The dazzling views of Central Park come with a dark side.

Each spring and fall, dead and injured birds litter the front sidewalk and interior courtyard of a glassy, crescent-shaped building of about 50 condominium units on the northwest corner of the park. The casualties are brightly colored travelers on migrations that would normally take them hundreds or thousands of miles.

From tiny yellow warblers to large, elegantly marked woodpeckers, their journeys end at the building, Circa Central Park, when they crash into glass they can’t see.

The deaths have brought outrage from bird advocates, shame on social media, disapproval from neighbors and even stronger disapproval from the residents’ own children. News articles have labeled the building a “death trap.” Online reviews became an embarrassment; a simple address check by dinner guests could lead to uncomfortable questions about dead birds.

Circa Central Park certainly isn’t the only bird-killing building in the city, but it appears to be among the worst. Last year, the number of window strikes at Circa put it in the top three among buildings monitored by NYC Audubon. Now residents are trying to fix the problem, joining a small but determined global push to make glass more bird-friendly.

“Circa is so important because it’s individuals,” said Dustin Partridge, the director of conservation and science at NYC Audubon, which worked with Circa on their addition of bird-deterring window film, a stick-on pattern that makes glass more visible to birds. “These are residents that are making a rare decision and they’re helping save hundreds or thousands of birds a year.”

Over the last several decades, as people have fallen in love with floor-to-ceiling windows and light-strewn spaces, birds have suffered the consequences. The amount of glass in a building is the strongest predictor of how dangerous it is to birds, according to a report on the issue published by the city of Toronto.

Earlier this month in Chicago, almost a thousand birds were killed on a single day at a single building, McCormick Place. Nationwide, researchers estimate that hundreds of millions of birds die crashing into windows each year. It’s one factor, along with problems like habitat loss, behind a sharp decline in North America’s bird population. Since 1970, numbers have dropped by about 30 percent.

Architects and companies are trying out solutions. New windows at the Javits Center, a convention space in Midtown Manhattan, are set with patterns that make them more visible to birds, and deaths have declined by 90 percent. Since most crashes happen within 100 feet of the ground, skyscrapers are advised to treat only the first 10 or so stories.

Advocates in cities across the country, meanwhile, are keeping up the pressure by counting the dead and posting photos on social media.

The move at Circa wasn’t a shoo-in. After all, to make existing glass bird-friendly, you’re spending money on something that could interfere with your views.

How much it interferes is a matter of debate. The dot stickers chosen by Circa’s residents are translucent but still visible, especially at certain angles or against certain backgrounds. One resident who was not involved on either side and declined to be quoted estimated that the building was split more or less evenly between those for and against the endeavor.

But James Levy, the president of the condo board, said there was broad consensus for the first phase: treating all the glass and clear railings surrounding the courtyard, where most of the building’s bird strikes occur. That’s almost done, at a cost of about $60,000.

“The visual interference is not really that significant,” Mr. Levy said.

Bright Lights, Big Hazards

Across cities like New York, Chicago and Washington, volunteers deploy to the streets during spring and fall migrations, collecting the data needed to push for bird-friendly glass with building owners and elected officials.

New York and San Francisco have passed laws requiring such measures for new construction and major renovations.

Many birds migrate at night, but lights lure them disproportionately to cities, researchers say. Lethal crashes appear to peak in the early morning as the birds start to look for food. Unlike urban birds, these migrators are used to living in forests and grasslands from Canada to South America. They don’t understand that reflections of sky and trees aren’t the real thing.

Melissa Breyer, an editorial director at a website who lives in Brooklyn, started volunteering with New York’s monitoring effort, Project Safe Flight, after seeing a social media post about 26 birds killed at Circa on a single spring morning in 2020. She found herself patrolling New York’s deadliest monitored area: The World Trade Center. Once, in 2021, she came upon a massacre of some 300 birds strewn between three buildings there — an unusually deadly day.

On a recent morning, her eyes scanned as she walked, paying close attention to corners and awnings.

Her first, predawn check at 1 World Trade Center was uneventful, perhaps because the building’s sweepers got there first (“It’s like an arms race here,” she said).

But upon returning 20 minutes later, she suddenly stopped. A tiny silhouette of a bird hunched perfectly still near an entrance, apparently stunned. Ms. Breyer took a brown paper bag out of her backpack and slowly approached. The bird didn’t move until she grabbed it, at which point it squawked in protest, a good sign.

“A pine warbler,” Ms. Breyer said, placing it in the bag for transport to New York’s bird rehabilitation clinic, the Wild Bird Fund.

On the awning just above, a lifeless ruby-throated hummingbird was splayed where it fell, too high to reach. Around the corner, another bird lay dead on the awning of another building. A moment after Ms. Breyer documented it, a gull flew over and swallowed it in one gulp. Researchers say they miss a lot of birds because of scavengers like rats, crows and even, as Ms. Breyer has noticed recently, an especially bloodthirsty squirrel.

Dara McQuillan, a spokesman for Silverstein Properties, which owns and manages 3, 4 and 7 World Trade Center, said the building met with NYC Audubon after a mass of bird strikes in 2021 and started encouraging tenants to turn out lights. Since then, he said, the number of strikes has been small enough that it didn’t seem worth imposing on tenants’ views with bird-friendly window treatments.

According to a statement from One World Trade Center, the building has textured glass on the vast majority of the building’s surface on lower floors, and the lights on the podium are dimmed year-round. “Our team continues to work with experts on various strategies to protect birds,” the statement said.

Even though volunteers like Ms. Breyer can cover only a tiny fraction of the city (they patrol 13 routes in New York), the data they gather is a potent tool. She herself has documented more than 1,600 dead or injured birds in three years, and takes comfort in giving some purpose to each bird’s death.

“I’m determined to collect any bird that I can find,” Ms. Breyer said. “The more I can record, the more data they have, the more power they have.”

‘More Architects Are on Board’

Companies that manufacture bird-friendly products say demand is growing.

“We see more architects are on board with designing bird friendly, even in areas that do not have legislation in place,” said Paul Groleau, a vice president at Feather Friendly, a company that makes pattern treatments for windows. In the past 5 years, business has increased 20-fold, he said, and this year alone has seen a jump of 20 percent over last year’s sales.

NYC Audubon keeps boxes of sample glass and film from various companies that developers or homeowners can peruse.

One of the biggest hurdles is existing buildings, since new laws often don’t cover them. But Christine Sheppard, a biologist and researcher who directs the glass collisions program at the American Bird Conservancy, a nonprofit group, said people are also demanding change as awareness of the issue grows.

“Having your building be a bird killer is more and more of a negative,” Dr. Sheppard said. “People really don’t like living or working in a building that’s killing birds.”

At Circa, Mr. Levy started looking into solutions after his wife, working from home during the pandemic, grew distressed by the repeated thumps she would hear during video calls. Another board member, Bradley Bennet, got involved after bird strikes interrupted one family breakfast after another. Dead, dying or injured birds would drop onto his terrace.

“Your kids are in their pajamas,” Mr. Bennett said. “Feathers are still on the glass and it’s twitching on the ground.”

While some residents genuinely care about the birds, Mr. Levy and Mr. Bennett say there’s also an economic argument to make: bird deaths are compromising the value of the building.

The next phase of window treatments would involve the front of the building, which faces Central Park. It’s the most controversial part of the project, since those are the Central Park views. And at Circa, they are multimillion dollar views.

Residents will have a chance to get used to the new glass in the courtyard and see how effective it is. If enough residents still support it, they will make the front bird friendly, too.