This article originally appeared in longer form in bioGraphic.
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. Many don’t, done in by brain injuries and internal bleeding. Per one 2014 analysis, glass kills as many as 1 billion birds every year in the United States alone.
Chicago, among the largest and brightest cities within North America’s midwestern flyway, is especially lethal—both during spring migration and again in fall, when the survivors fly south. The artificial lights that glow across the Windy City present as a galaxy of false stars, confusing migrant birds that orient themselves by starlight and potentially 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.
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 U.S.’s 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.
Before long, Prince arrived—a compact, competent-looking woman in a fluorescent raincoat.
She took me on a walk 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, has been part of the group nearly since its inception. Each morning during spring and fall migrations, its volunteers peruse the street for victims and respond to reports that members of the public call in to a hotline. Injured birds go to the Willowbrook Wildlife Center, a sanctuary outside the city, to receive treatment and eventually be released. Dead ones go to the city’s Field Museum to enter its collection. Most mornings, the dead outnumber the living three to one.
Every year, the monitors collect about 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 about 1,000 birds at McCormick Place, a convention center abutting Lake Michigan whose massive glass facade makes it a particularly egregious hot spot. 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, 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 reflected 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 fun-house 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 3 billion animals. The culprits are many—especially habitat loss and climate change—but glass is among the most catastrophic of direct, human-related killers, second only to cats, according to one 2015 study. Certain bird species are unusually susceptible, according to a 2020 analysis, including wood thrushes, yellowthroats, black-throated blue warblers, and sapsuckers. That may be because these forest-dwelling migrants are accustomed to darting through tree-canopy gaps. For these and other vulnerable species, glass poses an unignorable 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.”
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 that, 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, the longtime home of the mid-century architect Ludwig Mies van der Rohe, whose preference for vast glass facades still influences the city’s aesthetic. His 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 data from local bird monitors. 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 the 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 precipitously, and for 20 minutes I watched red-winged blackbirds 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 dropped dramatically. Still, the projects had been neither cheap nor perfect. Retrofitting existing buildings is crucial, no doubt; Chicago isn’t about to dismantle its existing skyline for the sake of birds. But “the best solutions are the ones that are designed into the building from the beginning,” Claire Halpin, an 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 has sprouted shelf fungi. The terraces lend the tower “visual noise,” warning birds that this otherwise reflective structure is in fact a solid object. The studio applied similar principles at Solstice on the Park, an apartment complex whose glass panels are angled toward the ground. The lobby’s windows are 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, incorporate enormous expanses of glass, yet they possess a visibility that other Chicago towers lack.
What’s more, avian safety doesn’t always require structural overhauls. During migration season, the FBI swaddles its Chicago headquarters in fine black mesh, off of which birds harmlessly bounce. At the Blue Cross Blue Shield tower, numerous birds used to die while trying to reach a potted ficus stationed invitingly in the lobby. Prince said that at her suggestion, the building’s managers moved the plant away from the window and the collisions virtually ceased.
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. 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 interior lighting that can lure birds to their death. The city’s Department of Planning and Development is continuing to implement the ordinance.
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 many advocates say the notion that protecting birds harms business is largely a myth. For one thing, because most bird collisions happen in the lowest hundred feet, architects don’t need to treat entire high-rises with bird-friendly glass. For another, glass represents a small portion of construction costs. 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 ordinances drive up demand for bird-friendly glass, glass manufacturers are likely to 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.”