The bluebird house is a specialized nest box designed for a species that cannot excavate its own nesting cavity. Bluebirds rely on existing holes or man-made boxes for reproduction. The specific architectural requirements of the box respond directly to the threat posed by aggressive, non-native competitors, primarily the European Starling and the House Sparrow. Adhering to precise dimensions creates a safe haven that excludes larger birds while providing optimal conditions for native bluebird populations.
Essential Architectural Specifications
The entrance hole is the defining feature of a bluebird house, engineered specifically to exclude larger, non-native birds. For Eastern Bluebirds, the standard diameter for a circular entrance hole is precisely one and one-half inches. This measurement allows the bluebird to pass through comfortably while physically blocking the starling. In regions with Mountain or Western Bluebirds, the recommended diameter is slightly larger at one and nine-sixteenths inches to accommodate body size variations.
The interior floor space must meet specific guidelines to support eggs and nestlings. A square floor measuring four inches by four inches or five inches by five inches is recommended to provide adequate space. Too small a floor can lead to overcrowding, while a floor that is too large may invite other cavity-nesting birds. The internal depth of the cavity, measured from the bottom of the entrance hole to the floor, is also a dimension.
This distance should be five to seven inches, ensuring the nest is deep enough to protect young birds from predators. The total internal height, from the floor to the ceiling, typically ranges from eight to twelve inches to accommodate nest material and adults. These dimensions distinguish a functional bluebird house.
Design Features for Safety and Function
Beyond the basic architectural dimensions, a functional bluebird house incorporates several features for safety and ease of maintenance. A cleanout door is needed for monitoring the nest and removing old nesting materials between broods or seasons. This access point is usually a hinged panel on the side or front, secured with a latch to prevent accidental opening. Cleaning the box prevents the accumulation of parasites and old nests, ensuring the box remains usable.
The design must also include provisions for temperature and moisture control to ensure the survival of eggs and nestlings. Small drainage holes, typically three to four quarter-inch holes, should be drilled into the floor to allow any water that enters the box to escape quickly. Ventilation is provided by small gaps or holes placed just beneath the roofline to allow hot air to escape during warm weather, which prevents the box from overheating. These vent holes should be positioned to avoid rain entry, often shielded by a generous roof overhang.
To protect young from predators like raccoons and squirrels, an extended entrance hole or predator guard is often attached. This guard, a block of wood or tube, extends the entrance tunnel by at least one and one-half inches, making it impossible for a predator to reach inside. Mounting the house on a pole equipped with a cone or stovepipe baffle prevents climbing mammals from accessing the box. No perch should be mounted on the front, as bluebirds do not require one, and a perch aids competitors like the House Sparrow.
Material Selection and Construction Details
The longevity and insulation of the bluebird house depend on the chosen materials. Untreated, rough-sawn lumber is the preferred material, with durable woods like cedar, redwood, or pine being common choices. The rough texture provides grip for fledglings to climb out of the cavity when they are ready to leave the nest. Walls should be constructed from wood at least three-quarters of an inch thick, which provides insulation against temperature fluctuations.
Pressure-treated lumber should be avoided, as the preservation chemicals can be toxic to birds. If a finish is desired, only exterior-grade, light-colored paint or stain should be applied to the exterior. Light colors reflect sunlight, maintaining a cooler internal temperature, while dark colors absorb heat and can cause overheating. All fasteners, such as screws or nails, should be weather-resistant (stainless steel or galvanized hardware) to prevent rust streaks and maintain structural integrity.
Optimal Siting and Orientation
Proper placement of the bluebird house is essential. The house should be mounted on a smooth metal pole, rather than a tree or wooden post, to deter climbing predators. The recommended mounting height is five to six feet from the ground, which is comfortable for monitoring and maintenance.
Bluebirds prefer open habitats, such as meadows, pastures, or areas with scattered trees, and they avoid dense forests. The entrance hole should be oriented away from prevailing winds and harsh afternoon sun, usually facing the box toward the east or southeast. This orientation keeps the interior shaded during the hottest part of the day and protects the entrance from storms. Boxes should be spaced at least 100 yards apart, as bluebirds are territorial and will not tolerate another nesting pair nearby.