Once upon a time, birds had the skies pretty much to themselves. They had to navigate through trees occasionally, but those trees were part of their natural habitat, and they were familiar with how they grew and how they looked.
Then, people began to build specific places where they could live and work. In early history, that wasn’t much of a problem because the structures tended to be low to the ground and didn’t intrude as much on avian airspace. As time passed, though, humans built narrower towers that weren’t as easy to detect and used new materials.
If we could change the way we construct bird-safe buildings and retrofit or renovate them to reduce the risk posed by glass, we would be making an important contribution to bird conservation.
And, in fact, we can do just that.
There are a number of ways to address the problem, from screens to etched or fritted windows to minimizing lighting that can distract and confuse migrating birds. These are just a couple of characteristics of some bird-safe buildings, but the true definition is more detailed than that. So, before we go any further, it would behoove us to define exactly what a bird-safe, or bird-friendly building is.
Table of Contents
- What causes birds to collide into buildings?
- What defines a bird-safe building?
- Why are bird-safe buildings important?
- Solutions for creating bird-safe buildings
- Costs of bird-safe buildings
- Examples of bird-safe buildings
- Bird-safe building legislation
What Causes Birds to Collide Into Buildings?
Collisions happen wherever birds and glass are found in the same vicinity, increasing by 19% for every 10% increase in glass area.
Glass surfaces, whether they’re reflective or transparent, can confuse birds. Reflective glass can create the illusion of a continuous natural environment on the other side of the glass. Birds won’t notice the glass itself and may attempt to fly through it to what they think is a backyard, garden, or meadow. Transparent or clear glass can appear invisible, so birds might attempt to simply fly through it, as well.
In 2014, the Smithsonian attempted to calculate the number of bird deaths caused by glass collisions each year and found that 44% of the fatalities involved buildings that were one to three stories tall, such as homes.
Most of the remaining bird collisions involved buildings four to 11 stories tall, with far less than 1% involving skyscrapers 11 floors or higher. This didn’t mean glass skyscrapers were safe; there are simply far fewer of them: 22,000 as compared to 15 million low-rise buildings and 123 residential structures. So, the percentage for high-rises was naturally lower.
Christine Sheppard, manager of the Bird Collisions Campaign for the American Bird Conservancy, put it this way in the BirdNote podcast: “Birds don’t see glass…People don’t see glass either, but people understand context. They understand window frames. They understand right angles.”
Birds are diurnal creatures, which means they’re active during the day. If you have any doubt of this, just start listening at dawn: that’s when songbirds start chirping and roosters tend to crow.
As such, their vision is best adapted to the natural light provided by the sun. Some birds, however, are active at night and use those hours for migration, waiting for daylight to feed. They can still see well enough to sense migration patterns and know where they need to go using a mechanism in their eye, but artificial lighting appears to interfere with this sensory mechanism. They then become disoriented, and their migration patterns are disrupted.
Although most bird-building collisions happen during daylight hours, the disorienting effects created by artificial light can cause birds to fly directly into a built environment without stopping. The problem can be compounded by weather conditions. Birds flying on an overcast night may descend to avoid low clouds or mist, thereby flying closer to the disorienting artificial light sources.
Artificial light not only disorients birds, it also attracts them, which increases the danger by bringing them closer to the buildings near the lights. If they avoid crashing into them during the night, they’ll likely perch there and depart in the morning, when they’ll be at risk of flying into reflective or transparent glass windows once again.
What Defines a Bird-Safe Building?
A bird-safe building is one that minimizes the dangers to birds through various means, which will be discussed further below. Bird-safe designs take into account a building’s:
- Lighting design
- Use of glass
The U.S. Green Building Council established guidelines for bird collision deterrence under a rating system known as Leadership in Energy and Environmental Design (LEED). This rating system assigns points to determine a “threat factor.” Structures that qualify as bird-friendly earn LEED Pilot Credit 55.
To understand what materials are threatening to birds, the American Bird Conservancy designed a tunnel test that does not injure birds. The threat factor is determined based on putting 80 birds through the tunnel test.
The birds fly through a tunnel toward two panes of glass: a clear panel that serves as a control and is invisible to the bird, and the panel with a pattern that’s being tested. The birds are stopped by a net before they reach the end of the tunnel and are then released. If a bird chooses a path toward the patterned panel and then switches paths to the control (clear) panel, then this indicates that they see the panel and are avoiding it.
To obtain the threat factor score, the total number of birds tested is divided by the number of birds that flew towards the patterned panel, indicating that they did not see it.
The American Bird Conservancy (ABC) defines materials as “bird-friendly” when they have a threat factor of 30 or less, thereby reducing collisions by at least 50%.
This baseline is crucial in defining the ABC’s standard for a bird-friendly building, which begins with the requirement that at least 90% of the material used in any façade has to have a threat score of 30 or less. That standard covers anything from ground level to 40 feet up, which is the primary bird collision zone.
Above this zone, at least 60% of the exposed façade has to meet the standard, and so does all the glass surrounding courtyards or atria.
In addition, bird-friendly buildings should contain no “see-through” passageways or corners, and landscaping should be designed without any features that boost the likelihood of collisions.
Outside lighting should be shielded to keep from attracting migrating birds at night. Inside lighting should be turned off after dark when not in use, so it can’t escape through windows.
Finally, bird mortality at the site should be monitored and offset by measures such as providing safe habitat elsewhere or reducing mortality from other sources.
Why Are Bird-Safe Buildings Important?
Birds are essential to our ecosystem. Their ecological value is immense. They pollinate plants, control pests, spread seeds, and clean up the landscape. Just one vulture, for example, contributes the equivalent of $11,600 worth of waste disposal during its lifetime.
Bird-safe buildings counteract this threat by reducing bird-window collisions.
Solutions for Creating Bird-Safe Buildings
Designing a bird-safe building obviously begins with mitigating the threat posed by glass to birds in flight. This can be done in a number of ways.
Thanks to the emergence of new materials, the costly process of retrofitting entire buildings is no longer necessary; targeted retrofit projects that focus on problem areas can be done at more manageable costs.
The term “green roof” doesn’t refer to the color, but to actual greenery that is planted on roofs of buildings. As might be expected, there are pluses and minuses to this practice.
On the positive side, green roofs, green walls, and rooftop gardens can create true new ecosystems and serve as functioning habitats for birds, where they can nest and find food. On the downside, it can attract them to areas that are closer to glass.
It’s therefore important that green roofs and similar features be constructed in such a way as to minimize the danger created by any proximity to glass, and that the glass is designed or shielded to minimize the possibility of collisions. When considering whether to install such features, the glass in nearby buildings should also be taken into account.
The term “patterned glass” can refer to a variety of products that provide birds with a visible cue that they’re approaching solid material.
- Frosted, etched, or opaque glass can all allow natural light to enter a building without creating problems of reflectivity or transparency that can prove fatal for birds.
- Stained glass can preserve and enhance the aesthetic value of a building while at the same time providing a clear visual cue for birds to steer clear.
- Ultraviolet (UV) patterned glass offers the prospect of keeping birds safe without any visible change in the glass’ aesthetics. Because some birds see light in the UV spectrum that isn’t detected by humans, it’s possible to create glass products with UV-reflecting patterns that can keep birds away without changing how the surface looks to us.
The process started out being more expensive than other alternatives, but the cost is expected to come down as demand increases.
Frits are ceramic dots that have been applied to the surface of glass to reduce the transmission of light, but can also serve as visual cues to warn birds away from windows. Patterns on the exterior surface of the windows that follow the 2-by-4 rule work best at deterring collisions because of their high visibility.
Screens are often installed outside glass on home windows primarily to keep insects from entering, but provide the added benefit of deterring bird window strikes.
They’re relatively inexpensive and are standard in many homes, but because they’re meant to be used as a barrier when windows are open, glass that’s fixed in place on windows that aren’t designed to be opened won’t have them. You may not see them, for example, on plate-glass windows covering office-building façades, although motorized screens may be used in some cases.
Some screens may not be visible to birds and may resemble darkened tunnel entrances, but they can still serve as protective barriers. For this reason, it’s best to preserve a decent gap between the screen and window when installing them, so birds that collide with them aren’t carried into the glass by their momentum.
Shutters are a slightly more expensive option than screens that can also be effective at deterring bird strikes.
Netting, like screens, is a relatively cost-effective way of making a building bird-friendly. Also like screens, it should be installed at some distance from the windowpane to avoid allowing birds’ momentum to carry them into the glass.
Interior Window Tape
Tape is another cost-effective solution. Some people have even resorted to sticking decals and post-it notes on windows. Affixed to windows at regular intervals consistent with the 2-by-4 rule, tape can deter bird strikes while preserving a view for humans looking out.
Regular household tape tends to peel and isn’t particularly durable. As an alternative, ABC offers its own BirdTape, which lasts as long as four years. The downside to using tape is that it may look tacky (especially the household variety) and can resemble window bars, which some homeowners may not care for.
Shielded Light Fixtures
Shielded light fixtures can reduce the amount of artificial light that’s visible to birds, which can disrupt migratory routes and create confusion. Fixtures should be fully shielded, that is, equipped with an opaque shield that only allows light to be cast downward, not skyward where it can distract birds.
The term “fully shielded” is synonymous with “dark sky compliant” and “zero up light.” Minimizing artificial light during the migratory season is the goal of the Lights Out Program described below.
The Audubon Society’s Lights Out Program is an effort to enlist the help of building owners and managers in helping keep birds safe at night during the season of their migration by reducing light pollution.
The Audubon Society is asking them to install motion sensors so lights aren’t operating continuously, and to turn off any unnecessary lights—such as decorative lighting and floodlights—during avian migrations.
Other steps to take include turning off interior lights that aren’t being used, especially on the upper stories of buildings; replacing full-room lighting with area lighting and/or drawing shades on rooms in which building occupants work late; and reducing atrium lighting where possible.
Building owners can also help by using fully shielded outdoor light fixtures that do away with upward-shining light and horizontal glare, and by avoiding bright lights when installing replacement lighting.
Examples of light fixtures that may be unsafe for birds and whose use is discouraged include unshielded streetlights and floodlights; period-style fixtures meant to resemble lanterns or early 20th century street lamps; louvered Marine-style fixtures; unshielded bollards; drop- and sag-lens fixtures with exposed bulbs; and unshielded or poorly shielded wall-mount fixtures.
Any fixture that’s fully shielded is likely safe, while those that are not probably aren’t.
Costs of Bird-Safe Buildings
The good news is that bird-safe buildings aren’t cost-prohibitive. In fact, you can construct a bird-safe building at no additional cost.
National Audubon estimated that it would have cost an extra $1 million to add bird-safe glass at the Minnesota Vikings Stadium (U.S. Bank Stadium), and not even that, if it had been undertaken earlier in the building process.
That may sound like a significant expense until you realize that it’s less than 0.1% of the $1 billion project’s total cost. And it would have been cheaper to do so upfront than to pay $10 million for a retrofit of the building that became notorious as the site of the most fatal bird strikes in Minneapolis.
This is because retrofitting typically costs more than altering the look of a building in the planning stages.
Buildings with less glass can actually save money for owners when it comes to energy costs. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the International Code Council have found that temperature control costs are higher in buildings where the façade is more than 30% to 40% glass.
In addition, improving outdoor lighting design is likely to produce savings greater than what the changes cost. The International Dark Skies Association estimates that wasteful lighting caused by poorly designed or installed outdoor fixtures boost electricity costs by $1 billion annually in the United States.
Retrofit options, from least to most expensive, run from temporary seasonal measures (least expensive) to netting and screens, followed by window film, shutters, and grilles. The most expensive step is fully replacing the glass.
Examples of Bird-Safe Buildings
As research and awareness continue to grow, more buildings are being constructed and retrofitted with bird-safe features.
For example, the 82-story Aqua Tower in Chicago is characterized by undulating terraces that shade its windows, thus minimizing reflection and discouraging birds from flying through.
1 South Dearborn
Frosted glass on the 40-story-tall office tower at 1 South Dearborn in downtown Chicago features a frosted glass façade behind a cluster of shade trees, which is a strategy that has resulted in fewer bird strikes since it was implemented.
Fritted glass on the Jacob Javits Center in Manhattan has made a big difference, too. The center saw a 90% reduction in bird collisions after it removed some dangerous glass and replaced other installations with glass containing a pattern of visible fritting. A green roof covers 6.75 acres on the roof, and the adjacent glass is translucent.
Similarly, the headquarters of Internet company IAC, also in New York City, is made up entirely of fritted glass, with the result being that no bird mortalities had been reported in the first two years of monitoring by the New York City Audubon.
Window film retrofits on Francis Searle Hall and the Kellogg School of Management at Northwestern University in 2017 reduced collisions by 95% between the spring semester that year and the spring of 2018.
University of British Columbia
The University of British Columbia (UBC) in Vancouver, Canada has pursued a number of projects as part of a comprehensive effort to reduce bird deaths. One of the most creative solutions involves the windows at the UBC Bookstore, which were fritted with text-pattern designs showcasing sentences from books.
Solar shades over windows on another building block reflections from the surrounding sky and vegetation, providing shade and wind screening in addition to the benefits to birds. And screens on other buildings such as the Campus Energy Centre and Beaty Biodiversity Research Center prevent birds from flying through.
UBC continues to monitor bird collisions on campus and report them through Global Bird Collision Mapper, which is a web app that provides a place to view and report the locations of bird collisions around the world. The information gathered is used to pilot further solutions.
Bird-Safe Building Legislation
Legislation to promote bird-safe glass and buildings and protect birds from collision has been introduced at the federal, state, and local levels.
In 2019, the Bird-Safe Buildings Act was introduced in the U.S. House to require that any public building constructed, acquired, or substantially altered by the General Services Administration meet the American Bird Conservancy’s list of standards defining a bird-friendly building.
This included the requirement that 90% of the exposed façade be made up of bird-safe glass or non-glass material up to 40 feet, and 60% above that. The bill, first proposed by Rep. Mike Quigley (D-Ill.) in 2011, passed the House of Representatives, but did not become law. Still, it has not been abandoned: it was reintroduced in 2021 by Quigley, U.S. Rep. Morgan Griffith (R-Va.), and Sen. Cory Booker (D-N.J.).
Additionally, several local jurisdictions have been proposed or passed. For example, New York City passed laws that took effect in December 2020 to require the lower levels of all buildings, up to 75 feet, to include bird-safe material.
Also in 2021, Governor Pritzker of Illinois signed the Bird Safe Buildings Act (HB 247), which requires bird-friendly design to be incorporated into the construction and renovation of state-owned buildings in Illinois.
As such efforts are adopted, they are altering the human-made landscape to the benefit of bird populations.
One billion bird deaths are far too many. This has led a number of individuals and organizations, including activists, engineers, and legislators, to create and promote standards for bird safety.
As research continues and technology evolves, less expensive and more effective means of keeping birds safe from building strikes will emerge.
It’s an important issue, not just for birds, but for the ecosystem in which they play an important role. This is not just their ecosystem, but ours, too. So anything we can do to protect birds not only safeguards them, but benefits us—their human cohabitants of Earth—as well.