Two beams of light illuminate a mass of flying birds.

Bright lights, big pity

City lights lure migratory birds, with lethal results, but weather radar and modeling can help reduce toll

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The Tribute in Lights installation in New York City lures thousands of migratory birds (bright specks) into twin towers of light. Lawrence Lucier/FilmMagic via Getty
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A version of this story appeared in Science, Vol 376, Issue 6591.Download PDF

Every 11 September at dusk, in memory of the 2001 attacks, New York City mounts the Tribute in Light, an art installation in lower Manhattan. And every year, as twin towers of light bloom skyward, they attract thousands of migrating birds, sucking in warblers, seabirds, and thrushes—along with predators such as peregrine falcons eager to take advantage of the confusion. On each anniversary, bird conservationists wait below, counting and listening to disoriented chirps. If the observers report too many birds circling aimlessly in the beams, organizers flip off the lights.

In recent years, on-site observers have also used a complementary tool to quantify the orbiting birds: weather radar, which bounces off birds as well as raindrops. In 2017, a group led by Cornell University ornithologist Andrew Farnsworth found that during seven previous anniversaries, the once-a-year installation had attracted a total of about 1.1 million birds. Within 20 minutes of lighting up, up to 16,000 birds crammed themselves into a half-kilometer radius. But when the lights flicked off, the dense clouds of birds on the radar screen dissipated just as fast, a finding later confirmed by on-site thermal cameras.

“That was really illuminating—pun not intended,” says that study’s lead author, Cornell ecologist Benjamin van Doren. “It really gives you a sense of the scale that light can impact bird migration.” The circling birds burn through time and precious body fat, are easy prey, and worst of all, can brain themselves on the windows of nearby buildings.

Flight risks

By using satellite maps to quantify light at night and radar to estimate the numbers of migratory birds streaming across the night sky, scientists have ranked the cities where birds face the most danger from light pollution. Chicago tops the list in both spring and fall.

Relative exposure of birds to light in U.S. cities, including (highest to lowest) Chicago, Houston, Dallas, New York City.
(Graphic) K. Franklin/Science; (Data) K. Horton, A. Farnsworth et al., Frontiers in Ecology and the Environment, 17, 4, (2019).

The finding came as scientists grew increasingly worried about slumping bird numbers. The skies above North America host some 3 billion fewer birds today than in 1970, according to one 2019 analysis. The flocks have faced death by a thousand cuts, including not just light pollution, but climate change, vanishing habitat, and pesticides. Ornithologists fear each added insult could be enough to bend once-abundant bird populations toward extinction.

The radar studies at the Tribute in Light helped lay the groundwork for a tool that could ease the toll: a program Farnsworth’s team calls BirdCast, which incorporates continent-scale weather radar and machine learning to forecast the exact nights when hundreds of millions of migratory birds will torrent over U.S. cities. The team then feeds those findings to conservationists and policymakers desperate to help the birds survive the journey by dimming lights along the way.

Such research, paired with existing conservation efforts, is beginning to make a difference: New York City recently passed an ordinance requiring city buildings to turn down lights during migration season, the world’s largest city to take such a step. It joins dozens of growing campaigns across the United States that aim to save tens or even hundreds of thousands of birds per city each year. “Given declines over the last 50 years, and what’s happening on the planet, everybody needs a win,” Farnsworth says.

Long before Thomas Edison turned on a light bulb, people were recording anecdotes of nighttime lights ensnaring birds, especially during migration. In 1880, for example, the Bulletin of the Nuttall Ornithological Club queried lighthouse keepers across the United States; one respondent, in Florida, reported shielding his face with his hat against an onslaught of “more than a million” birds heading south. He carried away two bushels of crumpled bodies the next morning.

Today’s researchers keep more detailed records. In New York City, for example, volunteer crews patrol city sidewalks morning after morning during migration seasons, cataloging birds that thwacked into buildings. Among them are songbirds, light as quarters, that are adapted to every part of their yearly trek between the Amazon and Canada’s boreal forests—except for human cities.

“It’s really bad to see injured birds that you know are about to die,” says Kaitlyn Parkins, a science consultant for New York City Audubon, which has tracked fallen birds since 1997. “I just—I can’t internalize it anymore.”

27 small dead birds carefully laid out on a neutral background.
These warblers and other birds died at the World Trade Center in New York City on 20 September 2021.Melissa Breyer

Some 3.5 billion birds fly through the southern border area of the contiguous United States each spring, then head north through the center of the country, Cornell ecologist Adriaan Dokter found in a 2018 study using weather radar. Come fall, after breeding season, roughly 4.7 billion birds travel south, this time along routes that cluster more toward the eastern United States. Collisions with buildings take out anywhere from 365 million to 1 billion of them each year, according to one landmark meta-analysis of papers and bird collision data sets published in 2014. That study put buildings behind only domestic cats on the list of top anthropogenic bird killers.

As biologist Pete Marra at Georgetown University puts it, big glass buildings might be more “efficient from an energy perspective [but] are extremely efficient from a killing perspective.”

In the daytime, reflective glass can trick birds into flying into what seems like clear, open space, which often ends in death by brain hemorrhage. Overnight, the chief killer is artificial light, which harms birds in several steps. First, when the cumulative glow of light above cities is bright enough to outshine the Milky Way, it sucks in passing migrants from up to 200 kilometers away, a 2018 study using weather radar found. Once in a city, birds are subject to degraded habitats and are prey for housecats, and individual light sources can trap them in pointless, calorie-wasting circles or tempt them headlong into windows. Exposure to light at night has also been linked to disruption of birds’ immune systems, microbiomes, foraging behavior, and of course sleep cycles.

To this day, researchers don’t know why so many birds find artificial light so alluring. Insect researchers face the same puzzle, debating why, exactly, moths and other insects are drawn to flames and streetlights. “If I could answer that question, I’d be happy,” says Kyle Horton, a BirdCast team member now at Colorado State University, Fort Collins. He speculates only that the impulse to fly to light must have once conferred an evolutionary advantage.

Whatever the reason light lures birds, Farnsworth has toiled since the 1990s to keep them safe from it by tracking and predicting their movements. He aimed to map migration not just as numbers of birds crossing a particular field site, but as flows of biomass operating on regional, even continental scales. On weather radar, multispecies rivers of passing birds can be filtered out from rainclouds, allowing their large scale movements to be forecast like weather. (Meteorologists face the same problem—telling apart birds and storm systems—in reverse.) But it took the advent of cloud computing and modern machine learning to crunch through enough data to make BirdCast work.

In 2017, the team did a small-scale test of its methods at the Tribute in Light, proving that a single radar station—even one on Long Island, relatively far from the art installation—could quantify how birds move on both large and fine-grained scales. A year later, van Doren and Horton let machine learning models loose on 23 years of continental U.S. weather data. They found peak migration nights regularly involve half a billion birds aloft over the sleeping United States. Given a weather forecast of upcoming temperatures and winds, their model could predict variations in when and where the birds would fly with 62% accuracy 1 week ahead of time, rising to 75% 1 day in advance.

Night moves

On peak nights during migration seasons, some half a billion birds fly over the sleeping United States. Based on weather and flight paths, models can forecast where their densities, and peril from city lights, will be highest.

548 million birds predicted for 8 September 2021. Migration intensity is highest in the Midwest.
(Graphic) K. Franklin/Science; (Data) B.M. Van Doren and K.G. Horton, Years of Migration Forecast Map; Birdcast Migration Forecast Map, 8 September 2021 20:00 EST; Cornell Lab of Ornithology; Colorado State University; University of Oxford

Since then, checking tomorrow’s BirdCast has become “common vernacular” for everyone from hobbyists planning birding trips to scientists, says Emily Cohen, a migration ecologist at the University of Maryland Center for Environmental Science. “This real-time forecasting of animal movements is really unprecedented and superexciting.” The Cornell team is working to leverage those predictions to help with the siting of future wind turbines, to forecast spikes in disease transmission from wild birds to farmed poultry, and to anticipate elevated risks of birds smashing into planes, which has bedeviled aviators since the Wright brothers.

At the same time, other teams are using GPS trackers mounted on individual animals to give a bird’s-eye view of the same big patterns. One study published this month in the Journal of Applied Ecology mapped migration paths in North Africa and Europe, identifying areas where new turbines or power lines would cause collisions.

The birdcasts also reveal where and when migratory flows collide with concentrations of light. In 2019, a study led by Horton ranked the U.S. cities where birds face the greatest collision risk from lights. Chicago, Houston, and Dallas came in on top, as the most light-polluted areas along heavy migration routes (see graphic, above). That information soon percolated to local conservation groups.

Bird advocates have long organized voluntary “Lights Out” campaigns during migration season in a few cities, starting as far back as the 1990s in Toronto, and spreading to at least 44 U.S. cities today. BirdCast data allow conservationists to amplify this message on the exact nights millions of birds are likely to cross an area, using tweets, newsletters, and targeted emails to everyone from bird rehabbers to downtown building managers. “It’s wonderful policy influenced by data,” Marra says.

Seeing the ignominious placement of Texan cities on Horton’s list—and a Lights Out program already in place in Chicago since the late ’90s—the BirdCast group decided to focus its own resources in Texas, joining a smaller campaign that had begun after 400 birds dashed themselves against a single Galveston skyscraper one night in 2017. In 2020, 14 major buildings participated across the state; in 2021, it was more than 100. Most of the iconic skyline buildings in Dallas and Houston turned off nonessential lights last spring and fall for several peak migration weeks during which at least 50% of migrating birds were expected to pass over. Other buildings, for example in downtown Fort Worth, kept their lights off for months to cover the full migratory season. Texas homeowners can sign up for BirdCast alerts that provide narrower night-by-night warnings.

In fall 2020, the city of Dallas turned out lights to spare migratory birds, somewhat dimming the city’s typically bright nightscape. Drag the slider to see the difference.Lloyd Clayton/Cornell Lab of Ornithology

The ability to quantify the risks with radar data has also encouraged advocates to push for deeper policy commitments. Newsworthy mass mortality events add to the pressure, such as on 14 September 2021, when heavy migration combined with stormy weather and low cloud cover in New York City led to what Parkins calls a daylong “collision massacre.” She says that “to get legislation passed, you have to have the science.”

In December 2021, after hearing testimony from Farnsworth and others, the New York City Council voted to require city-owned buildings to turn off lights on peak migration nights, joining several other U.S. cities like Austin, Texas, which passed a similar bill earlier last fall. New York state legislators have also introduced a more aggressive bill that would compel private structures, including residences statewide, to dim or turn off most nonessential lights by 11 p.m. all year. In Illinois, a law passed in summer 2021 requires new state-owned buildings to use bird-safe glass and dimmer, downcast lights shown to reduce collisions; another proposed state bill would ban nonessential lighting during migration season for new buildings near ecological preserves.

These measures are incremental, and researchers are eager to learn whether they will help even in the middle of light-polluted urban environments. In one study, published in summer 2021 in the Proceedings of the National Academy of Sciences, van Doren and colleagues drew on a unique data set: bird collision records compiled by volunteers from Chicago’s Field Museum during 40 years of morning walks around the perimeter of the city’s McCormick convention center, on the Lake Michigan waterfront.

From 2000 to 2020, almost 12,000 sparrows, warblers, thrushes, and other birds died at the convention center. The worst single nights left more than 100 bodies behind. Wind conditions and the intensity of migrating birds spotted in archival weather radar seemed to predict how many birds hit the convention center each night, the team found.

But another key factor was how many of the convention center’s windows had been illuminated. Each individual bright window left more dead birds for volunteers to find the next day. The correlation suggests halving the number of lit window bays would halve the number of bird strikes, the team estimated, saving thousands of birds at this one three-story building. “It really does seem that each window makes a difference,” van Doren says.

Despite the study and the negative press that resulted, the convention center hasn’t updated its lighting policy during migration season, says Annette Prince, part of a volunteer group that tracks bird collisions in downtown Chicago. “It’s disappointing, because they do have drapes.”

Cynthia McCafferty, a spokesperson for the convention center, says the customers who use the building for events dictate the hours the lights stay on. “We have to serve our clients.” She adds that closing drapes in the evening would require workers to bring in heavy equipment.

Harder data on the efficacy of Lights Out campaigns are still forthcoming. The BirdCast group, for example, is working with volunteers in Dallas and other Texan sites to build a bigger data set of collisions, as tracked by body counts in the morning. Between 100,000 and 200,000 birds perish in Dallas alone each year, Farnsworth says. Assuming van Doren’s findings scale up, their hope is that turning down the city’s glow by half could halve that casualty count.

Horton is developing sharper-than-ever migratory forecasts by incorporating information beyond atmospheric conditions, including an area’s vegetation and levels of light pollution. He’s also identifying public communication strategies to translate them into action. Light pollution harming wildlife is still an unfamiliar concept to many, he says. “Having your porch light on doesn’t sound the same as pouring paint down the sewer.”

But that means it’s also not politically polarized, unlike many environmental topics. “I don’t think there’s many people out there who are like, ‘I want to keep my lights on to kill colorful songbirds,’” Horton says.

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