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Dustin Partridge is all too familiar with the “thunk” of a bird hitting a window. Working in New York City as director of conservation and science for NYC Bird Alliance, he’s surrounded by a cityscape that kills as many as 230,000 of them a year.

Every time he’s heard that sound, he says, he’s hoped for the best—that the bird was only momentarily stunned and eventually bounced back. After three years spent investigating the aftermath of avian collisions with buildings, though, he harbors no illusions about the odds of a happy outcome.

The results of that work, and published last week in the journal PLOS ONE, analyzed the records of more than 3,100 collision victims brought to wildlife rehabilitators, representing 152 species. It found that about 60 percent of those birds ended up dying, far more than previously thought.

That’s in the best-case scenario, when an injured bird receives care, Partridge notes. The vast majority don’t—and now he’s rethinking those stunned birds he’s watched take flight after a crash: “That injured bird that flew off? That bird most likely didn’t survive.”

Based on that 60 percent death rate, the researchers reached a striking conclusion: Collisions with buildings could kill well over 1 billion birds per year in the United States alone. That adds significantly to the generally accepted range established by a , which estimated that the fatality figure sits between 365 million and 988 million.

The revision makes buildings an even greater factor than was previously known in the estimated loss of a quarter of North America’s birds over the past half-century. And unlike other sources of danger, windows take avian lives indiscriminately, Partridge says, killing birds that otherwise might have survived and reproduced for years. “The majority of the birds that we found in collisions were otherwise completely healthy,” he says. “And to me, that’s terrifying.”

Building collisions are not a new hazard for birds—they’ve been observed since the 19^th century. But the widespread use of glass in today’s built environment has created many more opportunities for misfortune, because birds to perceive it. They’re particularly liable to crash into windows that reflect nearby greenery or the sky. Buildings of any size can attract bird collisions but the vast majority occur at homes and low-rise buildings, not skyscrapers.

Artificial lighting, which is thought to disrupt birds’ ability to use the sky as a compass, exacerbates the danger to nocturnal migrants. When they’re drawn into areas with bright lights, they can be knocked off track, becoming disoriented and exhausted. Together, concentrations of glass and lighting can spell disaster. Last October, for example, nearly 1,000 birds fatally collided with the Lakeside Center at Chicago’s McCormick Place in a single night.

At NYC Bird Alliance, Partridge helps coordinate , an initiative that works to reduce collisions in New York by monitoring where and when they take place. That’s why he knew prior estimates of the scale of collisions have a blind spot: their reliance on carcass recoveries. That leaves out birds that don’t die on impact or are swept away, lost in shrubbery, or otherwise go undiscovered.

So, he and his colleagues took a different approach. They gathered documentation from wildlife rehabilitators across the Northeast, taking note of where and how collisions occurred, which species were involved, what injuries resulted, and the outcome of the care. The results showed that a majority of birds that succumbed to their injuries did so after at least a day. 

Daniel Klem, Jr., a Muhlenberg College ornithologist and expert on bird collisions, praised the researchers for using “responsible, sophisticated modeling to support their conclusions.” For his part, Klem believes that building strikes cause far more deaths than even the new paper suggests; he  earlier this year estimating U.S. collision mortality at somewhere between 1.3 billion and 3.5 billion birds, and possibly much higher. His study used 1,356 observations of window collisions to show that many strikes leave no mark on the glass and no nearby carcass, even if they frequently result in birds dying. “They’re indiscriminately being killed,” Klem says. “We're producing more and more glass in the environment in urban, suburban and rural areas that are killing these animals.”

Though collision estimates are rising, so is recognition of the problem. More architects and designers are employing bird-safe glass and decals or screens that make windows more visible. Local advocates are putting pressure on building owners and managers to employ best practices—efforts that bore fruit at McCormick Place, which is currently adding bird-friendly window film to the Lakeside Center, just in time for migration season.

Some legislators have also shown a willingness to intervene for bird safety. Last year, Connecticut Illinois and Minnesota in passing “lights out” bills to reduce non-essential outdoor lighting at state-managed buildings during migration seasons. After the beloved New York City Eurasian Eagle-Owl Flaco died by flying into a building—a collision possibly related to the high dose of rat poison in his system—lawmakers renamed a bill to require bird-friendly design the “.”

One reason to feel empowered: Anyone who lives near a window has an opportunity to take action, says Connie Sanchez, program manager for ���Ϲ���’s Bird-friendly Buildings initiative. She urges people to explore ways to of transparent and reflective surfaces in their homes and around their neighborhoods.

“It’s been great to see this traction as we build more awareness of the problem, and to get more people involved in this—not just those that are interested in birds and wildlife,” Sanchez says. “It’s a problem we can really do something about.”