Science: Migratory birds die in city lights, how weather radar saves them

City lights attract migratory birds and have lethal consequences. An article published April 21 in the journal Science tells how ornithologists in the United States are using weather radar and modeling to reduce bird mortality under the influence of artificial lighting.

Every year at dusk on September 11, New York City celebrates the 2001 terrorist attacks at the "Tribute in Light" art event in Manhattan. When the light of the twin towers is cast into the sky, it attracts thousands of migratory birds such as warblers, seabirds and thrushes, as well as predators like peregrine falcons. On anniversaries, bird conservationists wait beneath the sky, counting and listening to disoriented chirps. If observers report too many birds hovering aimlessly in the beam, organizers turn out the lights.

In recent years, event field observers have also used an auxiliary tool to quantify birds in orbit: weather radar, which reflects both from birds and raindrops. In 2017, a team of researchers led by Andrew Farnsworth, an ornithologist at Cornell University, found that the annual device attracted a total of about 1.1 million birds in the previous seven anniversaries. Within 20 minutes of the lights on, as many as 16,000 birds squeezed into an area half a kilometer in radius. And when the lights went out, the dense cloud of birds on the radar screen dissipated just as quickly, a discovery later confirmed by thermal cameras at the scene.

"It's really inspiring." The study's lead author, Benjamin van Doren, an ecologist at Cornell University, said. "It really gives you an idea that light can affect bird migration." Circling birds consume time and valuable body fat, are easy prey, and worst of all, crash their heads into the windows of nearby buildings.

Birds that died of light

Behind the study is scientists' concern about declining bird populations. According to a 2019 analysis, the number of birds over North America today is about 3 billion fewer than it was in 1970. Birds face numerous threats of death, including not only light pollution, but also climate change, habitat loss and pesticides. Ornithologists worry that each additional injury could lead to the extinction of once-rich bird populations.

Radar research at the Festival of Light laid the groundwork for a tool that could slow this mortality rate: Farnsworth's team named their project BirdCast, which combines weather radar and machine learning to accurately predict the nights on which thousands of migratory birds will fly over U.S. cities. The team fed back the findings to environmentalists and policymakers, helping the birds survive their journey by dimming the lights along the way.

Such research, combined with existing conservation measures, is starting to make a difference: New York City recently passed a decree requiring city buildings to dim their lights during the bird migration season. As the world's largest city, New York City has joined dozens of burgeoning movements across the country that aim to save tens of thousands, if not hundreds of thousands, of birds in each city each year. "Given the decline in bird populations over the last 50 years, and what's happening on Earth, everyone needs a win," Farnsworth said.

Long before Thomas Edison invented electric lights, anecdotes were recorded of trapping birds with nocturnal light, especially migratory birds. In 1880, for example, the Nathor Ornithological Club Gazette asked lighthouse keepers across the Country; one Florida interviewee said he covered his face with a hat to ward off attacks by "more than 1 million" birds migrating south. The next morning, he carried away two bushel-crumpled bird carcasses.

Today's researchers keep more detailed records. In New York City, for example, during the bird migration season, volunteers patrol the city's sidewalks each morning to classify birds that hit buildings. In 2018, Adriaan Dokter, an ecologist at Cornell University, used weather radar in a study to find that every spring, about 3.5 billion birds fly across the southern U.S. border and then head north through the central U.S. By the fall, after the breeding season, about 4.7 billion birds migrated south, this time with a more skewed route to the eastern United States. According to a meta-analysis of a paper published in 2014 and a dataset of bird collisions, between 365 million and 1 billion bird-to-building collisions occur each year. The study ranked the building second on the list of bird killers, after the domestic cat.

As Pete Marra, a biologist at Georgetown University, puts it, large glass buildings "may be more efficient from an energy perspective, and more efficient from the perspective of causing bird deaths." ”

During the day, reflective glass can trick birds into what appears to be pure and open spaces, causing birds to die from cerebral hemorrhage. At night, the main killer is artificial light, which harms birds in several steps. A 2018 study using weather radar found that, first, when the accumulated light over cities exceeds the milky way, these lights are able to attract birds 200 kilometers away. Once in the city, the bird habitat degrades and becomes the prey of domestic cats. Separate light sources can also trap them in meaningless, calorie-burning circles or induce them to rush towards windows. Exposure to light at night can also disrupt the birds' immune systems, microbiota, foraging behavior and, of course, sleep cycles.

To this day, researchers don't know why so many birds are so fascinated by artificial light. Kyle Horton, a member of the Bird Radio team at Colorado State University in Fort Collins, speculated that the impulse to fly to the light must have given birds an evolutionary advantage.

Whatever the reason for the light to lure birds, Since the 1990s, Farnsworth has struggled to keep birds safe by tracking and predicting their activity. His goal is to map migration, not just to calculate the number of birds crossing a particular wild site, but to mark the flow of biomass on regional or even continental scales. With weather radar, researchers can filter the flow of multiple birds out of rain clouds so that they can predict their large-scale activity like the weather. (Meteorologists face the same problem — in turn distinguishing between birds and storm systems.) But it wasn't until the advent of cloud computing and modern machine learning that "bird broadcasting" could handle enough data.

In 2017, the team conducted small-scale tests of its approach at the Festival of Light event site, demonstrating that a single radar station — even one on Long Island, far from the event site — could quantify bird movements on both large and fine-grained scales. A year later, Van Doron and Horton built a machine learning model based on 23 years of weather data from the continental United States. They found that on peak migration nights, 500 million birds typically flew over the sleeping United States. Based on weather forecasts for temperature and wind, their model can predict changes in the time and location of birds flying a week in advance with 62 percent accuracy and 75 percent one day ahead.

Emily Cohen, a migratory ecologist at the University of Maryland's Center for Environmental Sciences, said that since then, whether it's amateurs planning bird watching trips or scientists, checking out tomorrow's "bird radios" has become a "routine" for everyone. "The real-time prediction of this animal movement is really unprecedented and very exciting." Emily Cohen said. The Cornell research team is using these predictions to help determine the location of future wind turbines, predict peaks of disease transmission from wild birds to farmed poultry, and predict the risk of birds crashing into planes that have plagued pilots since the Wright brothers.

Bird Radio also reveals when and where bird migratory streams collide with light gathering points. In 2019, a Horton-led study ranked U.S. cities where birds are at high risk of light collisions. Chicago, Houston and Dallas are among the best, and they are the most light-polluted areas on bird migration routes. The news soon reached local environmental groups.

For a long time, during the bird migration season, bird protectors organized voluntary "lights out" campaigns in some cities. The campaign dates back to Toronto in the 1990s and has now spread to at least 44 U.S. cities. Bird Radio data allows environmentalists to use Twitter, newsletters and targeted emails to send that message to everyone from bird rescuers to downtown building managers on the exact night millions of birds might fly through an area. "It's a fantastic policy that's influenced by the data," Marra said.

The ability to use radar data to quantify risk also encourages advocates to push for deeper policy commitments. Newsworthy mass bird deaths add to this pressure. On September 14, 2021, a massive bird migration in New York, combined with stormy weather and low cloud cover, led to what ornithologists call a full day of "collision carnage."

In December 2021, after hearing testimony from Farnsworth and others, the New York City Council voted to require all of the city's buildings to turn off lights on the night of the peak of bird migration, joining several other U.S. cities, including Austin, Texas. Earlier last fall, the cities passed a similar bill. New York State lawmakers have also introduced a more aggressive bill that would require statewide private buildings, including residences, to dim or turn off most unnecessary lights by 11 p.m. throughout the year. In the summer of 2021, Illinois passed a law requiring new public buildings to use bird safety glass and dimming lights to reduce bird collisions; another bill in the state would ban new buildings near ecological reserves from using unnecessary lighting during bird migration season.

More reliable data on the effects of the "lights out" movement are still being released. For example, the Bird Broadcasts team is working with volunteers in Dallas and other parts of Texas to build a larger bird collision dataset. At the same time, Horton is developing more accurate migration predictions than ever before by synthesizing information beyond atmospheric conditions, including vegetation conditions and light pollution levels in an area. He is also defining public communication strategies that he hopes will translate into action. The harm of light pollution to wildlife remains an unfamiliar concept to many people, he said. "A lamp with the porch on doesn't sound like pouring paint into the sewer."

But it also means that, unlike many environmental topics, it has no political polarization. "I don't think there's a lot of people who say, 'I want to turn on the lights to kill colorful songbirds,'" Horton said. ”