Selecting the right hole size for a birdhouse might seem like a simple decision, but it carries significant implications for the well-being of our feathered friends. Choosing the right hole size is not just a matter of aesthetics; rather, it could be a matter of their life and death. There’s a fine line between offering sanctuary to avian families and rolling out the welcome mat for uninvited guests.
The Peep Show deliberately uses a 1-3/8” hole...welcoming smaller cavity-nesters while minimizing the risk of intrusion by larger, aggressive species.
Unfortunately, big box stores selling birdhouses in massive quantities use the “one size fits all” approach, carrying birdhouses with large holes that do not take into account vicious invaders. Backyard birding is about creating a safe space, not just any space, for native birds to raise their young. Using a hole size suited to local birds is a small but significant way to protect and restore native wildlife.
"Locals" Differ by Region
The Peep Show’s hole size caters to smaller cavity nesters such as Violet-Green Swallows, Chickadees, and Wrens. This choice is informed by research conducted by Cornell Lab's NestWatch. See Cornell's Guide to Nest Boxes.
Where do you live? Do the research to understand your region’s bird populations. Consult with local chapters of organizations like the National Audubon Society or American Birding Association to determine if invasive species are present in your area, then adjust the diameter of nesting box holes accordingly to welcome the locals, while restricting the ruffians.
Regardless of where you live in North America, chances are you will encounter at least one of the most common invasive bird species: English House Sparrows (EHS) and European Starlings. In the 19th century, well-intentioned but misguided efforts to introduce these species to the United States led to unintended consequences.
English House Sparrows
English House Sparrows (Passer domesticus) were introduced to North America in the 1850s. The introduction is often attributed to releases in Brooklyn, New York, with the intention of controlling insect pests and because they were thought to help reduce the spread of weed seeds. Additionally, in this era, homesick European settlers longed to make the American landscape feel more like Europe by introducing familiar wildlife.
European Starlings
Then in the early 1890s, the American Acclimatization Society, a group of New York businessmen, released 100 European Starlings (Sturnus vulgaris) in Central Park because, so the story goes, they wanted this continent to have all the birds mentioned in the works of William Shakespeare. This tampering of ecological balance must’ve seemed a good idea at the time. These stocky songbirds can fly almost 50 mph, and travel in synchronized mobs. From New York City, those 100 starlings proliferated and spread across North America till today, to over 200 million. (1) Many farmers consider them pests like magpies or crows since they destroy crops, taking a few pecks out of ripened fruit, causing enough damage to ruin entire harvests. Instead of aiding in pest control or adding to the poetic landscape, these marauders invaded the entire continent. DNA from starlings in Virginia is quite similar to that found in Californian starlings.1
...well-intentioned but misguided efforts to introduce European species to the United States led to unintended consequences.
Attack on Bluebirds
How did these invaders manage to decimate the bluebird population in North America? First, sparrows and starlings are notoriously brutal nest invaders. They forcibly evict bluebirds from their nests, destroy their eggs, and kill their young or the adult bluebirds attempting to defend their nests. The bullies may not even use the nest, but they will kill the inhabitants anyway. Their ruthless nature cannot be overstated.
Second, both invaders are highly adaptable, proliferating across a wide range of North American environments. Their sheer numbers and aggressive nature make it difficult for native species to compete for nesting sites and food.
Conservationists and bird enthusiasts are concerned about the decline of the bluebird population due to these invasive species. Efforts to protect and restore bluebird populations include the creation of bluebird trails (networks of monitored nest boxes), habitat restoration, and public education on the importance of choosing birdhouse and nest box designs (like the Peep Show) that prevent invasive species from roosting. These invaders are such a common problem for birders that you’ll find dozens of online articles with deterrence methods, from noxious smells to ultrasonic soundtracks, physical barriers, traps and poisons. Invasive species like sparrows, starlings and rock pigeons are not protected by the U.S. Fish and Wildlife Service’s Migratory Bird Treaty Act of 1918.2
Attracting Your Ideal Tenants
After much consideration and research, including insights from Cornell Lab’s NestWatch, we eventually chose a 1-3/8” opening for The Peep Show. This size reliably prevents starlings from entering while welcoming vast majority of smaller cavity nesters like Violet-Green Swallows, Chickadees, Wrens and Titmice.
What if you have English House Sparrows in your Neighborhood?
To block sparrows from nesting, install a “hole reducer” or predator guard. Some sparrows can fit through a 1.25” opening! Because we use real wood and really thick wood (3/4”) for The Peep Show’s front and back panels, a predator guard can easily be screwed into place without compromising the integrity of the panel—a crucial modification that is not possible with birdhouses of flimsy construction.
For fans of Bluebirds (and who isn't?)
For those dreaming of bluebirds (You’re not alone!), grab a drum sander and gently enlarge the opening to 1.5 inches. Remember, birds don’t look for architectural perfection the way we do—ovals and oblongs work just as well as circles.
Conclusion
In the realm of birdhouse construction, the size of the entry hole directly affects the well-being of the birds we aim to support. The Peep Show deliberately uses a 1-3/8” hole, a size that strikes a workable balance, welcoming smaller cavity nesters while minimizing the risk of intrusion by larger, aggressive species. The solid wood construction of the front panel allows modifications—the hole can be minimized using a restrictor or enlarged with a household drill, depending on the type of bird you hope to accommodate. By understanding and prioritizing the needs of our feathered friends, we can create environments that support thriving native bird populations and enrich our own lives in the process.
Sources
Photo credits
Mulberry Street, New York City, c. 1900, Detroit Publishing Company, Detroit, MI.
"Mountain Bluebird" by USFWS Mountain Prairie is licensed under CC BY 2.0.
"Eastern Bluebird" by Andy Morffew is licensed under CC BY 2.0.