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These Birds Got a Little Too Comfortable in Birdhouses

Whether it’s because we destroy their habitats, discombobulate them with city lights, or allow cats into their midst, most wild birds want nothing to do with humans. But purple martins—shimmery, blackish-bluish swallows native to North America—just can’t get enough. For centuries, the species has gradually abandoned its homes in the wild for birdhouses we’ve built. An entire subspecies of the bird now nests exclusively in human-made boxes; east of the Rocky Mountains, “there are officially no purple-martin colonies that exist outside of that,” says Joe Siegrist, the president of the Purple Martin Conservation Association.

Modern martins have become downright trusting of people. Some will even let humans reach into their nest and pick up their chicks—an intrusion that would send other birds into a screeching, pecking rage. “They’re the most docile species I’ve ever worked with,” says Blake Grisham, a wildlife biologist at Texas Tech University. And the more we build birdhouses and interact with martins, the more they seem to thrive. “It’s totally the opposite of our default in wildlife management,” Grisham told me. The martins’ reliance on us is a bit bizarre, but it’s also a boon: As habitat destruction, environmental contaminants, and invasive species continue to threaten wildlife across the world, an affinity for humans very well may have saved the purple martin.

But the birds’ attachment to us now seems to be transforming into a liability. With the birds facing more dangers in the natural world, their need for human-made homes has grown. At the same time, experts told me, fervor for building and maintaining martin birdhouses appears to be waning, especially as those most enthusiastic about the practice continue to age and die. The martins’ dependence on our structures is, at its heart, a dependence on our behavior. Their precarious housing situation is now many experts’ “No. 1 concern,” Grisham told me—and it threatens to hasten the species’ decline.

Martins have never been the architects of their own homes. As so-called secondary cavity nesters, they evolved to be tenants of tree cavities carved by woodpeckers and other birds, or crevices in the faces of cliffs. But at some point, the birds began to occupy structures hollowed out by humans.

Most experts believe that the shift began in precolonial North America, perhaps near the homes of the Choctaw, Chickasaw, and other Native American tribes that would hollow out gourds and hang them to dry for later storage use. For whatever reason, our proximity didn’t seem to much bother the birds. And humans may have quickly found good reason to embrace their new tenants: “We used them to ward off black birds and other species that would interfere with our gardens,” Deanna L. Byrd, of the Choctaw Nation of Oklahoma’s historic-preservation department, wrote to me in an email. The insect-munching martins may have also served important roles in pest control, Byrd said.

By the early 1800s, building nesting enclosures for martins had become commonplace among Indigenous people as well as colonists. But it was likely the Europeans that cemented the martins’ preference for us into dependence. They practiced unsustainable land-management practices that destroyed acres of forested habitat. They introduced invasive species, such as starlings and sparrows, that began to aggressively compete with martins for cavities. By 1900 or so, human-made houses weren’t just some of martins’ best options; they were, especially in eastern regions, some of the birds’ only options. (Western populations of the birds, though also keen on human-built boxes, still happily nest the old-fashioned way.) Purple martins might not be around today had they not had humans and their birdhouses to fall back on, Siegrist told me.

The martins seem fine with this outcome—maybe in part because, at the houses we build for them, they tend to encounter fewer competitor species and far more opportunities to mate, Grisham told me. Over the years, they’ve even grown to actively prefer living within about 100 feet of human housing; go too much farther than that, Siegrist said, and the birds will turn up their beaks. Although martins are not formally domesticated, behaviorally speaking, “it kind of feels like they’ve domesticated themselves,” says Heather Williams, an ecologist at the University at Buffalo.

And yet, purple martins may also be in trouble. Worldwide, “we’ve lost about a third of purple martins in the last 50 years,” Siegrist told me. The reasons are manifold, and probably include a decline in insects and increased migration perils on the way to the birds’ winter habitats in South America. Clarissa Oliveira Santos, a biologist studying purple martins at the University of São Paulo, is also investigating whether they may be imperiled by potential exposures to mercury, pesticides, and other contaminants. But Siegrist and others told me that, given just how much of the birds’ life cycle hinges on humans, a shortage of housing, especially for eastern martins, is probably playing an important role too.

The decline in human-made houses has been difficult to document and quantify. But Lori Jervis, an anthropologist at the University of Oklahoma, has found that purple-martin “landlords”—as the most enthusiastic housing providers describe themselves—are concerned that their practice is a dying art. The community—which surveys suggest is predominantly white, southern, and male—is also very much on the older side: Two recent surveys showed that a firm majority of landlords were over 50 years old.

Organizations such as the Purple Martin Conservation Association are actively trying to bring younger generations on board. But in a world where so many young people are moving frequently and settling down later, landlording—an activity that usually involves establishing and maintaining multiple-occupancy birdhouses on a tract of land that you probably need to own—is perhaps no longer as easy or palatable as it once was. Jervis told me that landlording can be quite a bit of work too: The birds’ boxes are usually elaborate, multi-room complexes that must be cleaned and inspected regularly; starlings and sparrows must be kept away. (Jervis and her colleagues have, through their work, interviewed people who are so hard-core about protecting their martins that they’ll shoot and strangle the invasive species that try to squat.) And as urbanization has increased, martins have also developed an unfortunate reputation as pests. Flocks of the migratory birds, sometimes as many as 100,000 strong, will occasionally congregate near city centers, leaving behind an unsightly mess.

On a landscape where wild, untouched habitats have grown only scarcer, a species able to wean itself off those venues might seem poised to survive. But from the beginning, purple martins were tying their fates to human caprice. The species could, in theory, revert to its old ways: Grisham is now trying to experiment with coaxing martins back into wild nests. But after so many centuries getting up close and personal with us, they might not know how to live on their own anymore.

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