Cities are killing birds. Activists and architects have solutions.

The bird lies on its side, a clump of feathers no bigger than a crumpled leaf. It’s just a dark speck on the concrete, with massive glass and steel skyscrapers rising above it in the pre-dawn light.

Annette Prince sees it at once. She hurries over and lifts it gently in her right hand. It has a slender bill, a tuft of yellow on its rump, and dark eyes that show no glimmer of life. A yellow-rumped warbler, bound for the warmth of the Caribbean or the American South, has met its end in Chicago’s Loop.

She stuffs it into a plastic bag and writes down the date and address. Then she moves on.

“Sometimes, you come downtown and there are birds everywhere,” says Ms. Prince, a volunteer who has monitored bird collisions for two decades. “It’s not quite like that yet.”

In the contest between birds and cities, the cities are winning. Scientists estimate that, on average, at least a million birds die in collisions with buildings each day in the United States – and as many as a billion a year. Most perish during the spring and fall migrations in which vast numbers journey up and down the continent, flying mainly at night. City lights attract and disorient them, and many end up crashing into windows, not just the sides of gleaming office towers but suburban patio doors as well.

The problem, then, is twofold: lights and glass. The light from ever-expanding cities is disrupting the movement of creatures that evolved to migrate in the dark, using the stars and the Earth’s magnetism as their guides. And the modern architectural penchant for glass has proved deadly for them.

Most glass is invisible to birds, appearing either as clear air to fly through or as a reflection of the trees and sky behind them. There are growing efforts to make cities safer for birds. The National Audubon Society’s Lights Out programs, in which owners and managers agree to switch off exterior lights during migration, have spread to 45 U.S. cities. Architects and developers are learning how to make buildings bird-friendly by using specially treated glass that birds can see. Grassroots activists like Ms. Prince are monitoring collisions, pressuring businesses and local officials to take bird safety seriously, and in some places asking homeowners to consider their own windows. Scientists say more birds die by hitting houses – urban and rural – than by striking downtown skyscrapers.

For many conservationists, the issue is far more than birds. They say that protecting birds is part of a larger imperative: the need to reduce the human impact on the natural world in an age of habitat destruction and climate change. To them, the mass death of birds is a symbol of how humans treat the world around them.

“It’s a proxy for a much bigger problem of our stewardship of the planet,” says Andrew Farnsworth, an ornithologist with the Cornell Lab of Ornithology and an expert on bird collisions. “How it relates to systemwide collapse, we don’t know. But I worry about that.”

Birds are clearly in trouble. A 2019 study by the Cornell Lab concluded that the North American bird population had declined by 29%, more than 3 billion birds, over the previous half-century. The biggest reason, scientists say, is probably habitat loss. Feral cats also kill birds – by some estimates more than windows – as do collisions with vehicles and power lines. But the combination of buildings and city lights is deadly.

By this measure, Chicago may be the deadliest city of all. According to a 2019 study, Chicago endangers more migrating birds than any American city, followed by Dallas and Houston. It’s a matter of lighting, but also geography. Chicago sits on the Lake Michigan shore and within the Mississippi Flyway, a broad path that funnels migrating birds from as far as the Arctic to the Gulf of Mexico, Central America, and beyond.

Yet if Chicago is one of the worst cities for birds, it’s also one of the best. It has produced a strong response in defense of avian migrants, including a well-established Lights Out program and architects who use bird-friendly designs. It also has some of the most determined advocates for bird safety in the country. Ms. Prince’s group started with just a handful of volunteers two decades ago and has grown to more than 150. These monitors take turns patrolling a square mile or more of downtown Chicago, searching at daybreak for dead or wounded birds. It’s difficult, labor-intensive work, and few cities can match the scale of the effort.

If a bird is alive, monitors take it to a rehabilitation center in the suburbs. They take the dead ones to Chicago’s Field Museum, where volunteers prepare them for storage in the museum’s collections. Over the years, the museum has acquired more than 100,000 birds this way. Songbirds, especially warblers and sparrows, are the most common, but bird kills encompass as many as 170 species.

The monitors also work with building managers to reduce collisions. Turning off exterior lighting is a start. The lights of entryways, lobbies, and glassed-in atria also attract birds. Moreover, birds drawn to a city typically spend a day or two there, pausing to rest and feed before continuing their journey. Most collisions happen on the lower floors, during the day. Monitors encourage building managers to dim interior lights, move plants away from windows, and apply speckled film to clear glass so birds can see it.

Geoffrey Credi was one of the first to embrace this effort. Two decades ago, Mr. Credi, director of operations at Blue Cross Blue Shield Tower in downtown Chicago, attended a symposium about birds and buildings. He was surprised to discover that his building, which overlooks Grant Park, was considered one of the worst in town. He already knew there was a problem. Collision monitors and custodial staff had found birds outside. Mr. Credi threw himself into efforts to make the building safer. He and his staff began to track where birds were hitting. They had speckled film applied to clear-glass entryways. An olive tree in an atrium attracted birds, so Mr. Credi had it moved.

“It wasn’t perfect, but it was a vast improvement,” he says.

The effect of city lights on birds is well established. One of the most dramatic examples involves the 9/11 memorial in New York. The National 9/11 Memorial & Museum’s “Tribute in Light” consists of two columns of light shining into the night, a symbol of the fallen towers. It’s switched on once a year to mark the anniversary.

Members of NYC Audubon and others were alarmed when it was first turned on in 2002. Videos show hundreds of birds circling and crossing through the light, like insects in a car beam. Radar and ground observation revealed that the number of birds in lower Manhattan increased from around 500 to as many as 15,700. Conservationists reached a compromise with the museum. Monitors would consult radar and watch the sky. When the number of birds in the beams exceeds 1,000 in 20 minutes, the organizers would turn off the lights for 20 minutes.

“There was an immediate reduction,” says Dr. Farnsworth. Some years, he says, the lights go off eight times on the tribute night. Other years, when migration is low, they stay on all night.

Meanwhile, architects are beginning to design buildings that reduce bird collisions. Jeanne Gang, a prominent Chicago architect, is well known for her efforts. Her designs do not eschew glass, but modify it in critical places to discourage collisions. On lower floors, the glass is fritted – printed in the factory with a ceramic pattern that is both durable and visible to birds.

A simple pattern consists of lots of small dots. But other patterns work, too. Glass on a dormitory complex that Ms. Gang designed for the University of Chicago is imprinted with pale white chevrons, making an aesthetic element out of a safety feature. Elsewhere in the building, decorative steel panels screen the glass. Retractable shades reduce transparency. At glass corners, vertical shields eliminate the see-through effect that is perilous for birds. This and other buildings, together with her outspoken advocacy of bird-friendly architecture, have won Ms. Gang the praise of many conservationists.

“She came out with this early on,” says Ms. Prince. “She talked truth in Chicago with us.”

The world of bird-friendly architecture is evolving rapidly. Glass companies are coming out with more products, including glass imprinted with patterns only visible to humans under ultraviolet light. Birds can see the patterns; people can’t. Architects also are finding new ways to reconcile the competing demands of function, aesthetics, and economics. At the Jacob K. Javits Convention Center in New York, architects used fritted glass to reduce sunlight into the building and save on energy bills. The pattern also reduces bird collisions.

In Evanston, Illinois, just north of Chicago, architects printed patterns of small dots on glass to make a new community center safer for birds. But they varied how they spaced the dots on different facades, using them to create more or less transparency and to enliven what might otherwise have been a dull, uniform pattern.

“The solutions are becoming more creative,” says Brian Foote, project manager for Woodhouse Tinucci Architects, which designed Evanston’s Robert Crown Community Center. “There’s now a whole cottage industry that is attempting to find technological solutions to this problem. So I think the solutions available are more attractive architecturally. They’re more integrated solutions. It’s not just something you add at the end.”

It’s also about learning as you go. In the early 2000s, Daniel Piselli’s architectural firm designed the Center for Global Conservation at the Bronx Zoo in New York, incorporating features that designers hoped would help birds. Not all of them did. Overhangs didn’t reduce collisions. Wooden slats installed as sun shades were too far apart to make a difference.

“The aesthetics of architecture have always changed,” says Mr. Piselli, who is director of sustainability for FXCollaborative in New York. “We are in a time when we need to be designing our buildings to deal with the issues of our times: climate change and destruction of the environment in general. To me, there needs to be a new concept of design aesthetics. ... This is part of it.”

One of the most diligent students of bird collisions is David Willard, an ornithologist and collections manager emeritus at the Field Museum. In 1978 he got a tip from a colleague that birds were hitting the windows of McCormick Place, a convention center that had been rebuilt seven years earlier after a fire. Designed by a protégé of architectural pioneer Ludwig Mies van der Rohe, the new McCormick Place was a monument of Modernist architecture, a giant rectangle of glass, brick, and steel, with a wide flat roof and windows 50 feet high. It was also a 10-minute walk from Dr. Willard’s office. One day he made the trip.

“I went out of curiosity, without any idea of what I was getting into,” he says.

What he was getting into was a job of collection and study that has lasted almost a half-century. That first time, Dr. Willard recalls, he did find a few birds – enough to spur him to come back. And back. He began going to McCormick Place almost every morning during the spring and fall migrations, each of which spans several months. He brought the dead birds back to his lab, where he recorded – by hand in a big ledger – their species, size, and other characteristics. Stuffed and preserved, the birds filled drawer after drawer in tall wooden cabinets in the museum’s musty backrooms.

Today, 44 years later, Dr. Willard is still going back, circling McCormick Place at dawn. On a recent morning, the building is brightly lit, and he picks up 13 birds, mostly sparrows. One, a tiny Lincoln’s sparrow, feels warm in his hand. Its small black eye, he notes, “still looks like there’s life in it.” Tall, soft-spoken, and graying now, he walks with his head bent, yet alert for the carts and forklifts of workers also up early to prepare the center for the day’s exhibition. Dr. Willard estimates he has picked up 30,000 birds at McCormick Place. In the early years of his visits, when the convention business was booming and the building in constant use, he found as many as 200 a day. Nowadays, he says, he averages about 20. (A spokesperson for the building’s managers, the Metropolitan Pier and Exposition Authority, says workers turn off lights when the building is not in use. Trees around the building have also matured, which may reduce collisions.)

But Dr. Willard has done more than gather up dead birds. He and his colleagues saw early on that McCormick Place offered an opportunity to study migration patterns, as well as one building’s effect on collisions. The most important revelation came when convention business declined in the early 2000s. They found that on nights when the center turned off its lights for lack of conference activity, bird deaths declined by 60%. This suggested that Lights Out programs worked.

Of course, big buildings are only one hazard for birds, and not the worst. Across the country, efforts have sprung up to encourage homeowners to do their part to prevent the thump of a warbler or thrush against their windows.

James Cubie, a lawyer who has worked for activist Ralph Nader and the U.S. Senate, started his own campaign in South Carolina several years ago after learning that North America had lost nearly a third of its birds. “I thought, ‘30% loss, I’ve got to get to work,’” he says.

He lived then in Sun City Hilton Head, a retirement community, where he belonged to a birding club. He started by encouraging members to plant native species like azaleas and hollies to increase bird habitat. “It was very successful,” he says. Then he shifted his focus to windows. He collected different products that were available: netting, decals applied to the glass, and arrangements of hanging monofilament line or parachute cord. He turned his sunroom into a display area. He made a video. He wrote a guide. “The whole shebang,” he says.

The results were disappointing. “People don’t want to spend any money,” says Mr. Cubie. “They’ll spend $500 on plants, but they don’t want to spend $100 on collision prevention.”

This feeling is widespread among those most concerned about bird deaths – that progress is far too slow. Chicago’s downtown skyline may dim noticeably on fall and spring nights, but the city still throws up an immense blaze of light into the night sky, as satellite images readily show. And many cities lack even a basic commitment to dimming lights.

“The first stage is light reduction,” says Ms. Prince, the bird volunteer in Chicago. “That in itself seems the simplest of things. But it can be a challenge. It doesn’t cost anything to turn the lights out. But people, buildings, city businesses, they want the lights on.”

Till now, most efforts have depended on individual initiative. In some places that’s changing. New York, Chicago, and San Francisco as well as some smaller cities have adopted ordinances that require bird-friendly features on new buildings. But these laws are recent and apply mainly to the largest structures. And such designs, especially patterned glass, are used mainly by public institutions and universities. Most private developers avoid it, either because of the extra expense or because of their attachment to transparency.

“The fact that architects and builders are using bird-friendly technology, that is a positive,” says the Cornell Lab’s Dr. Farnsworth. “But it’s not [happening] as fast as it needs to be.”

Daniel Klem, an ornithologist at Muhlenberg College in Allentown, Pennsylvania, has watched the long awakening of concern about birds and buildings. Starting in the 1970s, he became not only a leading scholar of bird collisions but also an apostle for bird safety – and at times a voice in the wilderness. He worries that too many “feel-good” stories about Lights Out programs have obscured the bigger issue of dangerous glass. He’s tried to interest a celebrity in the cause, but has had no success. He has even patented his own glass, with UV patterning that he says works better than other varieties on the market. That’s the future, he says. And yet he, too, is frustrated that more people don’t take bird collisions more seriously.

“This is something you can’t ignore,” he says. “It’s happening everywhere. All you have to do is open your eyes and see it. But you can’t twist arms.”

It’s not yet dawn when Ms. Prince climbs out of her van and begins her rounds. A cold wind blows from the north. After a few days of unsettled weather, radar suggests a heavy migration: 690 million birds on the move, most through the country’s heartland.

Ms. Prince moves briskly, scanning the base of each building she passes. She wears sneakers, a baseball cap, and a pale green vest with pockets big enough to hold small chickens. She carries a voluminous tote and a small white net. She strides along beneath cantilevered overhangs, climbs up and down concrete staircases, explores narrow passages, crosses plazas and patios, and sometimes pauses to look into doorways to see if a bird is trapped.

There aren’t dead birds everywhere, but Ms. Prince does find a few. She also spots a warbler sitting on the concrete, upright but dazed. She moves in quickly, traps it with a flick of her net, and thrusts it into a paper bag. “It probably hit one of the upper floors,” she says. “It’s a little wobbly.” She’ll take it to the rehab center.

Ms. Prince has learned to see the city with a bird’s eye. She imagines what it must be like to be a young warbler, growing up in a northern forest and then one morning finding itself in Chicago, exhausted and confused. “I wonder if it’s not the first time they’ve seen buildings,” she says. “It must be so weird for them.”

Ms. Prince’s sympathy for birds has also made her a sharp critic of buildings. Even watching television, she can’t help but notice the ones that look hazardous to birds. Now, as she reaches the Chicago River, she gestures toward a big glass cube – an Apple Store – on the opposite side. A dozen gulls stand on its flat roof.

“I have to look hard to see the walls,” she says, with a mixture of anger and exasperation. “It looks like a big open space.”

For Ms. Prince, it’s a reminder of how much remains to be done.