City of Glass

Meet the dedicated cadre of experts and volunteers working to protect birds from glass in the window-strike capital of the United States.

Every spring, as the daylight lengthens and the weather warms, rivers of birds flow north across the Midwest. They fly high and at night, navigating via the stars and their own internal compasses: kinglets and creepers, woodpeckers and warblers, sparrows and shrikes. 

They come from as far as Central America, bound for Minnesotan wetlands, Canadian boreal forests, and Arctic tundra. They migrate over towns and prairies and cornfields; they soar over the black tongue of Lake Michigan in such dense aggregations that they register on radar. Upon crossing the water, many encounter Chicago, where they alight in whatever greenery they can find—office parks and rooftop shrubs and scraggly street-trees and the sparse landscaping outside apartment-complex lobbies.

And, as they linger and forage in Chicago’s urban canyons, they collide with glass.

To us humans, glass is ubiquitous and banal; to birds, it’s one of the world’s most confounding materials. A tanager or flicker flying toward a transparent window perceives only the space and objects beyond, not the invisible forcefield in its way. The reflective glass that coats many modern skyscrapers is just as dangerous, a shimmering mirror of clouds and trees. Some birds survive collisions, dazed but unharmed. Most don’t, done in by brain injuries and internal bleeding. Per one 2014 analysis, glass kills as many as a billion birds every year in the United States alone.

Chicago, among the largest and brightest cities within North America’s Mississippi flyway, is especially lethal—both during spring migration and again in fall, when the survivors fly south. The millions of artificial lights that glow across the Windy City present as a galaxy of false stars, confusing migrant birds that orient themselves by starlight and enticing them toward the glassy buildings below. In 2019, researchers at the Cornell Lab of Ornithology ranked Chicago the country’s most perilous city for birds—a metropolis that doubles as an ecological trap.

The city’s residents aren’t blind to the tragedy. Some architects and building managers have taken measures to protect birds, and politicians have tried to alleviate the crisis through laws and regulations. But progress has been fitful, and new glass monoliths sprout every year. Chicago thus epitomizes both the severity of the United States’ glass problem and the difficulty of summoning the will to redress it. “We have so much urban lighting, so much glass, it just puts all the wrong things together for birds,” Annette Prince, the director of a conservation group called the Chicago Bird Collision Monitors, told me. “Chicago is the perfect storm.”

One morning at the outset of spring migration, I found myself pacing Federal Plaza in downtown Chicago, waiting to join Prince as she scoured the city for birds. I shivered in the predawn damp, the sky the pearly gray of a chickadee’s wing. Everywhere around me loomed glass, geologic in its permanence and grandeur: towers of glass, spires of glass, bluffs and fins and ravines of it, a million deceptive facets of sky glittering overhead.

I wandered along a courthouse facade as transparent as an Apple store. In the revolving door squatted a white-throated sparrow (Zonotrichia albicollis), head striped in jaunty cream and black, who’d blundered into the window on his way north from the Gulf Coast. I approached and he fluttered against the door, left wing drooping. I backed off and watched him; he watched back, head cocked. The workday’s first pedestrians stomped past, collars pulled up, oblivious to the tiny being struggling at their feet. A man burst out of the lobby, sending the door spinning, and the sparrow hopped along ahead of its sweeping pane.

Before long Prince arrived—a compact, competent-looking woman in a fluorescent raincoat. After greeting me, she took a moment to sweep the sparrow up in a butterfly net, slipped him into a paper bag, and laid the bag in a microwave-sized cardboard box that she carried under one arm. Although I’d feared that the sparrow would never fly again, Prince suspected he’d merely fractured his coracoid, a shoulder bone that a rehabber could heal with a wing wrap. “It was alert and moving—that’s a good sign,” she said.

Sparrow in tow, we walked through the city, canvassing storefronts and alleys where birds might be lying, stunned or dead. Elsewhere in Chicago, other volunteers searched their own neighborhoods. This was the Monitors’ 20th year in operation; Prince, a retired speech pathologist and avid birder, had been part of the group nearly since its inception.

Each morning during spring and fall migrations, its volunteers perused the street for victims and responded to reports that members of the public called in to a 24-hour hotline. Injured birds went to the Willowbrook Wildlife Center, a sanctuary outside the city, to receive treatment and eventually be released. Dead ones went to the city’s Field Museum to enter its permanent collection. Most mornings, the dead outnumbered the living three to one.

Every year, the monitors collect around 7,000 birds, doubtless a tiny fraction of the unknowable number that die every year. Some days the work is constant: One recent October morning, the Monitors scooped up around a thousand birds at McCormick Place, a convention center abutting Lake Michigan whose massive glass façade makes it a particularly egregious hotspot. Prince joked that the volunteers measured their busyness in Valium gulped. “People call and say, hey, is there some kind of disease outbreak going around?” she said wryly. “No, it’s just architectural design.”

Prince’s phone rang: a bird reported to the hotline, in a neighborhood without a monitor. We got into her car and tore off, Prince weaving through traffic with a cabbie’s reckless surety. When we arrived at the building—a preschool and hotel fronted by sheer glass—we found a female yellow-bellied sapsucker (Sphyrapicus varius), her eyes sunken and legs gone stiff.

“Every building has its own pathology,” Prince had told me earlier. Here, it was easy to diagnose. Mirrored glass abutted a few scrawny trees outside the building, creating a faux, fatal forest: an optical illusion perfectly designed to slaughter birds. “You can see what a funhouse mirror this city is,” Prince said as she stuffed the sapsucker into a bag—a plastic one.

The sapsucker’s death was tragic both for the individual bird, and for all of avian kind. Since 1970, according to one large-scale synthesis of national bird surveys, U.S. bird populations have declined by close to 30 percent, a loss of nearly three billion animals. The culprits are many, but glass is among the most catastrophic, second only to cats as a direct killer. Certain bird species are unusually susceptible, according to a 2020 analysis, including wood thrushes (Hylocichla mustelina), yellowthroats (Geothlypis trichas), black-throated blue warblers (Setophaga caerulescens), and sapsuckers. That may be because these forest-dwelling migrants are accustomed to darting through tree canopy gaps at high speeds. For these and other vulnerable species, glass poses an existential threat.

Moreover, whereas cats or hawks often take out weaker or less wary animals, glass is an undiscerning predator, as apt to eliminate healthy migrants as sick ones. Our dead sapsucker was a hale breeding female who would have reared chicks this summer and likely for several to come. No longer. “What we’ve done here is killed one of the strongest members of her species,” Prince said with a disgusted shake of her head. “We’re incrementally taking away their future.”

The history of bird-strike research in Chicago dates to 1978. That year, Dave Willard, an ornithologist at the city’s Field Museum, received a tip from a coworker that birds were dying in droves at McCormick Place. He wandered over and found several casualties, among them a Virginia rail (Rallus limicola), a species then absent from the museum’s collection. The birds slain at McCormick, Willard realized, were an untapped scientific resource—a near-endless font of data, just waiting to be harvested and analyzed.

Over time, Willard came to realize he’d also stumbled upon a catastrophe. Around 2,000 birds died at McCormick Place every year, in large measure because the brightly lit convention center lured nocturnal migrants into peril. Willard’s documentation of the carnage eventually helped to inspire Lights Out Chicago, a citywide campaign that encourages building managers to extinguish or dim their lights during migration seasons; most years, more than 90 percent of Chicago’s towers participate. Yet the harms of glass—a material, after all, that can’t be turned on and off with the flick of a switch—have proved harder to address. “Even with lights out, there’s a tremendous number of birds (being killed),” Willard told me. “There’s no change in our taste for glass as an architectural feature.”

Although technically retired, Willard still works in a sort of emeritus capacity in the Field Museum, where I met him one afternoon. He led me into a labyrinth of cabinets and pulled open a tray packed with birds, most of which he’d collected and dressed in the early 1980s. I nearly gasped at the menagerie within, plumage undulled by four decades in a drawer: the cherry of cardinals (Cardinalis cardinalis), the lemon of meadowlarks (Sturnella neglecta), the ultramarine of an indigo bunting (Passerina cyanea). Each almost unbearably bright and beautiful; each gone too soon.

Willard reached into the tray and extracted the collection’s most remarkable member: a brown creeper, an unassuming bird that makes its living plucking insects from bark. On April 17, 2008, he explained, volunteers had found this very creeper stunned on North Michigan Avenue. They’d taken it to the sanctuary, slipped a metal band around its ankle, and released it. A year later, precisely to the day, monitors picked up the exact same bird on the exact same street, this time dead. Now the bird lay in Willard’s palm, a delicate testament to the gobsmacking fidelity of migratory birds, and to the inescapable doom wrought by the glass that blocks their way.

Today Annette Prince and her fellow volunteers annually deliver several thousand birds to the museum. Willard doesn’t generally skin these specimens, as he once did; instead, he measures their weight and the length of their wings, bills, and feet, then feeds them to ravenous, flesh-devouring beetles to reduce them to skeletons. (The “beetle room” smells precisely as foul as you’d expect.) Once the insects have done their grisly work, Willard remands each set of bones into the kind of small beige box that, in another context, might contain a wedding ring. When I visited, boxes covered desks and tables in endless stacks, like the contents of a macabre and miniature Amazon warehouse.

As the museum’s collection has grown, its birds have supported valuable research. In a 2019 study, Willard and his colleagues drew upon four decades of specimens to show that migratory birds are simultaneously losing weight as the climate changes—perhaps because it’s advantageous to have a lighter body in a warmer world—and growing longer wings to compensate for their less robust physiques. He and others have also shown that species that call to each other in flight, like white-throated sparrows, are more likely to hit glass, possibly because flock members are attracted to light and then draw their comrades toward danger, like pilot whales beaching en masse. Windows are at once revelatory and tragic, offering insight into the lives of birds even as they obliterate them.

“We’d trade everything we’ve learned not to have them dead on the sidewalk.”

— Dave Willard, Field Museum

Yet Willard was that rare scientist who craved less data, not more. He opened a box to reveal the yellowed bones of a golden-crowned kinglet, so light and fragile it seemed they’d crumble to dust if I blew on them. He smiled sadly. “We’d trade everything we’ve learned,” he said, “not to have them dead on the sidewalk.”

For as long as buildings have sported glass, birds have likely collided with it. In an 1832 ornithology textbook, the naturalist Thomas Nuttall related the tale of a young male hawk who, while “descending furiously and blindly upon its quarry,” smashed through a greenhouse. Miraculously, the hawk was “little stunned,” though his “wing-feathers were much torn.”

In Nuttall’s day, glass was comparatively rare: windows tended to be small and set within brick or granite. Today it’s everywhere—particularly in Chicago, longtime home of the mid-century architect Ludwig Mies van der Rohe, whose preference for vast glass facades still influences the city’s aesthetic. van der Rohe’s purpose, he once said, was to fuse nature, humans, and structures in a “higher unity.” The virtue of glass was that it connected indoor spaces with outdoor ones. The irony is awful: We prize a material that kills birds because it makes us feel closer to nature.

Yet even a perilous building can be made safer. One day, I took a self-guided tour of the Chicago area’s bird-friendly architecture. I started in Evanston, home of Northwestern University, which had retrofitted a couple of particularly deadly buildings in response to monitors’ data. Most problematic was the Kellogg Global Hub, a business-school headquarters as colossal and vitreous as an airport terminal. In 2018, Northwestern had coated part of Kellogg’s facade with a translucent, dot-patterned film designed to make the building visible to birds. The dots, which were so faint that human passersby were unlikely to notice them, were spaced about as far apart as the width of my palm. Any wider than that, and birds would attempt to fly between the dots, as they flit through dense twigs and leaves. (A single hawk decal on a big pane? Essentially useless.)

The film seemed to be working: Collisions at the Kellogg Hub had declined by half, and for 20 minutes I watched red-winged blackbirds (Agelaius phoeniceus) alight easily on its railings and roof. Even better, at the nearby Frances Searle Building, whose windows the university had covered with faint horizontal stripes, bird deaths had virtually ceased. Yet the projects had been neither cheap nor perfect. The Searle Building alone had cost the university $250,000 to retrofit, and large swaths of the Kellogg Center remained unprotected. Retrofitting existing buildings is crucial, no doubt; Chicago isn’t about to dismantle its existing skyline for the sake of birds. Still, “the best solutions are the ones that are designed into the building from the beginning,” Claire Halpin, a landscape architect who sat on the board of the Chicago Ornithological Society until her recent death, told me later.

Few architectural firms do that better than Halpin’s former employer, Studio Gang, the firm behind some of Chicago’s bird-friendliest mega-structures. I visited two of them, starting with the Aqua Tower, an 82-story monolith frilled with curvaceous balconies, as though the building had sprouted shelf fungi. The terraces lent the tower “visual noise,” warning birds that this otherwise reflective structure was in fact a solid object. The studio had applied similar principles at Solstice on the Park, an apartment complex whose glass panels were angled toward the ground and thus reflected earth rather than sky. The lobby’s windows were also subtly covered with dashes—a material, known as “fritted” glass, with markings printed on the pane rather than added retroactively. Both buildings, I noticed, incorporated enormous expanses of glass, yet they possessed a visibility that other Chicago towers lacked.

“It’s a mistaken view that bird-friendly architecture means you have to build this opaque thing that nobody can see in or out of,” Halpin said. “Transparency does not have to be in conflict with birds.”

What’s more, avian safety doesn’t always require structural overhauls. During migration season, the Federal Bureau of Investigation swaddles its Chicago headquarters in fine black mesh, off of which birds harmlessly bounce. At the Blue Cross Blue Shield tower, birds used to die often while trying to reach a potted ficus stationed invitingly in the lobby. At Prince’s suggestion, the building’s managers moved the plant away from the window, and the collisions virtually ceased.

Not everyone was so receptive. During our rounds, Prince and I passed the Bank of America Tower, where three trees beckoned to birds from behind glass so clear it was surprising more humans didn’t hit it face-first. Prince had begged the building’s sustainability officer to put up a patterned film, or move the trees, or even hang a temporary banner during migration season — to do literally anything. “The answer,” she told me, “was ‘no, no, and no.’”

Fortunately, Chicago’s bird advocates are also attempting to influence policy and compel widespread change. In 2021, Illinois began to require new state-owned buildings to incorporate netting, screens, shutters, and other bird-friendly features, one of the country’s first such laws. Even more promising, in 2020 Chicago passed an ordinance mandating that new buildings limit their use of transparent and reflective glass, use patterned glass in high-risk areas, and reduce the night-time interior lighting that lures birds to their death. The city’s Department of Planning and Development expects to implement the ordinance by the end of this year.

This progress hasn’t been greeted with pleasure by all developers, some of whom fear that patterned glass will jack up construction costs and deter retailers from renting space. As one put it to Landscape Architecture Magazine, “There is a real big bird that this ordinance is going to kill: the biggest bird in town, the goose that laid the golden egg in real estate.”

Yet the notion that protecting birds harms business is largely a myth. For one thing, because nearly all bird collisions happen in the lowest hundred feet, architects don’t need to treat entire high-rises with bird-friendly glass; instead, the bottom seven stories generally suffice. For another, glass represents such a minuscule portion of construction costs that spending a little extra has virtually no effect on a building’s bottom line. In a 2022 report, Daniel Klem, an ornithologist at Muhlenberg College who’s studied window collisions since the 1970s, found that bird-safe glass adds less than four-tenths of a percent to the cost of a typical building. (An $8 million office tower, for example, would only pay an additional $30,000 or so.) Moreover, Klem argued, as pro-bird laws in Illinois, Minnesota, Maryland, and other states drive up demand for bird-friendly glass, glass manufacturers will inevitably produce more of it and lower their prices.

“Right now, the majority of developers and architects don’t have this issue on their radar, but many are changing,” Klem told me. Years ago, he said, a magazine had branded him the “Rodney Dangerfield of ornithology,” referencing the comedian whose trademark joke was that he never got the respect he deserved. Within the past decade, however, respect for bird collisions has arrived, if belatedly. “Members of these key constituencies are joining the cause of saving more lives from windows,” Klem said. “These are innocent creatures that need our help.”

At the end of our patrol, Prince took me to a parking lot where a car, one door emblazoned with a Bird Collision Monitors panel, had been left unlocked—with windows cracked and interior pleasantly cool—so that surveyors could drop off their bounty. In the backseat, a cardboard box brimmed with dead birds bound for the Field Museum: an eastern towhee (Pipilo erythrophthalmus), a yellow-rumped warbler (Setophaga coronata), a veritable flock of white-throated sparrows.

Happily, the hatchback was also full of paper bags that rustled faintly as their captives stirred—the live animals that a volunteer would soon drive out to the sanctuary, and that might yet continue interrupted journeys.

Prince inspected the bags’ sharpied labels. Woodcock. Virginia rail. Red-headed woodpecker (Melanerpes erythrocephalus). “That one’s probably a mislabeled sapsucker,” she said. She reached in and removed a gorgeous creature—wings and breast feathered in dapper black and white, vivid maroon from the shoulders up—sure enough, a red-headed woodpecker. He crankily clamped his bill around Prince’s thumb.

“Right in the middle of this dense urban area, we’ve got these exotic birds that people would be thrilled to see in the woods or a marsh.”

— Annette Prince, Chicago Bird Collision Monitors

“You are so beautiful. Oh, you’re so pretty,” Prince cooed. “You’re gonna go out to a nice forest preserve where you can be away from these scary buildings.” She turned to me, full of wonder. “Right in the middle of this dense urban area, we’ve got these exotic birds that people would be thrilled to see in the woods or a marsh.”

Like glass itself, bird collisions are often invisible. Songbirds are tiny and delicate; thousands of loafer-clad feet step over them, and rats and crows scavenge them. Clever gulls have even learned to chase songbirds into buildings and snatch them as they fall limp—resident birds exploiting the naïveté of migrants. Cities devour birds and grind them to pulp, a crisis both omnipresent and mostly unremarked.

The beauty of monitoring is that it reveals this hidden catastrophe: that it alerts a great many more people to the millions of small lives that wing through cities, and even converts them into advocates. At one point, as Prince and I drove around Chicago, she took a call from a doorman on Wacker Street. We raced over and found a red-breasted nuthatch (Sitta canadensis) huddled against a planter, so dazed that Prince could pick her up without a net. “Did you find it?” the doorman called, his voice anxious. His nametag read Fernando. Prince showed him the nuthatch in her palm and his eyes widened. “Cool, cool,” Fernando said, with evident relief.

Prince unfolded a paper bag, slipped the nuthatch inside, and stowed it in the trunk. Fernando watched, riveted. “Hey, could I get a couple of those bags?” he asked. “For next time?” Prince was delighted to oblige.

Ben Goldfarb

Ben Goldfarb

Ben Goldfarb is a Colorado-based environmental journalist whose writing has appeared in The Atlantic, National Geographic, The New York Times, and many other publications. He is the author of Crossings: How Road Ecology Is Shaping the Future of Our Planet and Eager: The Surprising, Secret Life of Beavers and Why They Matter, winner of the PEN/E.O. Wilson Literary Science Writing Award. Follow him on Twitter @ben_a_goldfarb and read more of his work at

Christy Frank

Christy Frank

Christy Frank is a freelance conservation and documentary photographer, writer, and filmmaker based in the Great Lakes region. She is passionate about storytelling that intersects environmental and human conditions, science communication, habitat restoration, and species recovery efforts. Her work has appeared in Audubon, National Wildlife Federation, CBS News, PBS Nature, and other outlets.

Patricia Homonylo

Radim Schreiber

Patricia Homonylo is an animal photojournalist and filmmaker based in Toronto, Canada. As habitats continue to dwindle, non-human beings are struggling to survive, and Homonylo’s work spotlights species caught in the crosshairs. She is a We Animals Media photographer and her documentary When Worlds Collide won top honors at the Discovery Film Festival (London).

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