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Imagine a single bird during a spring dusk moments before flight. Maybe an Upland Sandpiper standing on the vast Argentine pampas, ready to launch itself on an 8,000-mile trek to its breeding grounds on Alaska’s upland tundra. Or a Blackpoll Warbler in an orange grove in Colombia, set to make a 6,000-mile journey to Canada’s boreal forest. Or a Yellow-billed Cuckoo leaving the Bolivian plains, bound for the deciduous woodlands of Illinois, some 4,000 miles away. Or a Bobolink, a Scarlet Tanager, a Wilson’s Warbler ...Tonight, conditions seem right. The way ingrained inside. The script passed down for millennia. The birds will flow through the sky, a living river rolling through darkness, skirting storms and artificial light and a minefield of inhospitable landscapes below. Zoom in: That single bird spreads its wings, joining thousands upon thousands of birds spread across the night sky. Pull back: The continent becomes a beating heart, the birds its blood pulsing back and forth, a mystery we’re just beginning to understand.

Billions of birds travel north in spring and south in fall, hundreds of species keeping a cycle of movement, and they do so primarily at night. While some large birds like hawks, cranes, and waterfowl are daytime travelers, most migrants—including the vast majority of songbirds—are on the wing in the dark. The reasons include a calmer atmosphere, guidance from the stars, and safety from predators. Meanwhile we humans by and large have no inkling of the tremendous journeys made while we sleep. Even for those who study migration, the story is unfolding in ways never before possible. “The more you can see what’s going on, the more fascinating it becomes,” says Jeffrey Buler, a University of Delaware wildlife ecologist. “And we’re making new discoveries all the time.” 

In the past few years, scientists have gained an unprecedented window into the phenomenon of nocturnal migration. Advances in the use of weather surveillance radar now offer images of millions of birds on the move, while new tracking technologies trace the long aerial paths taken by individuals. Community science is helping fill in the picture on the ground, identifying previously unknown perils and places critical to migrants’ survival. “It really is a new golden age of observation,” says Cornell University ornithologist Andrew Farnsworth.

This age has dawned at a critical time for conservation. Some three billion birds have disappeared from North America in the past 50 years, and migrant species in particular have suffered significant losses. Habitat destruction, collisions with buildings, declines in insect abundance—the threats to migrants are many, and the question has become: Can new tracking technologies help to unravel the mysteries of nocturnal migration while we still have time to preserve one of the world’s great natural wonders?

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rom the moment humans first set artificial light against darkness, we have sensed that birds move at night. In the 19th century observers reported birds at oil-burning lighthouses and lightships. In the 20th century electric lights brought clearer confirmation, sometimes horribly so. The deaths of some 50,000 birds attracted to a Georgia airport’s ceilometer in 1954 is just one of many examples. But with radar’s invention we began to grasp the true abundance of life in our skies at night.

Based on German physicist Heinrich Hertz’s experiments with electromagnetic radiation, British scientists successfully implemented radar in the 1930s with national defense in mind. Radar bounced radio waves off flying objects that couldn’t otherwise be seen, such as German bombers in World War II. When radar operators in the 1940s and 1950s detected “unexplainable echoes” and “occasionally dense displays of small blobs,” the suggestion that these might be birds flying at night was initially rejected. Early radar ornithologists such as Sidney Gauthreaux used the technology in the 1960s in part to prove that birds migrate over the Gulf of Mexico.

By 1970 Gauthreaux published definitive studies correlating weather radar with the volume of bird migration; through the next decades he and others continued to build a foundation of knowledge on which today’s aeroecologists, who study living organisms in the lower atmosphere, stand. With the completion of the NEXRAD Doppler weather surveillance system in the 1990s, our ability to see nocturnal migration began to bloom in ways previous generations couldn’t have imagined.

Among those on the cutting edge of our ability to “see” nocturnal migration is Colorado State University aeroecologist Kyle Horton. Thanks to Horton and his colleagues, we’re now able to picture the nocturnal movements of almost inconceivable numbers of birds migrating between North, Central, and South America. Computer algorithms translate shapes that might appear to the untrained eye as bands of thunderstorms to show waves of living creatures—and lots of them, sometimes hundreds of millions a night.

To see these images and realize they are huge groups of birds is breathtaking. And sometimes, doubted. Horton has encountered critics within the meteorological community since he began presenting his images in 2018. “You know how many birds would have to be migrating to show up like this on radar?” they tell him. He’s undeterred: “That is exactly the point we are trying to make.”   

In addition to showing the magnitude of nocturnal migration, the use of NEXRAD radar—combined with revolutionary advances in computing power—is revealing how migration has shifted over time. Horton’s team has, for example, examined the impact of climate change on when birds migrate. Analyzing 24 years of radar data, they found that average peak migration in spring and fall came sooner and coincided with higher temperatures in the continental United States. While scientists have been tracking the arrival and departure dates of specific species at particular sites for decades, these findings paint an unparalleled picture of how the disruption of ancient traditions—timed to coincide with the blooming of certain flowers or the emergence of insects—could put birds increasingly at risk.

Horton is not just looking back: His work is also transforming our ability to predict specific nights migrants will pass over a location. By taking meteorological data and “spinning the forecast from weather to birds” with computer models, Horton can offer one-, two-, or three-day forecasts for more than 100 U.S. cities, color-coded based on low, medium, or high levels of movement. The models predicted, for instance, that on September 28, 2020, 594 million birds would fly along a corridor stretching from the Great Lakes to the Gulf of Mexico.

For bird lovers, these forecasts, available at BirdCast.info, have added to the natural excitement and anticipation that arrives with each migration season. “There’s this mystery and mystique around migration where you wake up and there’s this new suite of birds,” Horton says. “The power of the radar and the forecast is that we’re finally able to show what’s going on.”

Four thousand miles away, in southern Sweden, aeroecologist Cecilia Nilsson has similarly unveiled birds’ nighttime journeys in the Eastern Hemisphere. In 2019, using weather radar from 11 countries, she mapped the flyway of some of the two billion birds migrating between Europe and sub-Saharan Africa, a finding that thrills her. “It’s just the grandeur of it,” she says. “You have these huge waves of biomass moving over the planet.” But her excitement is tempered by a sense of urgency, one that Horton shares. “We need to start thinking about these large-scale patterns because we’re running out of time,” she says. “How can we make sure there are still birds migrating in 50 years?”

Gaining a comprehensive view of what has long been invisible to us is critical to understanding night migration. But while weather radar can provide deep insight into the density, direction, and speed of bird movements, the glowing shapes can’t provide another piece of critical information: which species are actually migrating.

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nder a low, waning gibbous moon, Jennifer Vieth and a small crew wait for owls to come down from the dark, lured by a recording of a male saw-whet call. When a little owl catches in their mist net, she gently untangles its wings and slips the bird into a soft cotton sack. Inside Carpenter Nature Center, 30 minutes from Minneapolis, we weigh the owl (“same as eight Oreos”), match its eyes to a paint chart (“Golden Orchards”), and band its leg as it clicks its little beak. “That’s her saying, ‘Get away—I’m big and scary and make weird noises,’ ” Vieth says. I carry it outside in a box, wait 15 minutes for its eyes to adjust, and lift the flaps.The owl rises into a nearby oak, its short spell in captivity providing a few more clues to the grand mystery in which it plays a small role.

Such banding operations are key for understanding migration on a more granular level. Banding ramped up in the 1900s, offering a sense of the scope of migration. Today banders upload their records to the USGS Bird Banding Lab’s Band-It database. Similarly, ornithologists and birders worldwide upload their avian sightings to eBird, adding to the global dataset used to create distribution maps showing which species are on the ground during migration. These types of ground-based observations are essential to documenting the movement of birds through the skies.

While eBird data can show which migrants stop during the day, sound can reveal which birds are on the go at night. “Acoustics can qualify migration traffic and tell us what’s moving as it’s moving,” Farnsworth says. Since mid-century, ornithologists have used acoustic monitoring—recording flight calls and later listening by ear or with spectrogram software—to determine which bird species were present. But as with weather radar, new developments promise to revolutionize the field.

Take BirdVox. The collaboration between the Cornell Lab of Ornithology and NYU’s Music and Audio Research Laboratory uses machine listening to automatically detect and classify birds in flight from their vocalizations, technology that could vividly describe nocturnal migration. At the community science level, apps such as BirdNet already allow anyone with a smartphone to identify birds by capturing their calls, an achievement Farnsworth calls “truly inspiring.” The night isn’t so far away, he says, when a vast network of community scientists will decipher through flight calls the birds passing overhead in real time.

Meanwhile, our ability to track individual migrants has grown dramatically. One example: a Bar-tailed Godwit’s record-breaking 7,500-mile nonstop flight from Alaska to New Zealand last fall. Tracking technology is not one size fits all, which has hampered science. The gadgets can be no more than 5 percent of a bird’s body weight. Over time, both geolocators (simple devices with a light sensor, clock, and memory chip) and satellite tags have gotten lighter, allowing ornithologists to track increasingly smaller birds throughout their travels—and revealing to scientists basic aspects of migration they never had a clue about.

That was a breakthrough for Peter Marra, an ornithologist and director of the Georgetown Environment Initiative at Georgetown University. When geolocator tags dropped from two grams to one to three-tenths in the past decade, he was able to incorporate the technology into his 30-year study of American Redstarts overwintering in Jamaica. “We thought our birds were going to New England; turns out the little bastards are going to Iowa and Wisconsin,” he says. Tags connected to the Argos satellite system revealing secrets, too. No one knew, for instance, where Yellow-billed Cuckoos spend much of the year. Now, Marra knows precisely: “They’re all going to the area called the Gran Chaco,” in South America.

A system called ICARUS—which uses the International Space Station to receive data from GPS tags—has the potential to usher in a new chapter in wildlife tracking. Two decades in the making,  ICARUS will enable scientists to track more animals—smaller ones, especially—across larger areas, and to do so faster and cheaper than previous technology allowed. “I’ve been thinking about it like the race to the moon,” Marra says, “this race to create an inexpensive, reliable device that is really small.”

Buler compares the trove of new information about migration stemming from technological advances to deeper explorations of the sea made possible by Jacques Cousteau and Émile Gagnan’s invention of the modern demand regulator decades ago. “You discover there’s this whole other world,” he says of the birds, bats, and insects that inhabit the lower atmosphere. “There are animals in the air everywhere.”

And yet we have only recently begun to think of the airspace as habitat—to con- sider, as we have long done for terrestrial and aquatic environments, how development and pollution might put migratory birds in peril.

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wice a year for millennia, hundreds of thousands of birds representing hundreds of species have passed through what is now Minneapolis-Saint Paul, drawn to the rich habitat along the Mississippi River. But while the region has long been a key stopover site, now it’s a deadly one. Like so many urban areas set in ancient flyways, these cities are beacons that lure all too many migrants to their ends.

Last year a study of 13 buildings in downtown Minneapolis found that bird-building collisions increase with greater amounts of artificial light. Most of the casualties—nocturnal migrants, including White-throated Sparrows, Soras, Ovenbirds, Nashville Warblers, and Virginia Rails, drawn and disoriented by the lights—died the next morning, when they mistook reflections of vegetation in glass facades for the real thing.

“It’s sad to think they were traveling hundreds or thousands of miles and that incredible journey was cut short by a largely avoidable problem,” says lead author Sirena Lao, of Oklahoma State University.

A driving force behind the up to one billion birds killed globally each year by human-built structures is artificial light at night (ALAN), or light pollution. The new technology telling us so much about migration has also helped illuminate the magnitude of ALAN’s impact: For the September night that BirdCast predicted 594 million birds would be in the air, an overlaid map of light pollution showed the gauntlet those birds would have to pass through. “Suddenly the whole system of evolving to move at night is under potentially extreme pressure,” says Farnsworth, “pressure that birds would not experience otherwise, and it is creating chaos.”

Light pollution harms night migrants in other ways. A 2020 review of 150 studies concluded, “Artificial light at night—in combination with habitat loss, chemical pollution, invasive species, and climate change—is driving insect declines.” And a 2021 study found that ALAN exposure on their wintering grounds may cause Purple Martins to migrate early, reach their nesting sites too soon, and starve before the insects they rely on emerge. These unsettling findings follow the news that of the nearly three billion North American birds lost in the past half-century, native migrants in particular have suffered steep declines, with insectivores such as swallows down 32 percent.

Fortunately ALAN is a problem readily in our control. Shielding lights downward and reducing usage can make a significant difference. Weather surveillance radar can help, too. Lights Out programs have long encouraged cities to flip the switch on municipal buildings during migratory seasons. Horton hopes BirdCast will spur managers to take precautions on specific nights when the greatest numbers of birds fly through.

Advances in tracking nocturnal migrants are also helping focus efforts to preserve critical habitat. “Nocturnal migration is like The Adventures of Huckleberry Finn,” says University of Maryland Center for Environmental Science ecologist Emily Cohen. “You think of them floating on a raft down the river, but really three quarters of the book are their adventures as they stop along the way.” It’s the same with migration, as birds spend up to three-quarters of the time on the ground.

In 2020 Cohen led a group that, for the first time, synthesized data from weather radar with mapping of stopover sites to identify vital hotspots along the U.S. Gulf Coast to prioritize for conservation. Cohen’s work emphasizes that migratory concentrations are often a mix of birds, and she argues that protecting high-use stopovers will likely benefit a bevy of migratory species.

Meanwhile, satellite technology has been key to revealing previously unknown locales where migrants overwinter, places conservationists didn’t know to safeguard. The Gran Chaco, where tracking revealed Yellow-billed Cuckoos spend much of their lives, is being deforested for soy cultivation and cattle grazing, which may be contributing to the bird’s decline. “It really helps us target where we do conservation,” Marra says.

For Jill Deppe, senior director of Լ����Ƶ’s Migratory Bird Initiative, the cuckoo is a prime example of reasons for optimism: “That’s the beauty of migration, these fantastic stories, and this tracking data brings those stories to life.” By combining data such as Marra’s with abundance estimates derived from the Cornell Lab of Ornithology project eBird; USGS banding data; and tracking records contributed by more than 100 academics, institutions, and organizations, Deppe and her team hope to reveal a more holistic picture of migration. Visualizations they are producing will depict what we know about migratory routes—and help identify critical gaps in knowledge. This summer the initiative plans to launch an open-access online platform that she says will “show people how spectacular migration is.” 

In the end, this may be what the advances in night-migration science are doing most powerfully—shifting our perspective, giving us a greater appreciation for this ancient phenomenon. When I think of meeting the saw-whet owl and how it felt as though we’d dipped our hands into the timeless stream of life passing overhead, John Muir’s words from a century ago come to mind: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” Here is yet another reason to head outside at night, look toward the sky, and feel amazed.

This story originally ran in the Spring 2021 issue as “Night Moves.”​ To receive our print magazine, become a member by .​