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Returning bluebirds are a welcome sign of spring—but in many places, it’s up to people to give them a home. To keep populations healthy, naturalists are urging people across the country to erect and tend bluebird boxes. Over the past few decades, they’ve helped all three bluebird species—Eastern, Western, and Mountain—recover from a declining population, says Geoff LeBaron, director of Audubon’s Christmas Bird Count, and in the years to come, they could help secure the bird’s future.
Bluebirds—and more than 60 other species in North America—are cavity nesters. Rather than make their own nests, these birds take advantage of existing nooks and crannies in trees. But today, as natural cavities have become fewer and farther between, bluebirds increasingly rely on human-made homes.
“There is no one single perfect, ultimate bluebird nestbox,” according to the North American Bluebird Society (NABS). But a good box should be made of untreated wood, and be able to be opened for monitoring and cleaning. Use the NABS guidelines to select an inexpensive prebuilt birdbox, or use this box blueprint to make your own. If you place your box on a pole—and experts recommend poles, rather than trees to better protect against insects—make sure to add a predator baffler: raccoons and cats love bluebirds, too. (Bluebirds prefer to nest at the edges of forests and fields; they’re less inclined to shack up in city boxes.)
It’s important to monitor the boxes for unwanted residents—check in on the box at least once a week to make sure other birds haven’t taken over the bluebirds’ home. European Starlings and House Sparrows are unwelcome in a bluebird house; these are both invasive species brought over from Europe in the 1800s.
One way you can tell what species has taken up residence in your nestbox is by checking the color of the eggs or the variety of nesting materials. Bluebirds generally favor grasses and pine needles, though the specifics vary by geographic area. Mountain Bluebirds often add wool and deer hair, while Western Bluebirds like to add feathers and ribbons. In comparison, starling nests are bulky while sparrows use a looser jumble of materials.
If an invasive species takes over your bluebird box, you can remove the nests by hand (some birds will keep coming back and laying new eggs, so you can render eggs infertile by refrigerating them). But the invasive birds don’t like human activity, so just your presence might be enough to scare them off, LeBaron says. Putting up a pair of nest boxes, at least several feet away from each other, can ensure that there is a spot for the bluebirds even if starlings or sparrows set up camp, too.
Need one more reason why it’s particularly important to help these native birds? Audubon’s Birds and Climate Report spells bad news for both the Western Bluebird—projected to lose the majority of its winter range by 2080—and the Mountain Bluebird—projected to experience severe loss in its summer range.
“Climate change adds to the pressure on many of these species,” says Audubon’s chief scientist, Gary Langham. “Nestboxes [are] more important than ever to the long-term future of these vulnerable species.”
Once you set up your boxes, report any residents to Cornell’s NestWatch to help scientists and other birders understand the cavity nester population.