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At 2 a.m. on a warm September night, Molly Adams, a conservation educator at the New York Aquarium, stares up at a swarm of white specks whirling in the air. As she lies on her sleeping pad atop the Battery Parking Garage, she quietly names them: Common Yellowthroat, Yellow-billed Cuckoo, American Kestrel, Northern Flicker, Chimney Swift. She’ll do this for two hours, all part of her job as a tribute counter for .

Every year on September 11, two searing columns of light connect the sky to the ground in lower Manhattan—part of that commemorates those who were killed or injured by the 2001 terrorist attack on the World Trade Center. But in those xenon beams, one can see hundreds of confused birds circling endlessly, as though they were trapped inside.

"Birds have to use things to orient. One of the tools in their kit is celestial cues, so they can use the star maps like early navigators," Susan Elbin, director of conservation and science at NYC ���Ϲ���, says. Believing they’re flying toward starlight or something similar, nocturnal migrants are drawn to the dazzling display, where they end up wasting crucial energy flying around and sounding off in distress.

Although the phenomenon is visible to naked eyes, it’s difficult to directly measure how the lights affect birds. Not only is it hard to identify species at night, but it’s also tough to measure how birds respond to lights, Elbin says. Now, a study published in the justifies her concerns: that birds veer off their regular migratory routes to gather at the tribute—and may end up dead because of it. 

To better understand this pattern and collect data, researchers and volunteers from the Cornell Lab of Ornithology and NYC ���Ϲ��� have been working with the group behind the "Tribute in Light," the National September 11 Memorial & Museum, to monitor it. They use radar to detect high densities of birds, track how many are in the beams, and record vocalizations to measure stress levels and species composition. If the count reaches a dangerous high, or there are more than 1,000 birds circling the site, they advise technicians to turn off the bulbs for 20 to 30 minutes to let the travelers disperse.

For the study, Elbin and her collaborators looked at data collected by volunteers like Adams between 2010 and 2016 to compare bird activity when the lights were illuminated and turned off. Over the study span, an estimated 1.1 million birds—primarily passerines—were influenced by the installation. During the tribute, the number of avians in the area soared by more than threefold compared to a typical fall night in New York City, and distorted many of their flight paths. "[It’s] not just birds that happen to be in the neighborhood; they're coming to the light from a distance," says Elbin, an author on the study. "[Bright light] has a strong effect on birds and their behavior."

Scientists hypothesize that the luminescence leads to higher risks of fatal collisions, too. “The light itself isn't a problem: The problem is the [birds’] energy and going on a detour,” Elbin says. “What happens, most likely, is that they wind up landing somewhere until the next day.” The migrants usually rest in the city overnight, then risk flying into glass in the morning.

And while it was previously thought that artificial lighting only affected birds in inclement conditions, the results of this study show otherwise. The lead author, Oxford University graduate student Benjamin Van Doren, who previously worked with at Cornell University, was surprised that the beams of light had real consequences even when the weather was clear on 9/11.

"Early ornithologists writing about birds killed by lighthouses noted that the largest mortality rates occurred when it was foggy or stormy," Van Doren says. "What we observed was that these lights still had a strong effect even in clear skies."

The observations offer a clearer idea about the relationship between artificial lights and birds, and they also provide evidence that quick actions can minimize harm. The scientists found that turning off the tribute eliminated its disruptive effect almost immediately: Birds emitted fewer calls, flew faster, and moved away from the site within minutes.

With proof that artificial lights do alter bird migrations, researchers are hoping to extend the collaborative effort and work with other public organizations to spread the message beyond this annual event. NYC ���Ϲ���, for instance, recently teamed up with Kings County Brewers Collective in Brooklyn to create the . Each can of beer, which can be purchased at the brewery and in local shops and restaurants, features facts on avian collisions, along with tips on how to make the Big Apple .

Elsewhere, advocates are taking legal action to address birds' night-light problem. In Kauai, Earthjustice, the Conservation Council for Hawai‘i, and other groups are suing the Hawaii Department of Transportation for violating the federal Endangered Species Act. Their  that the state failed to adjust beacons at airports and harbors to prevent injuries and deaths of three imperiled seabird species, including the Newell's Shearwater.

Each of these efforts takes a different approach, but the message is the same. “The darker we can keep the sky at night,” Elbin says, “the safer the birds will be when they're flying to get to where they need to go to."

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