Attracting bluebirds to a yard or trail is a rewarding experience, but the presence of the non-native House Sparrow (HOSP) poses a serious threat to their nesting success. These invasive birds compete fiercely for nesting cavities, often resulting in the destruction of bluebird eggs and young, and sometimes even the death of adult bluebirds. Protecting native bluebirds requires a proactive, multi-faceted strategy that combines structural modifications with consistent management.
Identifying the House Sparrow Threat
The House Sparrow, originally introduced to North America in the mid-1800s, is an invasive species not protected by the Migratory Bird Treaty Act. Male House Sparrows are identified by their black bib, white cheeks, and chestnut-colored nape, while females are a duller, plain brown with a distinct stripe behind the eye. Distinguishing them from native, protected species like the Tree Sparrow or Song Sparrow is necessary before intervention.
These aggressive birds claim nest boxes early in the season, preventing bluebirds from starting a nesting cycle. Their nests are messy, typically filling the entire cavity with coarse grass, trash, and often topped with a tunnel-like entrance. Unlike the neat, woven grass nests built by bluebirds, the sparrow’s nest is bulky and disorganized. House Sparrows destroy eggs and kill young bluebirds, sometimes building their own nest directly on top of the victims.
Passive and Structural Exclusion Techniques
Modifying the bluebird house structure and placement can significantly reduce its appeal to House Sparrows without requiring daily action. For Eastern Bluebirds, the standard round entrance hole should be 1 1/2 inches in diameter to exclude larger competitors like European Starlings. Since the hole size cannot exclude the smaller House Sparrow, a metal restrictor plate is recommended to prevent them from chewing the opening wider.
Another structural approach is the use of non-standard entrance holes that bluebirds tolerate more readily than House Sparrows. Eastern Bluebirds will use a vertical oval hole measuring 1 3/8 inches wide by 2 1/4 inches tall, or a horizontal slot entrance around 1 1/8 inches high. House Sparrows tend to prefer the traditional round hole design over these alternative shapes. Some bluebird house designs, such as the Gilbertson box made from PVC pipe, are also less preferred by House Sparrows.
Box placement is an effective passive deterrent, as House Sparrows prefer to nest near human habitation, dense shrubbery, or bird feeders. Bluebird houses should be mounted in open, grassy areas, ideally 50 to 100 feet away from houses, barns, or heavy tree lines. Mounting the box on a smooth metal pole with a predator baffle, rather than a wooden post, further reduces accessibility and helps deter climbing predators.
A Sparrow Spooker is a physical, visual deterrent installed once a bluebird has laid its first egg. This device consists of strips of shiny, reflective material, such as Mylar streamers, suspended above the entrance hole. The fluttering strips scare the House Sparrows away from the box, providing protection during the egg and nestling stages. To maintain effectiveness, the Spooker must be removed immediately after the bluebirds fledge.
Active Management and Intervention Methods
When passive measures fail and House Sparrows begin to claim a box, active intervention becomes necessary for the safety of native birds. Since House Sparrows are an unprotected, non-native species, it is legal to remove their nests, eggs, and young immediately upon discovery. Consistent monitoring is essential, as House Sparrows can quickly rebuild a nest, sometimes within a single day. The nest and any contents should be removed and destroyed to prevent successful fledging.
Plugging the entrance hole of a contested nest box for a few days can sometimes encourage a persistently territorial male House Sparrow to abandon the site. This short-term exclusion works best when the male has not yet completed a nest or laid eggs. If the House Sparrow continues to be a problem, trapping is often the only permanent solution to remove the aggressive male bonded to the nest box location.
Specific in-box traps, such as the Van Ert trap, are designed to be temporarily installed inside the bluebird house. When a House Sparrow enters the box, the mechanism is triggered, covering the entrance hole and safely trapping the bird inside. Any trap, especially in-box models, must be monitored frequently, ideally every 30 to 60 minutes, to ensure that no native birds are accidentally caught and harmed. Ground traps, like the Repeating Sparrow Trap, can also be used near the box to reduce the overall local population by luring them with bait like white proso millet.