Clear & Present Danger

Millions of birds perish every year from crashing into glass windows. After decades of inattention, biologists, builders, and architects are joining together on solutions that will benefit both people and the birds.

By David Malakoff
Photograph by Robert McCaw

Darragh Brady wouldn't hurt a fly, much less a flycatcher. So the architect was devastated recently when she discovered that a breathtaking, $10 million building she had helped design was a death trap for songbirds. "I was appalled," she says. "I'm a birder, and I had inadvertently created this building that was killing birds." The culprit is as clear as the day: A glittering glass entryway that guides visitors to the Maryland Historical Society in downtown Baltimore to an array of treasures, including a priceless original manuscript of Francis Scott Key's Star-Spangled Banner. It is, literally, a stunning piece of architecture. But some unsuspecting birds, it seems, simply don't see its transparent, unyielding walls and crash into them at full speed. Others may be fooled by a tempting reflection of trees or sky. Many are killed instantly. But even dazed survivors aren't out of danger, explains a mortified Brady: "We put a reflecting pool at the base of one of the curtain walls—so some of the birds fall in and drown." About two dozen birds have died at the building since it opened last fall, she estimates, including hermit thrushes and warblers.

That toll may not seem like much. But even such small glass kills can add up to big trouble, believes ornithologist Daniel Klem of Muhlenberg College, in Allentown, Pennsylvania. Between 100 million and 1 billion birds die in glass collisions every year in North America alone, Klem estimates. At the very least, that's an average of one bird a year slamming into each of the roughly 100 million homes, apartment buildings, office towers, schools, and storefronts that dot the American landscape. "Glass is one of the world's great bird killers," rivaled only by habitat destruction and perhaps cats, says the blunt-spoken, 57-year-old ornithologist.

Glass gets less attention than other threats, however, because "most kills are pretty small, just a bird here or there," says Frank Gill, Audubon's director of science. "Few people see the big picture."

That may be changing—in part owing to two decades of work by Klem, whom Gill calls "perhaps the only scientist to have gone at this problem systematically." Conservationists and architects interested in reducing glass's death toll are increasingly calling on Klem for help. His bird-friendly building ideas are being incorporated into construction projects, and may also find their way into evolving guidelines for "green" buildings. Bird advocates, including New York City Audubon, have even cited Klem's work in their bid to shape the rebuilding effort at New York's World Trade Center site.

But efforts to reduce glass kills face formidable obstacles. Like Brady, most architects and builders are unaware of the problem, or of what they can do about it. Consumers often balk at solutions that mar their views. Ironically, building codes intended to improve our quality of life or to save energy sometimes encourage greater use of windows. And Klem says we still have a great deal to learn about why birds hit windows, and how to prevent this.

Meanwhile, the hazard grows: Factories worldwide will churn out about 40 billion square feet of sheet glass this year, enough to cover the state of Rhode Island. And much of it will go into new homes—some 1.4 million in the United States alone—that are featuring unprecedented amounts of glass, from panoramic picture windows to towering patio doors. "Just when you think designers have exhausted the possibilities, they find new uses for glass," says Sandy Isenstadt, a Yale University architect who has studied the history of windows.

Forged from humble sand melted at more than 1,500 degrees Fahrenheit, sheet glass was once an almost magical material available only to the wealthy. But by the early 19th century, glass had become, well, as cheap as sand. Soon naturalists were tallying up the avian casualties at greenhouses and garden windows. In 1876, for instance, the naturalist C.S. Townsend collected a yellow-billed cuckoo that had been killed flying into glass. The species "seems especially prone to run its head against windows," he later wrote in The Auk, a leading bird-science journal.

That was only the beginning of the crystal era, however. Between 1900 and 1930 window areas on typical American homes jumped by about 50 percent, Isenstadt notes. A few bold architects built totally glass houses, transparent cubes that are now considered historic landmarks. In big cities glass-clad skyscrapers began to dominate skylines. Today the world's tallest building—the 1,483-foot-tall Petronas Towers in Kuala Lumpur, Malaysia—boasts no fewer than 32,000 windows.

Such buildings often become "bird slaughterhouses," says Klem. In Chicago, for example, researchers have collected more than 26,000 dead birds over the past two decades from the footings of the McCormick Place Convention Center. New York City's Twin Towers and several Toronto skyscrapers also became infamous bird hazards. In all three cities the biggest kills typically occurred at night during spring and fall migrations, when building lights appeared to lure the birds into deadly collisions. Light-dimming campaigns, such as those led by the Toronto-based Fatal Light Awareness Program, have helped reduce the problem. (See "The Dark Side of Light," Audubon, March-April 2000.)

But birds aren't at risk just during night migrations, says Klem, because most glass kills occur in daylight, and "winter is probably the biggest season." In part that's because "most people put their feeders at some convenient place—on a tree or a deck, so they can watch the action—but generally not close enough to protect the birds, at three feet or less from a window."

Buildings don't have to be big to be big killers, he adds. In the late 1970s and early 1980s, Klem studied glass kills at two typical single-family homes, one in rural southern Illinois and the other in suburban Purchase, New York. He documented roughly 100 bird strikes, about half of them fatal. Ironically, even buildings built to promote conservation—from glass-walled nature centers to hawk-watching outposts—have become hazards. For instance, at the Strawberry Plains Audubon Center in Holly Springs, Mississippi, "We have the viewing-window catch-22—a large window overlooking a beautiful garden that attracts many birds, combined with that horrible thud of the viewed bird hitting the window," says Kristin Lamberson, a horticulturalist at the center.

Such anecdotes, along with statistics from an array of studies, ultimately became the basis for Klem's glass-kill estimates, which he first published in 1990 in the Journal of Field Ornithology. He admits that some of his assumptions—such as a key estimate that 1 to 10 birds die per year at the average structure may be "a bit soft." But other researchers say Klem's assessment isn't wildly off the mark. Gill, for one, believes that experts "can and will debate the exact number, but I think Dan's been very conservative in his calculations."

If Klem's billion-death figure is accurate, it would mean that glass kills roughly 5 percent of the 20 billion birds that scientists estimate live in the United States each fall, right after the breeding season. And it's roughly equivalent to the higher estimates for the number of birds killed by cats. Even Klem's lower, 100-million-death estimate would mean that glass kills about as many birds as hunters, and far more than are killed by cars, pollution, poisons, and communications towers.

"Nobody sets out to intentionally kill birds," says one ornithologist. "they just like a nice view."

Worldwide, Klem figures, at least 556 kinds of birds—or 6 percent of all known species—have had run-ins with glass. In North America the number is 225, or about a quarter of all species. The most frequent victims? Relatively common songbirds that are comfortable around civilization, such as dark-eyed juncos, American robins, cedar waxwings, and several kinds of thrushes. But rare species aren't immune. In Australia glass collisions in one recent year killed about 30 of the 2,000 remaining swift parrots.

Still, even after decades of tallying such grim statistics, Klem is understanding. "Nobody sets out to intentionally kill birds," he says. "They just like a nice view." And he has spent thousands of hours running clever experiments to identify solutions. In one, for instance, he erected a row of different windows on the edge of a woodland to see which ones wild birds would avoid. Other times he used captive birds to test an array of strike-prevention tactics, from window decals shaped like falcons to wind chimes and blinking lights. The ongoing research has helped identify a range of options for making windows bird-friendlier, from whitewashing glass to make it visible, to hanging showy streamers or netting to head off fliers.

The studies, however, have cast some doubt on one of the most commonly recommended solutions: pasting bird silhouettes on problematic panes. While the Strawberry Plains Audubon Center eventually solved its bird-strike problem by using hummingbird silhouettes, Klem has found that the decals typically aren't big enough to deter birds, although using more than one can help. In general, he has discovered that birds respond best to patterns—such as stripes or dots—that are composed of elements just two to four inches apart and that cover a majority of the glass.

"Designers deal with trade-offs all the time. This is the kind of challenge a good architect would revel in."

A few designers have experimented with other approaches. At the Oak Hammock Marsh in Manitoba—a renowned birding destination—Canadian architect Robert Eastwood designed an award-winning interpretive center that uses deeply recessed windows to reduce reflections, and minimizes areas where windows create "see-through" corridors. The building also sports hardware for protective nets, "but we haven't used them—we've only had 14 fatal strikes a year since we opened [in 1993]," says naturalist Paula Grieef. Other designers have noted some success in reducing strikes by using glass that doesn't reflect much light or by installing windows at downward angles so that they reflect the ground.

Perhaps the highest-profile and most ambitious bird-friendly building experiment, however, is currently under way on the campus of Swarthmore College, in Swarthmore, Pennsylvania. There, builders are installing "fritted"panes—which use small dots of opaque glass to achieve a semifrosted look—in a new $71 million science building. The move came after Klem had discussed his findings at the school and after a campus biology professor had reported that another building project appeared to have killed off a campus population of hummingbirds.

"Klem totally convinced us that we had to do something," says engineering professor Carr Everbach, who then ran studies and a campus poll to identify a dotted pattern that would ward off birds but still be aesthetically acceptable. Once the building is complete in April, Everbach hopes to use it as a massive experiment. For instance, he wants to compare bird strikes at the building's fritted and clear panes—and even hopes to install "thump" sensors that will allow researchers to videotape the behavior of birds that collide with the clear panes.

The Swarthmore project is inspiring others. New York City Audubon, for example, acquired Everbach's help—and a chunk of the college's fritted glass—as part of its campaign to make sure that whatever replaces the World Trade Center is bird-friendlier than the fallen towers. "This is an opportunity to set a world-class example of green design," says E.J. McAdams, chapter director. Conservationists are also working to incorporate bird conservation issues into influential energy and environmental guidelines developed by the Washington, D.C.–based Green Building Council.

Such efforts will require difficult trade-offs. Mirrored glass, for instance, can help conserve energy. And some European building codes encourage the use of glass as a way to reduce lighting costs and improve worker morale by providing natural light. New technologies, such as glass that may look opaque from just one side or that holds particles that allow panes to change from transparent to translucent with the flick of a switch, could also help ease conflicts. But such products will be used only if consumers demand them—and so far "there's been less than zero demand from customers," reports Chris Barry, director of services for Pilkington Glass in Toledo, Ohio, one of the world's largest producers of flat glass.

If the pressure for bird-friendly structures does rise, though, Yale's Isenstadt believes designers will be up to the task. "Designers deal with constraints and trade-offs all the time," he says. "This is the kind of challenge a good architect would revel in."

Brady, the Baltimore architect, agrees. With Klem's help, for instance, she's exploring aesthetically acceptable ways to prevent birds from crashing into the Baltimore museum. So far, she notes, the problem appears to be limited to migration season, so altering lighting or installing temporary netting might do the trick. She's also briefed colleagues at her firm, Ziger/Snead, on the issue, and has begun thinking about how to incorporate the lessons she has learned into future projects. She has also been talking up the issue with glass providers, challenging them to do more to educate builders.

The fact that such discussions are even taking place has made Klem cautiously optimistic that the problem may get wider recognition. "It's a situation that causes a lot of guilt and anxiety, partly because there's no single, easy fix," he says. But a united effort, he adds, would go a long way toward finding solutions acceptable to all sides. "If we can get conservationists, builders, architects, and property owners working together," he says, "we could make life a whole lot better for people and birds."


David Malakoff, a writer for Science magazine, lives in Alexandria, Virginia. While he was researching this story, a house sparrow flew into his kitchen window—and survived.




© 2004  NASI

Sound off! Send a letter to the editor
about this piece.

Enjoy Audubon on-line? Check out our print edition!


Here are five tips for keeping birds away from your windows.

1. Put feeders within three feet of windows; at this distance, approaching birds aren't moving fast enough to get hurt.

2. Hang obstacles in front of the glass: tree limbs, strips of cloth or shiny materials, old CDs, toilet-paper rolls, or feathers on a string.

3. Spray the glass with vegetable oil or fake snow to make it opaque. Or stick on plastic wrap. Consider products used on greenhouses to make glass translucent.

4. Put decals—including dots or bird silhouettes—on the outer glass face. Space decals uniformly, two to four inches apart, to transform the window into an obstacle birds will see and avoid.

5. Hang netting, or buy special see-through screens. Also consider installing awnings that can be lowered when you go to bed or raised when you want a view.

To purchase decals, go to www.wpines.com.
For additional information, go to www.rlrouse.com/window-protector.html.