August 2005

Volume 14, Number 8

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Article Contents

Design for the Birds: Protecting Birds from the Hazards of Glass

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3M Canada donated this window film to the Earth Rangers Centre in Woodbridge, Ontario. Designed by architectural consultant John Butner, the tree-patterned film was applied to more than 100 second-floor windows specifically to prevent bird collisions.

Photo: Fatal Light Awareness Program

“There is unbelievable carnage taking place,” says Daniel Klem Jr., Ph.D., a biology professor at Pennsylvania’s Muhlenberg College and the world’s foremost expert on the phenomenon of birds colliding with buildings. “If you take the number of birds killed from the Exxon Valdez oil spill and compare it to my lowest estimate of the number of birds killed flying into windows,” says Klem, “it would take 333 Valdezes every year to equal that number. The Exxon Valdez is nothing compared to the animals dying at glass.”

Those of us engaged in the design and construction of buildings are increasingly aware of the intimacy between the built environment and the natural world. And, through our adoption of a philosophy of sustainability and our implementation of green design principles, we are beginning to assume responsibility for the negative consequences of that bond. We limit energy consumption, incorporate sustainably harvested wood, and avoid ozone-depleting refrigerants in the hopes that our actions will help move humanity toward a sustainable relationship with other species. Yet, for the most part, we have ignored one of the most direct threats our buildings pose to the natural world. Because birds collide with buildings as a result of design decisions, the design community possesses a unique capability to solve the problem.

The Problem

Although windows have always posed a threat to birds, the severity of the collision problem has grown significantly in recent decades. Beyond the ever-increasing size and number of buildings, glass itself has become a far more popular building material. Before World War II, the size of windows was limited by technology and cost. With the adoption of float-glass manufacturing, however, glazing panels became stronger and cheaper, and the design aesthetic began to favor large expanses of glass. “Here you have a product that’s so universally enjoyed and prolifically used that there are very few human structures that don’t have a piece of glass in them,” says Klem. “The situation applies anywhere on Earth to any type of building with a window,” says Randi Doeker, self-described evangelist for bird-safe buildings and founding director of the Birds and Buildings Forum. “Birds are everywhere.”

Daytime dangers


Birds often fail to recognize reflective glass as a solid surface, flying straight into reflections of safe habitat.

Photo: Jessica Boehland

Birds often strike windows because they see trees and the sky reflected in glass and mistake it for the real thing. Clear glass can be quite reflective, depending on lighting conditions and the angle at which it is viewed. Mirrored glass, lauded for its aesthetics, low cost, and effectiveness at blocking heat gain, is even worse. “Mirrored glass is to songbirds what DDT is to birds of prey,” says Michael Mesure, executive director of the Fatal Light Awareness Program (FLAP), in Toronto, Ontario, a nonprofit organization dedicated to raising awareness of this problem. Bruce Fowle, FAIA, of Fox & Fowle Architects in New York City, points out that “reflective glass is designed to be as minimally visible in the skyline as possible.” It often functions as nature-patterned camouflage for buildings.

Birds also strike windows when they see through clear glass—clear windows are especially problematic when they align so that the line of vision continues in one window and out another (such as through lobbies or walkways) or when indoor vegetation is clearly visible through the glass.

Nighttime dangers

The night brings another set of risks for birds, particularly for those migrating over cities. For reasons not well understood, birds are attracted to light. Ornithologists speculate that birds navigate by the stars and the moon, and that light from buildings confuses them. The problem worsens during inclement weather, when low clouds force migrating birds to fly at a lower altitude. Disoriented by lights, birds can fly into windows, walls, and even other birds. Some become trapped by beams of light, unable or unwilling to land or leave the beam. “Birds get exhausted and literally drop to the ground,” Doeker told EBN. Others land nearby, often “where they shouldn’t be—in the middle of the downtown business district, for example.” These weary birds are more vulnerable to being preyed upon. Those that survive the night face an unfriendly environment in the morning, including all of the daytime dangers described above and, in dense urban locations, the threat of starvation.

The magnitude of the problem

Though most of us can recall the sickening thud of a bird hitting a window, few of us are aware of the magnitude of the problem. “Even the scientific community has ignored this,” Klem told EBN. Window strikes are “a silent killer,” he says, largely because we rarely see the carnage. Vegetation around homes hides the dead and dying. Meanwhile, seagulls, crows, and vultures; rats, cats, and dogs remove the casualties. These scavengers learn to recognize the most bountiful buildings and stake out favorable positions; seagulls cruise by waiting for opportunity to strike. Maintenance personnel at commercial and institutional buildings also hide evidence by clearing away birds before employees and visitors arrive each morning. “Building staff at an illuminated skyscraper have reported filling a 55-gallon barrel with dead birds in the morning,” says Doeker.

“Nobody has a way of assessing how many individuals are dying,” says Klem. But his research, supported by other studies, suggests that somewhere between 100 million and one billion birds die each year in the U.S. as a result of striking windows. That range represents between one and ten birds per building per year. Some experts, including Doeker, believe the toll is in the billions each year. About 225 bird species—a fourth of all species in the U.S.—are known to have hit windows. Second only to habitat loss, collision with windows is now considered the largest human cause of bird mortality, according to Klem, beating out hunting, cat predation, pesticide exposure, and communication-tower collisions.

“Glass is an indiscriminate killer of the fittest members of the population as well as the weak,” according to Klem. Windows kill young and old individuals, from common, rare, and endangered species alike. About half of the birds that hit windows die on impact, Klem has found, most often from hemorrhaging in the brain. (Contrary to popular belief, birds rarely break their necks.) Birds that survive a crash may sustain concussions, broken wings and beaks, and other injuries. Even if these injuries are not fatal, they make the birds more vulnerable to predators.

Why it matters

Ignorant of their biological roles, or perhaps indifferent to them, “we take birds for granted,” says Mesure. Birds distribute seeds, pollinate flowers, eat rodents, and consume billions and billions of insects in their breeding territories, he says. “You wouldn’t be able to set foot out your door if we didn’t have the birds to eat these insects.” According to Klem, birds are not only an integral part of the ecosystem but also indicators of environmental health. “The old canary in the mine stuff—it’s hackneyed, but these animals, from a purely utilitarian point of view, are very serviceable.”

Through the market for birdseed, binoculars, and field guides, and the catering to birding tourists, birdwatching represents a multi-billion-dollar industry, according to Mesure. It’s the second most popular outdoor activity in North America, after gardening, and the fastest growing, he told EBN. Birds also serve a cultural role, Klem points out, “as a means of stimulating a conservation and environmental ethic in the citizenry. Birds are something people identify with. There are birds on cave walls, in spiritual texts, in music. Culturally, birds pervade our lives in ways people don’t even think about.”

To most people engaged in this cause, however, the problem is not one of strictly science or ecology, or one of economics or culture, but rather one of morality. The prospect of a “silent spring,” described by Rachel Carson’s landmark 1962 book about the threat pesticides pose to songbirds, launched the modern environmental movement. “Even if you know for a scientific fact that the species of birds that will hit your next building are in great abundance,” asks the Birds and Buildings Forum website, “do you really want to do that?” Klem believes that the most powerful motivation for change is the “guilt and anxiety associated with killing something you didn’t intend to kill.”

The Solutions

“There is a great crisis in the conservation field that only architects and designers can solve,” says Doeker. Mesure explains the challenge this way: “We have to make sure that windows are nonreflective and nontransparent, while at the same time allowing the windows to serve their purpose—to let light in and let people see through them.” A variety of strategies are emerging to meet that challenge. While initial bird-friendly design is preferable, building owners are experimenting with renovations that remedy problematic buildings.

While many of these strategies complement other green design strategies, some may necessitate compromise. Incorporating bird-friendly strategies on south-facing glazing can diminish overall energy performance by blocking passive solar gain, for example. The most significant compromise may be in the connection with nature that large expanses of clear glass can offer to occupants. “It is not a simple equation, and there can be conflicts” between bird-friendly design criteria and other green design criteria, notes Jeanne Gang, AIA, principal of Studio Gang Architects in Chicago. “Architects need to read up and fully understand both.”

Design for bird protection



A ceramic tubular screen is planned for the New York Times headquarters in New York City. The screen was designed primarily for aesthetics and sun control, according to Fowle, but bird-friendly design was also a factor.

Photo (top): David Joseph

Rendering: Fox

The most effective and cost-effective time to address a building’s bird friendliness is during initial design. It also tends to yield the most aesthetically satisfying results. “On new construction, no one will ever know,” says Doeker. Following the March 2005 Birds and Buildings Forum conference (which BuildingGreen cosponsored), “one of the architects wrote back,” Doeker remembers, “incredibly surprised, because he thought we’d be asking an arm and a leg. ‘You recognized that I do a million things already,’ he said, ‘and simply asked me to do a million and one.’ This is not testing anyone’s brain cells, or overpowering their skill set,” she told EBN. “It’s just one more factor to put in the design criteria.”

Glass selection and placement. “Do away entirely with mirrored glass,” says Mesure. “Let that fad pass!” Doeker is less emphatic, explaining that “you can have reflective glass, but you have to break it up with architectural details so a bird flying by doesn’t confuse the reflection for the sun and sky.” If birds do get close to the glass, they can be deceived, but simply breaking up the glass—creating visual noise with mullions, for example—can deter birds from approaching the building in the first place. Glass corners, view-through corridors, and visible indoor vegetation pose added threats to birds. Tilting glass so that it reflects the ground can be effective, but only, Klem has found, if the glass is oriented at angles between 20 and 40 degrees.

Patterned and etched glass. Etched and ceramic-fritted glass are often used to diffuse light, provide privacy, or decorate a façade, but they can simultaneously make glass visible to birds. Not all patterned glass prevents reflection, however; selecting a pattern large enough to be visible from a distance is important, so that birds notice the surface in time to change course. To be most effective, the pattern must also be tight: birds have evolved a stunning adroitness at flying between tree branches and through other tight voids, and they will try to fly through spaces as small as 4" wide by 2" high (10 by 5 cm)—about the size of a horizontal human handprint.

Photovoltaic panels. Different types of building-integrated photovoltaic (BIPV) panels affect a building’s bird friendliness in different ways. Though transparency is generally not a problem with BIPV panels, reflection can be. Generally, the more visual noise the panel creates, the more visible it will be to birds. See EBN Vol. 10, No. 3 for more on BIPV.

Shading devices. Lightshelves, shutters, sunscreens, and other shading devices mute reflections and provide visual noise, making it easier for birds to recognize the façade as a solid surface.

Retrofit for bird protection

Viewing an existing collision problem as a design opportunity allows the solutions to become integral to the buildings. “When a building has been fixed, the owners and residents should look at it and say, oh, you redecorated, or, oh, you did that for energy conservation,” says Doeker. “This does not have to be an aesthetic sacrifice.” Fixing a problematic building can easily involve a combination of the strategies described here. Window film can make the lower portion of a window visible, for example, while an awning shades the upper region.

Screens. Traditional screens installed on the outside of windows do an excellent job of muting both reflectivity and transparency. The renewed popularity of operable windows is especially beneficial to birds for this reason.

Window film. Nonreflective window films are an effective, inexpensive means of making glass visible to birds by reducing both the reflectivity and the transparency of the glass. Noting that a lot of window films worsen the problem, FLAP recommends Scotchprint®, produced by 3M, and CollidEscape, produced by Large Format Digital, Inc., in Madison, Wisconsin. Though this type of film is marketed for commercial and retail advertising, primarily on the sides of busses and vans, architects and building owners are applying it to buildings. Tiny perforations or a transparent surface allows light to pass through the film; viewed from inside, windows treated with the film appear shaded.

Other interior visual noise. “Get into using what you’ve got,” suggests Doeker. “If you’ve got blinds, crack ‘em three-quarters so they present a visual pattern.” She also recommends using different colors on the exterior of alternating blinds to create stripes. Banners or flags hung in lobbies can block see-through corridors. Artwork on the glass itself can also be effective. “A light paste made from Bon Ami soap is the ‘paint’ of choice,” says Doeker. “On windows, it looks like a light glaze or etching. Create a design that mimics your window covering or decorative style. If you don’t like it, wash it off and start again.” Interior solutions can help but are not always as effective as those applied to the building exterior—even if the remedy is visible from straight-on, reflection may render it invisible from an angle. Also, when a building’s interior is darker than its exterior, clear and tinted glass can reflect like mirrored glass.

Other exterior visual noise. Anything that blocks the view between a bird and glass can prevent window strikes. Fencing—even far from the building, as long as it blocks the bird’s view to the window—can be effective. FLAP recommends hanging ribbons or other artwork 2" (5 cm) apart on the outside of windows along the full width of the glass. If birds look down on the windows, awnings can hide reflective and transparent glass. If birds look up at windows, however, from a stream bank or shrubs, for example, an awning will have little effect on bird kills.

Netting. “People use netting in places where they don’t care what it looks like,” says Doeker, who has dubbed netting the official band-aid for problematic buildings. Netting, usually strung from poles several inches from the building, can mute both reflection and transparency to some degree; its real purpose, though, is to protect birds that do fly into the building. The weave must be tight enough to prevent birds from getting stuck, and the net must be firm enough to allow birds to bounce away safely. A few projects, notably nature centers, have used netting as an educational design element.

Birdfeeders. Moving birdfeeders to very near or right against windows can also reduce the number of deaths, as birds alighting from these feeders are unable to build up enough speed to hurt themselves if they hit the windows. About 70% of window strikes from feeders between 13 and 33 feet (4 and 10 m) from the glass are fatal, Klem’s research has shown. Bird baths and some types of vegetation can also be problematic if they encourage birds to be near unprotected windows.

Future solutions for bird protection

Those people engaged in bird-friendly design are pinning their hopes on the development of glass with an embedded or applied ultraviolet (UV) pattern. Birds would be able to see this pattern, and recognize the glass as a barrier, the theory goes, while the human eye would render it invisible. “If that works, we’ve got it made,” says Mesure. As long as the UV glass is relatively price-competitive with conventional clear glass, Fowle says, “it would allow me as an architect to easily say to the owner, we’re going to put bird-safe glass in this project.” He envisions a sticker in the corner as the only identification that the glass is bird-safe.

“If you leave it to the bird conservationists, we’ll tell everybody to put strips of tape on their windows two inches apart,” jokes E. J. McAdams, executive dirctor of New York City Audubon, pointing out that scientific solutions and architectural solutions reflect different priorities. The beauty of UV glass is that it neatly satisfies both agendas. While other technologies and strategies can be effective, “I think this is the most elegant solution,” he told EBN.

New York City Audubon, FLAP, the Birds and Buildings Forum, and the Humane Society of the United States—together with technical advisors Fowle, Klem, and glass artists Tom Patti and Marilyn Holtz Patti—have banded together with the goal of bringing such a glass product to market. Dubbed the Bird Safe Glass Working Group, the team met for the fist time in July 2005. “It felt like a piece of history,” Fowle told EBN. Working with a grant from the U.S. Fish and Wildlife Service, the group also plans to set up guidelines to help educate architects, building owners, and glass technicians. “Our goal is to go out of business,” says McAdams.

Shy of dismissing the promise of UV glass, Doeker is wary of any supposed silver-bullet solution. “It may turn out to not work,” she says. If it does work, it may take several years to become widely available, or it may prove prohibitively expensive for some projects. Even if the UV clear glass lives up to the Bird Safe Glass Working Group’s dreams, notes Fowle, “there’s also the reflectivity problem.” Doeker notes that other solutions already exist. “People need to design to avoid bird collisions,” she says.

Operations, regulation, and education for bird protection

Operations. Some of the most effective strategies, however, address not the buildings themselves but the ways in which they are operated. Long-term research at Chicago’s McCormick Place convention center shows that keeping lights off at night results in an 80% reduction in fatalities and that each light makes a difference. Even when darkness is impractical, other solutions may help. Selecting full-cutoff lamps for site lighting minimizes the amount of light thrown skyward, protecting birds and other wildlife. Dimming lights in lobbies and atria and closing blinds in offices have a positive effect. Using occupancy sensors and timer-automated control systems reduces the chance of unnecessarily lighting unoccupied spaces. Using task lighting in place of blanket lighting limits the amount of light that escapes the building, and locating nighttime work in interior spaces prevents light from escaping altogether. All of these strategies have the added benefits of saving energy and money.

Regulation. Both the Endangered Species Act and the Migratory Bird Treaty Act (MBTA) provide some protection for birds. MBTA is especially apt, as it disallows both intentional and unintentional harming or killing of even a single migratory bird without a permit from the U.S. Fish and Wildlife Service. Though it has never been used to prosecute designers, building owners, or glass manufacturers for window kills, MBTA has been used against power utilities for the unintentional electrocution of migratory birds.

“The judges are convinced that to implement the law would be impractical, that it would be an assault on common sense,” says Klem. “But let’s forget about enforcing the letter of the law. McCormick Convention Center can kill 200 birds a day—don’t you think we should do something about that?” Even if federal regulation, or the threat of federal regulation, continues to be ineffective at reforming the design of new buildings or encouraging the retrofit of existing buildings to protect birds, Klem hopes it can be used to sanction mandatory labels on all sheet glass, “to alert building-industry professionals that this product is a proven lethal hazard for birds.”

Education and advocacy. Concerned citizens have implemented “Lights Out” programs in Toronto, Chicago, New York City, and Milwaukee to educate building owners about the threat their buildings pose to birds and to encourage them to darken their buildings at night—especially during spring and autumn migration seasons. Building owners have been remarkably responsive. “Bright lights are left on overnight for decorative and promotional purposes,” says Doeker, noting a few exceptions, such as hospitals and police stations. “Some owners are relieved to finally have an excuse to turn out the lights and stop wasting electricity. Even people indifferent to conservation issues appreciate that turning out unnecessary lights saves money.”

The design community also has a key role to play in the support of bird-friendly design. “Architects are such intense problem solvers and creative thinkers that they’re in a unique position to help solve the problem,” says McAdams. “Architects have to recognize the problem,” says Fowle. “We have to put pressure on industry—particularly the glass industry—to develop solutions, and on regulation agencies to develop mandates, and on LEED to recognize the issue,” he continues. “And, of course, we have to convince our clients.” Doeker also places hope in designers. “I’m fully confident that once the architectural community understands the problem,” she says, “they will come up with solutions.”

Putting It All Together

While the problem of birds flying into windows is not well understood by the general public, the phenomenon is well documented. And it is clearly devastating the bird population. In order to design and retrofit our buildings to curtail the calamity, we must look at these buildings with a bird’s-eye view. Where birds look through glass, we must begin to see danger. The best approach to preventing bird collisions will likely be found in a combination of the strategies described here, and it will be unique for each project.

The first step to alleviating the problem is acknowledging its gravity. “People are paying more attention now,” says Klem, “and I’m grateful for that.” But solving this problem will require more than awareness. It calls for creativity from designers, flexibility from building owners, information from scientists, innovation from the glass industry, concern from the general public, and inspiration from the natural world. Klem hopes that the combined creative powers will yield solutions sufficient to confine the problem to the past. “We might be able to solve this thing together.”

– Jessica Boehland

For more information:

Randi Doeker, Director

Birds and Buildings Forum

Chicago, Illinois


Michael Mesure, Executive Director

Fatal Light Awareness Program

Toronto, Ontario


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August 1, 2005