The 9/11 Tribute in Light Is Helping Us Learn About Bird Migration

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Even in a city of skyscrapers, the Tribute in Light is colossal: twin columns of blue-white light shining four miles high into the bruised orange-black bowl of the night sky. With clear weather in New York City, the beams are crisply visible from 60 miles away. Illuminated every September 11 since 2002, the Tribute is an iconic and emotional memorial to the lives lost in the 9/11 terrorist attacks and a symbol of New York City’s unbreakable spirit. It is a beacon in more ways than one. Birds are drawn to the lights, at times by the thousands. On September 11th, 2017, I was drawn to the lights by the birds.

At sunset, I join a small gathering of people on top of a six-story parking ga­rage near Wall Street. At the entrance to the upper deck, I pass the event pro­duction staff supervising the display, conferring over a bevy of laptops and switchboards. To the eastern side of the roof, rescue workers and families of the victims of 9/11 congregate near a long white tent over a dinner buffet. Oppo­site, to the west, New York City Audu­bon staff and volunteers gather. Domi­nating the roof are the spotlights: 88 in total, separated into two square arrays to the north and south, twenty parking spaces apart. As lighting technicians check their snaking cables, the beams wink with dust motes and the looping flight of insects. There are few vertical light installations in North America of the magnitude of the Tribute. Only the Luxor Sky Beam in Las Vegas, Ne­vada, compares. I crane my head back, looking for birds, but quickly drop my gaze, dizzy. From afar, the Tribute is a powerful spectacle. Standing below the beams, it’s staggering.

I cross the roof to join Susan Elbin, director of conservation and science for NYC Audubon, as she greets her volun­teers. In coordination with the National September 11 Memorial & Museum, NYC Audubon maintains an annu­al presence monitoring the Tribute to ensure that the beauty of the memorial does not compromise the safe passage of nocturnally migrating birds. The vol­unteers arrive prepared for a night of looking up, unrolling sleeping bags and yoga mats to lie down on and unpack­ing snacks to share. This year, some of the best birders in the New York City area are here. South Brooklyn patch birders Doug Gochfeld, Sean Sime, and Heather Wolf come with cameras to help with bird identification. Cornell University alumni Luke Seitz and Ben­jamin Barkley arrive with binoculars in hand. Manhattan competitive birder David Barrett is scheduled for a mid­night shift. Michael O’Brien, a noctur­nal flight call expert, drove in from Cape May, New Jersey. After greeting the group, O’Brien turns his attention sky­ward. Hands cupped behind his ears, he quietly names the birds by their calls as they arrive overhead: “Redstart. North­ern Parula. Black-and-white Warbler.”

Over the last century and a half, sci­entists have catalogued the detrimental effects to nocturnally migrating birds from manmade light sources, from lighthouses to communication towers, and the ever-growing sprawl of city skyglow. When the Tribute was first proposed, NYC Audubon anticipated similar problems of attraction, aggrega­tion, and possible collisions in the night sky over lower Manhattan. By 11 p.m., most of the rescue workers here for the memorial leave the site, but the Tribute stays illuminated all night long. New York City is south of the boreal forest. If weather conditions are right, millions of birds could be moving regionally in mid-September throughout the night.

“Migration isn’t something that you set your clock by. The environmental variables have to be right. Birds are better at predicting weather than we are,” Elbin says. “My takeaway … I always dread it. You never really know what the migration is going to be like until you’re up on the roof, experiencing it with the birds.”

NYC Audubon started communication with city officials in 2002 and negotiated access to the site, a distinction already granted to the families of those lost in the terrorist attacks. But it was not until 2005 that a partnership arose, with the production team agreeing to turn off lights if need be. In 2007, NYC Audubon proposed the official protocol: If one or more birds crashes to the ground, dead; if the birds appear to be trapped (flying low in the beams and calling); or, if 1,000 birds are in the beams for more than a 20-minute period; then the lights are shut off for 20 minutes, to allow them to fly on.

“It needed to be a big number because we’re asking a lot,” Elbin says, “I can hear our members screaming at me, ‘They’re not asking enough!’ But this is a number that satisfies everyone that doesn’t necessarily care about birds.”

Elbin identifies 2010 as the year that “really drove home the issue” to everyone involved. That year, poor weather leading up to September 11 held up migration. With clear weather the night of the Tribute, birds came early and in huge numbers. Elbin was at a family reunion when she received a phone call from John Rowden, then the director of community conservation for NYC Audubon.

“John called and said, ‘It’s crazy, Susan,’” Elbin recalls. “He said, ‘Birds are so low I can see them directly.’”

That night, NYC Audubon found their first fatality: a Pine Warbler, dead on the street below the parking garage.

By 8 P.M., the silhouettes of birds flash, uplit, as they thread through the spotlights’ beams. A Chimney Swift flies in low with shallow, stiff wingbeats. A Northern Flicker shoots by, its wings outstretched in a brilliant revelation of yellow. While the birds are familiar, their behavior is strange. They seem to be caught in a gyre, looping the blue-white lights.

Near the parapet wall, Andrew Farnsworth, a research associate at the Cornell Lab of Ornithology, adjusts the tripods of two cameras a meter apart. Both cameras are pointed straight up, a laptop between them. The monitor displays a pair of grainy images of birds circling in the beams. With basic trigonometry, Farnsworth can calculate the altitude of the birds as their flight slows and descends under the influence of the Tribute. (Read Farnsworth’s and Kyle Horton’s New York Times op-ed about their research at Tribute in Light.)

Farnsworth first joined Elbin on the roof in 2011, following years of regular conversation with NYC Audubon about the lights. Farnsworth—who serves as lead biologist for the Cornell Lab’s BirdCast project, which uses radar monitoring to predict big movements in seasonal bird migrations—brought to the site his expertise in bioacoustic monitoring, radar ornithology, and a longtime familiarity with local migration ecology.

Both Farnsworth and Elbin realized the Tribute offered an unprecedented opportunity to study the effect of a powerful vertical light in an already photopolluted urban environment. At the Tribute, they had nearly all the makings of a controlled experiment, with a variable illumination that could literally be switched on and off. With the ability to turn the lights off temporarily, they could document strong evidence of the almost instantaneous benefit of turning off an intense source of artificial light.

Elbin, Farnsworth, and other Cornell Lab-affiliated researchers gathered data and observations from the Tribute in Light from 2008 to 2016, and after analysis, published their findings in the Proceedings of the National Academy of Sciences in 2017. In seven nights over the nine years of study (two years were omitted due to poor weather), the lights were shut off 22 times. With each episode of temporary darkness, the birds’ behavior—including circling at decreased speeds and lower altitudes, and increased vocalization and aggregation at the lights—stopped within minutes.

At 8:30 P.M., there are 400 birds wheeling a few hundred feet above us. Spotlit, many flash with gold: Canada and Yellow Warblers, the latter’s buzzy, inflected chirp easy to pick out from the steadily growing cacophony. One warbler is too high to see its white patch flash as its wings stretch in flight, but its call clearly pierces the air: the short, abrupt stips of a black-throated blue. During nocturnal migration many songbirds fly not in tight flocks, but loosely or singly. Periodically, they check in with one another using short, species-specific flight calls. On a good migration night away from the lights, a sky flush with migrants riding a favorable wind can have the call-and-response quality of a game of Marco Polo. What I hear is a traffic jam.

Of the 102 species recorded at the Tribute, the majority are Neotropical migrant songbirds, each only the weight of a couple of coins. Many are young birds that just hatched the previous summer, embarking on their first southbound migration and particularly susceptible to the draw of skyglow.

I raise my binoculars and look north, where the floor-to-ceiling windows of a new residential building reflect 60-plus stories of pure white light. As I scan, a Blackburnian Warbler strikes the glass at 30 stories up, its wings spread wide. It falters, then flies straight back into the spotlights and continues circling the beams. Farnsworth grimaces.

Measuring the Effect of Lights on Migrating Birds

Radar images show how migrating birds are drawn to the Tribute in Light when the beams are on (red shades in top image). With the lights turned off, the birds continue on their way and do not aggregate in the lights, as shown by the blue shades in the bottom radar image. Here’s more on how this radar study worked.

Above image is adapted from a figure in this paper, courtesy Proceedings of the National Academy of Sciences.

By 10:45 p.m., the count of birds in the beams is over a thousand. At the point in the sky where the rays of light seem to converge, the birds are so dense they appear as a unified swirl. They cartwheel through the beams and around them, flashing like sparks. It is impossible not to recognize the beauty of birds in flight. But Elbin says, “These are not angels and not confetti. These are birds in distress.”

As NYC Audubon volunteers confirm their count with Elbin, Farnsworth pulls out his cellphone. With a tap, he cues up a radar image from the weather station KOKX, 50 miles to the east on Long Island. On the small screen, a stippled blue-green circle blooms on a map of New York City, radar evidence of the birds aggregating above. Southern Manhattan is obscured by pixels bright and growing brighter.

“Boom,” he says, “[The birds] just lit up the radar.”

Radar reveals what darkness conceals: the numbers, spacing, and timing of birds winging through the dark. The quantity of birds visible above us is staggering, but radar confirms that many more are flying our way. If the weather on September 11 is favorable for migration, the regional influence of the Tribute is tremendous, drawing in densities of migrating birds at 20 times (and up to 150 times) their normal levels. In seven nights of study over seven non-consecutive years of the Tribute in Light, the scientists estimate that 1.1 million birds were influenced by the beams.

With the bird density count in hand from her volunteers, Elbin walks off to find the production staff. It’s time to turn out the lights.

Just past midnight, with the lights illuminated once again, more birds appear in the beams. One of the volunteers recruited by Farnsworth, birder Anders Peltomaa, seeks out Elbin. Peltomaa is holding a dead male American Redstart—jet black, with a white belly and vivid orange patches on its wings, sides and tail. The bird is small enough to fit neatly into his open palm.

I have watched American Redstarts burst off of trees with midair sallies and lively, flycatcher-like behavior. To see such an active bird inanimate is jarring. Disoriented by the lights, the bird must have struck a building. Its neck appears to be broken.

Elbin takes the dead redstart gently, her jaw set, and strides away to find the light production staff, corpse in hand. Typically, a shutdown takes 15 minutes to negotiate. This time, it takes five.

“I just opened up my hand,” Elbin says, “I just took the bird and held it out. ‘Oh my God,’ [they] said, ‘Turn the lights out.’”

The American Redstart is the first mortality of the night, and only the second bird ever found dead by NYC Audubon monitors at the Tribute. But it’s difficult to calculate the actual number of bird deaths due to the lights. At the Tribute and throughout downtown Manhattan, bird carcasses from collisions are often scooped up by building custodians or devoured by animal scavengers such as rats before they can be discovered in the morning light.

Building collisions claim an estimated 90,000 birds a year in New York City. A study by scientists at the Smithsonian Migratory Bird Center estimated that nearly 600 million birds die every year in the United States due to building collisions, and another study estimated that figure to be nearly 25 million in Canada. The low number of bird fatalities at the Tribute speaks to the dedication of all stakeholders and the powerful impact of turning off the lights, allowing the birds to fly on.

Two lighting technicians confer and then separate between the north and south arrays. As power cuts from each of the 88 spotlights one by one, it looks as though a curtain is sliding across the Tribute. The beams grow steadily more slender like a waning moon gone crescent, then new.

Twenty minutes of darkness begin. Silence follows. Near the western parapet wall, an acoustic monitoring device records the night’s vocal migration activity. Sound analysis of the chip notes will allow scientists to identify the species vocalizing by their flight calls. It also captures powerful evidence of altered behavior under the influence of intense artificial light at night: when the lights are on, cacophony. When they are off, quiet.

During the 20 minutes of darkness after the lights are shut off, the pixelation on the radar above Lower Manhattan disappears. High up, further than we can see from the roof deck, the birds have dissipated, southbound again.

As new pulses of birds migrate overhead, the lights are snuffed out four more times throughout the night. At last, in the hours toward dawn, the night seems calmer, emptier. I check the radar again. The blue and green pattern is now gray, indicating slower-moving biota: insects. Throughout the night, wisps of smoke coil off the spotlights as hundreds of insects, charmed by the lights, strike them repeatedly. Scattered around the arrays are the burnt bodies of stinkbugs and wasps, beetles and mantises, moths and flies. On the half-hour, Graham Montgomery, an entomologist from Cornell recruited by Farnsworth, discreetly collects them into neatly labeled vials. By dawn, Montgomery will find insects from around 40 families in 10 orders. A warmer night would have encouraged the movement of tens of thousands of insects.

During the final shutoff of the evening, I step over thick, snaking cables into the north array. As I stand shivering in the center of the darkened lights, appreciating the borrowed warmth of their afterglow, a line of a favorite poem by Wendell Berry comes to mind:

To go in the dark with a light is to know the light. To know the dark, go dark.

As my eyes adjust, the bright star Sirius appears in New York City’s photopolluted night sky. Aldebaran, the rufous eye in the constellation Taurus, the bull, glows furiously above Wall Street near a waning gibbous moon. There is no Milky Way.

The dark, too, blooms and sings,
and is traveled by dark feet and dark wings.

The sky is silent as the birds reorient, and resume their course.

To live in New York City—and, increasingly, to live on our planet in the places where most humans live—is to live in light, day and night. Over three-quarters of the world’s human population and all but 1 percent of Europeans and North Americans sleep under the luminous fog of artificial light, most of which is inefficiently used, improperly shielded, overly bright, and often unnecessary. What a loss of stars will mean for human health and culture is still unfolding, but it is increasingly obvious that it is detrimental to the nightlife, sex life, and migratory journeys of amphibians, fish, insects, and especially birds. Birds now navigate a new and deeply confusing world, the guidance of the sunrise, sunset, moon, and stars replaced by a nocturnal landscape dominated by electric light.

Light pollution increased by about 2 percent annually between 2012 and 2016. But over the last few decades many North American cities—including Houston, Boston, and Atlanta—have adapted “lights out” practices, promoting the darkening of their skylines to protect the biology and ecology of local insects and animals. Toronto is a model for best practices, with its Fatal Light Awareness Program and collision prevention guidelines for the city’s corporate and residential structures.

Farnsworth envisions a future in which real-time radar filtered for birds could help inform the use of outdoor lighting during periods of heavy migration. He says that, based on models relating decades of radar and weather data, migration forecasts can now predict 12 hours to a week in advance when and where migrating birds may encounter increased risks of collisions with structures from exposure to light pollution. Near-real-time alerts from live migration maps of radar data can identify situations where, as with the Tribute, dramatic increases in birds aloft are occurring near heavily illuminated areas.

“With these tools a program manager could advise industries, airports, or municipalities when to turn off lights in advance to minimize risks where high-intensity migration is predicted or in response when concentrations on radar above illuminated areas exceed some predetermined threshold,” Farnsworth says.

At sunrise, Great Black-backed and Ring-billed Gulls arrive on the roof to pick through the insects barbecued on the lights. Elbin takes her leave, shaking hands with the event staff and small group of rescue workers who stayed through the night. The Tribute is a necessary and beautiful memorial. The opportunity to study nocturnal bird migration here, and the gravity of this night, is not lost on the scientists.

“It’s the first project I’ve worked on that has an immediate intersection of research and society,” says Kyle Horton, another Cornell Lab researcher on the Tribute in Light bird study. “We’re grateful for the opportunity to have NYC Audubon diplomatically negotiate the opportunity for us to do this study.”

Earlier in the day, before joining Farnsworth at the Tribute, Horton had visited the World Trade Memorial Site for the first time, a somber visit that made a big impression on the 29-year-old.

“The Tribute in Light is an important icon, a symbol of remembrance,” he says. “The fact that organizers are willing to turn the lights off for migratory birds is encouraging for conservation.”

Farnsworth echoes that sentiment: “There’s another intention with this site, deep and profound. That will always color the science of what we’re doing. We’re lucky to have people who are thoughtful enough, caring enough, to let us do this.”

Annie Novak writes about the connections between people, agriculture, and ecology for The Atlantic and Organic Life magazine. She is currently working on a book about the scientific study of nocturnal bird migration.