Three species of bluebirds—the Eastern, Western, and Mountain Bluebird—rely on natural cavities for nesting across North America. Due to habitat loss, these birds increasingly depend on man-made nest boxes for successful reproduction. Attracting bluebirds and protecting them from aggressive competitors requires precision in nest box construction. Every measurement, especially the entrance hole size, must be exact to ensure the box functions as a safe shelter.
Required Entrance Hole Dimensions
The diameter of the entrance hole is the most important factor determining which species can use the nesting cavity. This measurement must be tailored to the specific bluebird species in your region to ensure proper fit and exclude larger birds. For the widely distributed Eastern Bluebird, the recommended circular entrance hole diameter is exactly 1 1/2 inches.
The slightly larger Western and Mountain Bluebirds require a marginally wider opening. For these species, the precise diameter of the entrance hole should be 1 9/16 inches. Using a specialized hole saw is advised to achieve these exact measurements, as a minor deviation can compromise the box’s effectiveness.
The placement of the hole also matters, affecting the bluebird’s ability to access the nesting space. The top of the entrance hole should be positioned about 1 1/2 inches below the underside of the roof. This space helps shield the opening from direct rain and provides a minimal overhang.
The hole size must be consistently maintained and checked, as weathering or pests can enlarge the opening over time. If a hole becomes too large, a metal plate or guard with the correct diameter can be affixed to the exterior to restore the required dimension.
How Precise Hole Size Excludes Competitors
Precise measurements are necessary due to the biological differences between bluebirds and their aggressive cavity-nesting competitors. Specific diameters, such as the 1 1/2-inch hole, are calibrated to the maximum width of a bluebird’s body. This size allows the bluebird entry while creating a barrier for larger, non-native species.
The primary target of this sizing strategy is the European Starling, an invasive bird known for aggressively taking over nesting cavities. Starlings possess a bulkier body size and cannot pass through a circular opening of 1 9/16 inches or smaller. This strict size limitation turns the nest box into a starling-proof structure.
However, the precise hole size is not a universal solution for all competitors, particularly the House Sparrow. House Sparrows are smaller than bluebirds and can easily enter the standard 1 1/2-inch hole. Due to their aggressive behavior, other management strategies are necessary to deter sparrows from colonizing the box.
The hole size acts as a selective filter based on body mass and shoulder width. This measurement successfully screens out the larger starling, aiding bluebird conservation efforts.
Other Critical Dimensions for Nesting Success
While the entrance hole is important, several other dimensions contribute to the functionality and safety of a bluebird house. The interior floor size provides the foundation for the nest cup and space for the growing brood. For Eastern Bluebirds, a floor size of approximately 4 inches by 4 inches to 4 inches by 5 inches is recommended.
Western and Mountain Bluebirds typically have slightly larger clutches, making a floor dimension of 5 inches by 5 inches more suitable. This floor area is large enough for nesting but small enough to discourage other, larger cavity-nesting birds from using the box.
The distance between the bottom of the entrance hole and the floor, known as the box depth, is also an important measurement. This depth should fall within the range of 5 to 7 inches. This vertical distance acts as a defense mechanism, making it difficult for predators, such as raccoons or cats, to reach the nest inside.
Finally, the overall height of the mounted box is a factor in successful use. Nest boxes should be placed on a pole approximately 5 1/2 to 6 1/2 feet off the ground. Mounting the box at this height, ideally with a predator baffle, provides protection from climbing animals and makes the box easier to monitor.