplied birdhouses an important resource.
Essential Design Elements of a Birdhouse
The success of a birdhouse is determined by its structural specifications, which must be tailored to the target species. The entrance hole diameter is the most important feature, as its precise measurement dictates which birds can enter and which larger competitors cannot. For example, a 1 1/8-inch hole is suitable for a Black-capped Chickadee, while a 1 1/2-inch hole is appropriate for an Eastern Bluebird. Both sizes exclude aggressive species like European Starlings.
The box must feature a sufficient floor size and internal depth to safely house an entire brood without overcrowding. A depth of six to ten inches from the entrance hole to the floor is recommended to prevent predators from easily reaching the nestlings. Proper air circulation is maintained through small ventilation holes placed near the top of the box, which prevent the interior from overheating.
Drainage holes, typically four small holes drilled into the floor, are necessary to allow moisture accumulation to escape, keeping the nesting materials dry. Untreated wood, such as cedar or pine, is the material of choice because it provides insulation and is non-toxic, unlike pressure-treated lumber. An extended, sloped roof helps to shed rain and provides additional shade, contributing to a safe environment.
Optimal Placement for Successful Nesting
The location and orientation of a birdhouse are important in determining whether it is accepted by a nesting pair. Boxes should be mounted on a smooth metal pole rather than a tree, as a pole makes it difficult for climbing predators to access the nest. For additional security, installing a stovepipe baffle on the pole below the house is an effective method for deterring raccoons and squirrels.
Mounting height varies by species, but a range of five to fifteen feet above the ground accommodates most common backyard cavity nesters. Wrens and chickadees are content with lower heights, while bluebirds prefer a slightly higher mount around six feet. The entrance hole should face east or northeast to shelter the nest from the intense heat of the afternoon sun and prevailing winds.
Placing the birdhouse in an open area, often near a field edge or scattered trees, is preferred, but it should be kept away from bird feeders and bird baths. Feeders can attract a higher concentration of potential predators, making the nesting site less secure. Providing a clear flight path to the entrance is helpful, as birds prefer an unobstructed approach.
Common Birds That Do Not Use Birdhouses
Many common backyard species are not cavity nesters and will not use a traditional birdhouse, regardless of its design or placement. These birds are open-cup or platform nesters, meaning they build their homes on open branches, ledges, or in dense foliage. American Robins, for example, construct their mud-reinforced nests on horizontal limbs or sheltered eaves, valuing open visibility over enclosure.
Northern Cardinals build their open, cup-shaped nests in dense shrubs or small trees, relying on camouflage and thick cover for protection. Mourning Doves and finch species, like the American Goldfinch, prefer open nesting sites in trees or on ledges and will ignore any enclosed nest box. Understanding these species-specific preferences prevents misplaced effort and allows bird enthusiasts to focus on providing suitable habitat, such as dense landscaping.