This course is part of the Glass in Architecture Academy
When we think about building designs that are both energy efficient and enhance comfort and control of the occupants, our minds may automatically jump to large, foundational concepts like renewable energy and building material transparency and certifications. But in addition to creating spaces that are made with these foundational concepts in mind, architects should take into consideration how their buildings impact the flora and fauna in the direct vicinity. One of the biggest considerations in this realm of modern building design is bird collision prevention.
Photo courtesy of Guardian Glass; Whitney Starbuck Boykin
Windows and glass are a vital aspect of design that not only provide natural light, but also situate the building within—and as part of—the surrounding landscape. Bird-friendly glass products, such as the UV-coated, low-E glass on 160 Water Street in New York City, allow architects and designers to freely integrate glass into their work while reducing bird collisions.
A DEVASTATING ISSUE FOR BIRD POPULATIONS
According to the National Audubon Society, more than half of all bird species in the U.S. suffered from population decline in 2022, with 67 species showing a steep slide of 50% population loss or more in the past 50 years. While loss of habitat and domestic cat predation are the two biggest causes of shrinking bird populations around the world, the third leading cause is bird strikes.
As pointed out by Daniel Klem, Jr., Sarkis Acopian professor of ornithology and conservation biology at Muhlenburg College in Allentown, Pennsylvania, collisions may happen to birds of both sexes and of all ages, regardless of season, location, time of day, and weather condition. They also involve windows of most every size, height, and orientation, in buildings everywhere, from the most urban to the most rural location. This issue is not unique to the U.S. In fact, the overall bird population in all of North America has declined by nearly 30% in the past 50 years, and 90% of that decline happened with those bird species most impacted by glass collisions.
Why Birds Matter
First and foremost, birds are a vital part of our ecosystem. They are a link in the food chain, cycle nutrients, and even contribute to soil formation. They also help maintain a balanced ecosystem, which includes scavenging carcasses and waste, controlling populations of invertebrate and vertebrate pests, pollinating plants, and dispersing seeds. Aside from their enormous role in nature, they also significantly affect the economy. Bird watching is a multi-billion-dollar industry, with related tourism boosting local economies all over the world. They are also a prominent cultural fixture, with their images and symbolism widely used in art and religion.
One compelling specific example of avian ecosystem services is how a few passerine species of birds, like the northern parula and yellow warbler, migrate to shade-grown coffee plantations in Costa Rica for the winter. They eat beetles that attack coffee beans, improving crop yield and reducing the use of pesticides on the crops. Both yellow warblers and northern parulas are common collision victims, which means that the health of these two species may have some impact on the cost and quality of coffee.
Bird Collisions and Occupant Health
Bird-friendly design doesn’t just benefit bird populations and the environment—far from it. Bird collisions also greatly influence the health and well-being of building occupants. Witnessing a bird collision is a shocking interruption to any activity taking place in a building, greatly reducing any calm, peaceful, or positive feelings evoked from an occupant’s aesthetic surroundings. In addition, the aftermath of collision fatalities can compromise a building’s landscape and further diminish the occupant experience.
Photo courtesy of Guardian Glass; Gavin Peters Photography
The key to bird-friendly glass products is creating contrasts that allow birds to understand that a dangerous barrier is approaching. Contrasting lines and dots, spaced two inches apart, can be achieved via fritting, etching, or UV patterned coating. Here, we see the new Sedgwick County Zoo entrance complex in Wichita, Kansas, where UV and low-E coated glass creates a light-filled environment while reducing bird collisions with a low-visibility solution that’s also energy efficient.
In some cases, occupants or potential occupants will even speak out or protest against structures that cause a large number of bird fatalities. This was the case in 2014, when public outcry caused the Minnesota Vikings to reevaluate their stadium, which was causing birds to collide with the highly reflective structure, impacting entire migratory flocks. Since the protests, the stadium has invested in an in-depth study to determine how to reduce collisions. The 2019 study concluded the need for mitigation strategies using films and visual markers to help birds see the stadium. It recommended prioritizing glass treatments where trees and other vegetation are reflected. In addition, it recommended monitoring weather and reducing lighting at night during key bird migration periods. It's worth noting that the local community had requested bird-friendly glass even before the stadium was built. It would have been much less expensive and better for PR for the developers to invest in bird-friendly glass from the beginning, as soon as the community requested it. Bird-friendly glass allows architects to preserve the benefits of integrating natural and outside elements into their designs, benefiting the occupant without sacrificing their architectural vision.
Helping Solve the Problem of Bird-Glass Collisions
While we are still learning how to reduce bird-glass impacts, the number and effectiveness of bird-friendly glass products and design elements have grown exponentially in recent years, giving designers more freedom and more opportunity to create spaces that match their wishes and requirements while protecting avian wildlife.
Treatment methods available on the market today for new constructions include fritted glass, etched glass, patterned specular coated glass (a subset of which is patterned UV coatings) and patterned reflective interlayers. All of these technologies use visual contrast to make the glass more visible to birds, though each have their advantages and disadvantages regarding aesthetics and cost.
These solutions come at a time when ever-increasing bird-friendly legislation, on federal, state, and municipal levels, requires builders to take bird safety into consideration on many of their projects so that they can meet specific standards.
The Evolution of Bird-Friendly Design
High numbers of bird collisions are a modern problem that does not reach back as far as the invention of glass. Bird strikes are an issue of image formation─when the animals can make out a scene that they want to go towards in the glass, whether that’s transmitted or reflected. It wasn’t until the 1950s, when very flat and large pieces of glass started getting used at scale, that bird deaths increased to the point that populations were affected. Before then, both the distortions in the glass (because windows were made from blown glass), as well as the smaller framed pieces, added a significant degree of safety by limiting image formation.
The Fatal Light Awareness Program (FLAP) was formed in 1993 to reduce nocturnal migratory birth deaths via education and policy about the dangers of lighted interiors at night. Since then, scientific research and advancements in bird-safe products have given us many more options to reduce collisions beyond simply turning off the lights to mitigate nighttime collisions.
As architects and designers, it is critical for us to understand how and why bird-window collisions happen and to have a working knowledge of the latest research and technology to reduce these unfortunate events. Further knowledge of the products currently available to builders makes it possible to meet legal standards, protect the environment, and create spaces that bolster and preserve occupant health.
When considering a building’s overall environmental impact, bird collisions shouldn’t be overlooked, and we should not underestimate the emotional disruptions caused to occupants who witness collisions. New and available bird-friendly glass products make it possible to create designs that stay true to architects’ visions and that take advantage of the natural world while reducing bird injury and loss.
HOW BIRD COLLISIONS HAPPEN
To understand and develop the best bird-friendly glazing, glass products, and designs, we must first understand how and why bird collisions happen. Specifically, there are four main causes of bird collisions: reflection, transmission, lighting and design.
Reflection
In design, reflection allows architects to extend or blend a structure in with its natural surroundings in an aesthetically pleasing way—a skyscraper may reflect the surrounding sky, while lower buildings may reflect surrounding trees and landscaping. At the same time, reflection can increase energy efficiency.
Reflection—image creation—is a risk to birds, who process reflection as a natural extension of their environment. Birds cannot differentiate between a reflection in mirrored glass or highly reflective glass and its natural surroundings and may mistake the reflection of the sky or trees for the real thing. Some birds will learn from painful experience that certain areas are "force fields" that should be avoided, however the mistake is often fatal. Even glass with low reflectivity creates image formation, which, in turn, can attract birds seeking desirable environments to fly into the glass. Reflection is inherent in pretty much all construction glass, and once image creation is achieved, the risk is present. Even a single monolithic piece of glass with no coating has enough reflection to risk bird collision.
Transmission
Transmission, a clear and direct line of sight through a window, can be just as dangerous as reflection. In reflection, the glass acts as a mirror reflecting nature; in transmission, the glass acts as a completely invisible barrier. Like reflection, transmission is often used by designers and architects to increase connectivity between the indoor and outdoor worlds, allowing occupants to feel an interaction with nature and the landscape around them. But this blending of interiors and exteriors can cause optical illusions for birds that result in bird-glass strikes.
Transmission occurs when a viewer can see clearly through glass without obstruction. It is especially confusing for birds when they have a direct line of sight through one window and out another (an occurrence common in corner offices and glass walkways) or when the interior design of a building includes natural elements like potted plants and trees. As with the other leading causes of bird strikes, transmission can become a larger issue when paired with other design and lighting choices.
Lighting
When the issue of bird collisions first became apparent in the past century, lighting was seen as both the leading cause of the issue as well as the best solution to the issue. While both interior and exterior lighting is used extensively by designers to express or accentuate design elements and create aesthetic spaces, lighting choices can significantly affect a building’s propensity to bird collisions.
Exterior lighting can confuse birds while interior lighting can attract birds. Some bird species migrate by night to avoid predators and keep cool. They use the light of the sunrise to know when to stop and feed. Urban lighting, especially upward lighting and lit atriums, can create the same lighting cues for birds, causing them to get lost in cities where there is little food and no natural habitat.
At the same time, interior lights can attract birds looking for food, a habitat, nesting site, or a mate. Lit interiors that contain outside elements may be particularly inviting—and birds often cannot detect the glass barrier between them and the area they want to explore.
Today, lighting design choices coupled with lights-out programs during migration periods can help prevent birds from colliding because of light. Lights-out programs also reduce light pollution and allow more stars to be visible for people in urban environments.
Design
Lighting, transmission, and reflection aren’t the only elements responsible for bird strikes. Both the design and the location of a building can have a significant effect on the number and rate of bird collisions. For example, a curved or U-shaped building can corral or funnel birds into a place where it is difficult for them to leave the area of the building without getting confused by reflection, transmission, and lighting, or a building might simply be located on a site that is near a specific bird population and habitat (and a habitat can be something as simple as a flowerbed or a tree). In another example, a building may also be in the line of flight for specific species or migrations.
It’s vital to understand that the overall design of a building can accentuate other factors that cause bird collisions, such as reflective glass or lighting choices. For example, a U-shaped building made of glass, or any design where birds can drop into the structure from above but cannot easily fly out horizontally, will result in major bird strike issues. A large opaque area with only a few areas of glass can also cause problems. Because many animals will perceive that one window as a shortcut around the entire wall, they will be attracted to and fly into the glass. Some monitoring studies have found that these designs can tally more strikes than on neighboring buildings with more glazing.
Buildings with certain designs can make even the best bird-friendly products less effective. In the same way, though, making smart design choices paired with bird-friendly glass can optimally reduce bird collisions.
UNDERSTANDING BIRD-FRIENDLY PRODUCT TESTING
The only way to develop and rate bird-friendly glazing and other products is through testing. Note that this section is not a comprehensive list of bird-friendly product testing options, and that various building owners, jurisdictions, and certifications may require different tests and ratings on those tests. Inclusion in this list is not an endorsement of one test over another—all testing methods have pros and cons, and testing effectiveness may depend on the type of product in question.
ABC Tunnel Test
Developed by the American Bird Conservancy (ABC), the tunnel test is the best known and the one most often used in the United States. Located at an avian research and banding center in Powdermill, Pennsylvania, as well as at Foreman’s Brand Bird Observatory in Maryland, these tunnels are tested with birds caught humanely in nets in a nearby forested trail. “Birds are placed in a ‘tunnel’ where they can view two pieces of glass: one unmodified pane of glass (the control) and the other with the pattern to be tested,” ABC explains. “Birds fly down the tunnel and are scored according to whether they try to exit through the control or the patterned glass. A mist net keeps the bird from hitting the glass, and it is then released. The project focuses not only on finding patterns effective for deterring collisions, but also on effective patterns that cover a minimal part of the glass surface.”
Credit: American Bird Conservancy
In the ABC tunnel test, birds fly down a tunnel until they reach a pivot point that allows them two options: flying toward the test glass or flying toward the control glass. A mist net protects the birds from harm.
The test is used to determine what is called the Material Threat Factor (commonly known as the Threat Factor or TF) and is based on flying at least 80 individual birds down the tunnel and recording whether they fly toward the control or toward the patterned test pane. For example, if 80 birds flew down the tunnel, with 20 flying toward the test pattern and 60 toward the control, 25% (20/80) of the birds flew toward the test pattern and it would therefore have TF=25.
ABC stresses that a material’s Threat Factor doesn’t translate to a specific perfect reduction in collisions when a product is installed on a building. This is because the same glass product will perform differently depending on a number of different factors, including the design of the building, the sun’s angle, or the reflected habitat. But, that being said, ABC writes that, “the lower the threat factor, the fewer collisions.” ABC also defines “bird-friendly” materials conservatively, as materials having a Threat Factor of ≤30, which they believe corresponds to a reduction of collisions of at least 50% under real-world conditions. TF is used by both New York City and Leadership in Energy and Environmental Design (LEED) in order to determine if a facade meets bird-friendly requirements.
The ABC tunnel test is a preferred test because it provides informative scores for glass products and because the test is safe and nonlethal to birds. However, it’s important to note that the test centers are testing for transmission as opposed to reflection and that the testing season is limited to migration months in the spring and fall. With most buildings, reflection is primarily responsible for the image formation on the facade under average circumstances.
The Threat Factor of Other Materials
Not all materials with a Threat Factor score are tested in the bird tunnel. The Bird-Safe Building Alliance (BSBA), a group of architects experienced in bird-friendly design, conservation biologists, and other collision experts, are tasked with assigning TF scores to a number of materials via board review.
According to their website, products may be scored by review if they meet one of three criteria: they “a) were tested using other, peer-reviewed protocols that ABC and BSBA have determined to be equivalent or translatable to tunnel testing scores, b) were studied by scientists or experienced building collision monitors with a documented reduction in collisions of at least 50%, or c) meet ABC's Prescriptive Standards, created using principles derived from tunnel scores, data from collisions monitoring, and information from the literature on avian perception.” For an easy example, the BSBA can assign a solid wall a TF score of 1 without having to conduct an 80-bird tunnel test.
Austrian Tunnel Test
The Collabs-Collision Laboratories nonlethal tunnel test in Austria was the inspiration for the ABC tunnel test. After its initial development, lead researcher Martin Rössler also created a new version of the Collabs- Collision Laboratories tunnel test that added the ability to test in either reflection or transmission, an aspect of testing that ABC has yet to add. Testing is also limited to July-November for the Collabs-Collision tunnel test, which measures the percentage of birds that fly toward the test glass:
- Over 45% = (ineffective)
- 20-45% = (not very suitable)
- 10%-20% = (conditionally suitable)
- 10% or less = (highly effective / "bird protection glass")
Retrofit Monitoring Studies
One shortcoming of the above tests is that specific products are tested in a very controlled environment; they do not simulate a building as a whole, which has a very specific shape, height, design, and geographic location. They also do not take into account dynamically changing sky lighting conditions. However, it’s possible to expand our knowledge of bird-safe glazing by monitoring buildings before and after building renovations and retrofits. Researchers daily scanning the area at the foot of buildings for bird fatalities both before and after a retrofit involving bird-friendly glass can elucidate not only the effectiveness of the product but also the performance of the specific building in question.
Monitoring studies can help determine if a building should be retrofitted with a bird-friendly solution. For example, at Western University, in London, Ontario, a bird collision monitoring study of 40 buildings on campus between 2019-2021 helped the administration retrofit five high-collision buildings as well as integrate bird-friendly design into their three new builds.
And retrofit monitoring studies have shown just how effective the addition of one bird-friendly product can be to an existing building. One such study of the Columbia building in North Portland, Oregon, found a 94% reduction in bird collisions after a retrofit by monitoring the building before and after the change. In this case, the retrofit involved the application of a striped bird-safe film to the building’s extensive floor-to-ceiling window glass.
Finally, monitoring studies can help determine the species of affected birds, the areas of the buildings that pose a danger, and the times of year that collisions might increase or decrease due to factors like migration and changing light. That said, keep in mind that while monitoring studies are part of both FLAP and LEED credit recommendations, two major disadvantages exist: they can only take place after a bird-friendly product is installed and only in existing or completed buildings. However, developers do not need to do an extensive monitoring study to know if a building is going to have a lot of collisions. An expert can evaluate the area for bird habitat, look at how the building sits in that landscape, and give a directionally correct evaluation of the building's collision risks.
When we think about building designs that are both energy efficient and enhance comfort and control of the occupants, our minds may automatically jump to large, foundational concepts like renewable energy and building material transparency and certifications. But in addition to creating spaces that are made with these foundational concepts in mind, architects should take into consideration how their buildings impact the flora and fauna in the direct vicinity. One of the biggest considerations in this realm of modern building design is bird collision prevention.
Photo courtesy of Guardian Glass; Whitney Starbuck Boykin
Windows and glass are a vital aspect of design that not only provide natural light, but also situate the building within—and as part of—the surrounding landscape. Bird-friendly glass products, such as the UV-coated, low-E glass on 160 Water Street in New York City, allow architects and designers to freely integrate glass into their work while reducing bird collisions.
A DEVASTATING ISSUE FOR BIRD POPULATIONS
According to the National Audubon Society, more than half of all bird species in the U.S. suffered from population decline in 2022, with 67 species showing a steep slide of 50% population loss or more in the past 50 years. While loss of habitat and domestic cat predation are the two biggest causes of shrinking bird populations around the world, the third leading cause is bird strikes.
As pointed out by Daniel Klem, Jr., Sarkis Acopian professor of ornithology and conservation biology at Muhlenburg College in Allentown, Pennsylvania, collisions may happen to birds of both sexes and of all ages, regardless of season, location, time of day, and weather condition. They also involve windows of most every size, height, and orientation, in buildings everywhere, from the most urban to the most rural location. This issue is not unique to the U.S. In fact, the overall bird population in all of North America has declined by nearly 30% in the past 50 years, and 90% of that decline happened with those bird species most impacted by glass collisions.
Why Birds Matter
First and foremost, birds are a vital part of our ecosystem. They are a link in the food chain, cycle nutrients, and even contribute to soil formation. They also help maintain a balanced ecosystem, which includes scavenging carcasses and waste, controlling populations of invertebrate and vertebrate pests, pollinating plants, and dispersing seeds. Aside from their enormous role in nature, they also significantly affect the economy. Bird watching is a multi-billion-dollar industry, with related tourism boosting local economies all over the world. They are also a prominent cultural fixture, with their images and symbolism widely used in art and religion.
One compelling specific example of avian ecosystem services is how a few passerine species of birds, like the northern parula and yellow warbler, migrate to shade-grown coffee plantations in Costa Rica for the winter. They eat beetles that attack coffee beans, improving crop yield and reducing the use of pesticides on the crops. Both yellow warblers and northern parulas are common collision victims, which means that the health of these two species may have some impact on the cost and quality of coffee.
Bird Collisions and Occupant Health
Bird-friendly design doesn’t just benefit bird populations and the environment—far from it. Bird collisions also greatly influence the health and well-being of building occupants. Witnessing a bird collision is a shocking interruption to any activity taking place in a building, greatly reducing any calm, peaceful, or positive feelings evoked from an occupant’s aesthetic surroundings. In addition, the aftermath of collision fatalities can compromise a building’s landscape and further diminish the occupant experience.
Photo courtesy of Guardian Glass; Gavin Peters Photography
The key to bird-friendly glass products is creating contrasts that allow birds to understand that a dangerous barrier is approaching. Contrasting lines and dots, spaced two inches apart, can be achieved via fritting, etching, or UV patterned coating. Here, we see the new Sedgwick County Zoo entrance complex in Wichita, Kansas, where UV and low-E coated glass creates a light-filled environment while reducing bird collisions with a low-visibility solution that’s also energy efficient.
In some cases, occupants or potential occupants will even speak out or protest against structures that cause a large number of bird fatalities. This was the case in 2014, when public outcry caused the Minnesota Vikings to reevaluate their stadium, which was causing birds to collide with the highly reflective structure, impacting entire migratory flocks. Since the protests, the stadium has invested in an in-depth study to determine how to reduce collisions. The 2019 study concluded the need for mitigation strategies using films and visual markers to help birds see the stadium. It recommended prioritizing glass treatments where trees and other vegetation are reflected. In addition, it recommended monitoring weather and reducing lighting at night during key bird migration periods. It's worth noting that the local community had requested bird-friendly glass even before the stadium was built. It would have been much less expensive and better for PR for the developers to invest in bird-friendly glass from the beginning, as soon as the community requested it. Bird-friendly glass allows architects to preserve the benefits of integrating natural and outside elements into their designs, benefiting the occupant without sacrificing their architectural vision.
Helping Solve the Problem of Bird-Glass Collisions
While we are still learning how to reduce bird-glass impacts, the number and effectiveness of bird-friendly glass products and design elements have grown exponentially in recent years, giving designers more freedom and more opportunity to create spaces that match their wishes and requirements while protecting avian wildlife.
Treatment methods available on the market today for new constructions include fritted glass, etched glass, patterned specular coated glass (a subset of which is patterned UV coatings) and patterned reflective interlayers. All of these technologies use visual contrast to make the glass more visible to birds, though each have their advantages and disadvantages regarding aesthetics and cost.
These solutions come at a time when ever-increasing bird-friendly legislation, on federal, state, and municipal levels, requires builders to take bird safety into consideration on many of their projects so that they can meet specific standards.
The Evolution of Bird-Friendly Design
High numbers of bird collisions are a modern problem that does not reach back as far as the invention of glass. Bird strikes are an issue of image formation─when the animals can make out a scene that they want to go towards in the glass, whether that’s transmitted or reflected. It wasn’t until the 1950s, when very flat and large pieces of glass started getting used at scale, that bird deaths increased to the point that populations were affected. Before then, both the distortions in the glass (because windows were made from blown glass), as well as the smaller framed pieces, added a significant degree of safety by limiting image formation.
The Fatal Light Awareness Program (FLAP) was formed in 1993 to reduce nocturnal migratory birth deaths via education and policy about the dangers of lighted interiors at night. Since then, scientific research and advancements in bird-safe products have given us many more options to reduce collisions beyond simply turning off the lights to mitigate nighttime collisions.
As architects and designers, it is critical for us to understand how and why bird-window collisions happen and to have a working knowledge of the latest research and technology to reduce these unfortunate events. Further knowledge of the products currently available to builders makes it possible to meet legal standards, protect the environment, and create spaces that bolster and preserve occupant health.
When considering a building’s overall environmental impact, bird collisions shouldn’t be overlooked, and we should not underestimate the emotional disruptions caused to occupants who witness collisions. New and available bird-friendly glass products make it possible to create designs that stay true to architects’ visions and that take advantage of the natural world while reducing bird injury and loss.
HOW BIRD COLLISIONS HAPPEN
To understand and develop the best bird-friendly glazing, glass products, and designs, we must first understand how and why bird collisions happen. Specifically, there are four main causes of bird collisions: reflection, transmission, lighting and design.
Reflection
In design, reflection allows architects to extend or blend a structure in with its natural surroundings in an aesthetically pleasing way—a skyscraper may reflect the surrounding sky, while lower buildings may reflect surrounding trees and landscaping. At the same time, reflection can increase energy efficiency.
Reflection—image creation—is a risk to birds, who process reflection as a natural extension of their environment. Birds cannot differentiate between a reflection in mirrored glass or highly reflective glass and its natural surroundings and may mistake the reflection of the sky or trees for the real thing. Some birds will learn from painful experience that certain areas are "force fields" that should be avoided, however the mistake is often fatal. Even glass with low reflectivity creates image formation, which, in turn, can attract birds seeking desirable environments to fly into the glass. Reflection is inherent in pretty much all construction glass, and once image creation is achieved, the risk is present. Even a single monolithic piece of glass with no coating has enough reflection to risk bird collision.
Transmission
Transmission, a clear and direct line of sight through a window, can be just as dangerous as reflection. In reflection, the glass acts as a mirror reflecting nature; in transmission, the glass acts as a completely invisible barrier. Like reflection, transmission is often used by designers and architects to increase connectivity between the indoor and outdoor worlds, allowing occupants to feel an interaction with nature and the landscape around them. But this blending of interiors and exteriors can cause optical illusions for birds that result in bird-glass strikes.
Transmission occurs when a viewer can see clearly through glass without obstruction. It is especially confusing for birds when they have a direct line of sight through one window and out another (an occurrence common in corner offices and glass walkways) or when the interior design of a building includes natural elements like potted plants and trees. As with the other leading causes of bird strikes, transmission can become a larger issue when paired with other design and lighting choices.
Lighting
When the issue of bird collisions first became apparent in the past century, lighting was seen as both the leading cause of the issue as well as the best solution to the issue. While both interior and exterior lighting is used extensively by designers to express or accentuate design elements and create aesthetic spaces, lighting choices can significantly affect a building’s propensity to bird collisions.
Exterior lighting can confuse birds while interior lighting can attract birds. Some bird species migrate by night to avoid predators and keep cool. They use the light of the sunrise to know when to stop and feed. Urban lighting, especially upward lighting and lit atriums, can create the same lighting cues for birds, causing them to get lost in cities where there is little food and no natural habitat.
At the same time, interior lights can attract birds looking for food, a habitat, nesting site, or a mate. Lit interiors that contain outside elements may be particularly inviting—and birds often cannot detect the glass barrier between them and the area they want to explore.
Today, lighting design choices coupled with lights-out programs during migration periods can help prevent birds from colliding because of light. Lights-out programs also reduce light pollution and allow more stars to be visible for people in urban environments.
Design
Lighting, transmission, and reflection aren’t the only elements responsible for bird strikes. Both the design and the location of a building can have a significant effect on the number and rate of bird collisions. For example, a curved or U-shaped building can corral or funnel birds into a place where it is difficult for them to leave the area of the building without getting confused by reflection, transmission, and lighting, or a building might simply be located on a site that is near a specific bird population and habitat (and a habitat can be something as simple as a flowerbed or a tree). In another example, a building may also be in the line of flight for specific species or migrations.
It’s vital to understand that the overall design of a building can accentuate other factors that cause bird collisions, such as reflective glass or lighting choices. For example, a U-shaped building made of glass, or any design where birds can drop into the structure from above but cannot easily fly out horizontally, will result in major bird strike issues. A large opaque area with only a few areas of glass can also cause problems. Because many animals will perceive that one window as a shortcut around the entire wall, they will be attracted to and fly into the glass. Some monitoring studies have found that these designs can tally more strikes than on neighboring buildings with more glazing.
Buildings with certain designs can make even the best bird-friendly products less effective. In the same way, though, making smart design choices paired with bird-friendly glass can optimally reduce bird collisions.
UNDERSTANDING BIRD-FRIENDLY PRODUCT TESTING
The only way to develop and rate bird-friendly glazing and other products is through testing. Note that this section is not a comprehensive list of bird-friendly product testing options, and that various building owners, jurisdictions, and certifications may require different tests and ratings on those tests. Inclusion in this list is not an endorsement of one test over another—all testing methods have pros and cons, and testing effectiveness may depend on the type of product in question.
ABC Tunnel Test
Developed by the American Bird Conservancy (ABC), the tunnel test is the best known and the one most often used in the United States. Located at an avian research and banding center in Powdermill, Pennsylvania, as well as at Foreman’s Brand Bird Observatory in Maryland, these tunnels are tested with birds caught humanely in nets in a nearby forested trail. “Birds are placed in a ‘tunnel’ where they can view two pieces of glass: one unmodified pane of glass (the control) and the other with the pattern to be tested,” ABC explains. “Birds fly down the tunnel and are scored according to whether they try to exit through the control or the patterned glass. A mist net keeps the bird from hitting the glass, and it is then released. The project focuses not only on finding patterns effective for deterring collisions, but also on effective patterns that cover a minimal part of the glass surface.”
Credit: American Bird Conservancy
In the ABC tunnel test, birds fly down a tunnel until they reach a pivot point that allows them two options: flying toward the test glass or flying toward the control glass. A mist net protects the birds from harm.
The test is used to determine what is called the Material Threat Factor (commonly known as the Threat Factor or TF) and is based on flying at least 80 individual birds down the tunnel and recording whether they fly toward the control or toward the patterned test pane. For example, if 80 birds flew down the tunnel, with 20 flying toward the test pattern and 60 toward the control, 25% (20/80) of the birds flew toward the test pattern and it would therefore have TF=25.
ABC stresses that a material’s Threat Factor doesn’t translate to a specific perfect reduction in collisions when a product is installed on a building. This is because the same glass product will perform differently depending on a number of different factors, including the design of the building, the sun’s angle, or the reflected habitat. But, that being said, ABC writes that, “the lower the threat factor, the fewer collisions.” ABC also defines “bird-friendly” materials conservatively, as materials having a Threat Factor of ≤30, which they believe corresponds to a reduction of collisions of at least 50% under real-world conditions. TF is used by both New York City and Leadership in Energy and Environmental Design (LEED) in order to determine if a facade meets bird-friendly requirements.
The ABC tunnel test is a preferred test because it provides informative scores for glass products and because the test is safe and nonlethal to birds. However, it’s important to note that the test centers are testing for transmission as opposed to reflection and that the testing season is limited to migration months in the spring and fall. With most buildings, reflection is primarily responsible for the image formation on the facade under average circumstances.
The Threat Factor of Other Materials
Not all materials with a Threat Factor score are tested in the bird tunnel. The Bird-Safe Building Alliance (BSBA), a group of architects experienced in bird-friendly design, conservation biologists, and other collision experts, are tasked with assigning TF scores to a number of materials via board review.
According to their website, products may be scored by review if they meet one of three criteria: they “a) were tested using other, peer-reviewed protocols that ABC and BSBA have determined to be equivalent or translatable to tunnel testing scores, b) were studied by scientists or experienced building collision monitors with a documented reduction in collisions of at least 50%, or c) meet ABC's Prescriptive Standards, created using principles derived from tunnel scores, data from collisions monitoring, and information from the literature on avian perception.” For an easy example, the BSBA can assign a solid wall a TF score of 1 without having to conduct an 80-bird tunnel test.
Austrian Tunnel Test
The Collabs-Collision Laboratories nonlethal tunnel test in Austria was the inspiration for the ABC tunnel test. After its initial development, lead researcher Martin Rössler also created a new version of the Collabs- Collision Laboratories tunnel test that added the ability to test in either reflection or transmission, an aspect of testing that ABC has yet to add. Testing is also limited to July-November for the Collabs-Collision tunnel test, which measures the percentage of birds that fly toward the test glass:
- Over 45% = (ineffective)
- 20-45% = (not very suitable)
- 10%-20% = (conditionally suitable)
- 10% or less = (highly effective / "bird protection glass")
Retrofit Monitoring Studies
One shortcoming of the above tests is that specific products are tested in a very controlled environment; they do not simulate a building as a whole, which has a very specific shape, height, design, and geographic location. They also do not take into account dynamically changing sky lighting conditions. However, it’s possible to expand our knowledge of bird-safe glazing by monitoring buildings before and after building renovations and retrofits. Researchers daily scanning the area at the foot of buildings for bird fatalities both before and after a retrofit involving bird-friendly glass can elucidate not only the effectiveness of the product but also the performance of the specific building in question.
Monitoring studies can help determine if a building should be retrofitted with a bird-friendly solution. For example, at Western University, in London, Ontario, a bird collision monitoring study of 40 buildings on campus between 2019-2021 helped the administration retrofit five high-collision buildings as well as integrate bird-friendly design into their three new builds.
And retrofit monitoring studies have shown just how effective the addition of one bird-friendly product can be to an existing building. One such study of the Columbia building in North Portland, Oregon, found a 94% reduction in bird collisions after a retrofit by monitoring the building before and after the change. In this case, the retrofit involved the application of a striped bird-safe film to the building’s extensive floor-to-ceiling window glass.
Finally, monitoring studies can help determine the species of affected birds, the areas of the buildings that pose a danger, and the times of year that collisions might increase or decrease due to factors like migration and changing light. That said, keep in mind that while monitoring studies are part of both FLAP and LEED credit recommendations, two major disadvantages exist: they can only take place after a bird-friendly product is installed and only in existing or completed buildings. However, developers do not need to do an extensive monitoring study to know if a building is going to have a lot of collisions. An expert can evaluate the area for bird habitat, look at how the building sits in that landscape, and give a directionally correct evaluation of the building's collision risks.
BIRD-FRIENDLY GLASS AND OCCUPANT HEALTH
We've established that bird strikes are an issue impacting bird populations. But on a project-based level, it's also vital to be cognizant of the effect of bird collisions on occupant well-being and health.
First and foremost, a bird collision is a shocking event. The impact is usually sudden and loud. In some cases, a bird’s imprint, blood, feathers, or other debris are left behind. Imagine being in a meeting room that is affected by a bird collision: the result is disruptive, upsetting, and violent. Whatever was happening is suddenly and abruptly interrupted, and observers are not only forced off-task, but could also be traumatized or triggered. According to a Carnegie Mellon study, it can take 25 minutes or more to refocus on a task after being distracted, and it likely follows that the ability to refocus could be even longer if the distraction involves the sudden death of an animal.
Bird collisions also leave behind the bodies of birds on the building’s campus, a sight that is disturbing to anyone passing by in addition to being a potential health hazard. Building maintenance staff are left to deal with the issue regularly. No matter the resources and effort put into landscaping, bird collisions can jeopardize their careful design.
Building occupants deserve a space that is peaceful and not at odds with natural surroundings, and multiple studies show that occupants benefit from architecture that prioritizes natural views and daylight as well as that which promotes an overall connection with nature. Specifically, biophilic designs that integrate natural elements can significantly reduce occupant illness, absenteeism, presenteeism, and fatigue while increasing occupant learning, performance, and retention. They’ve also been correlated with lower rates of crime and violence and higher profits in retail spaces.
BIRD-FRIENDLY GLASS DESIGNS AND OPTIONS
It’s clear that designers need to walk a line between creating spaces that assimilate nature and interior spaces while still clearly delineating interior and exterior spaces from a bird’s point of view. Bird-friendly glass makes those spaces more possible. The most effective bird-friendly glass products available today are based on what researchers have learned from bird physiology and behavior as well as from the tunnel and monitoring research described above.
Vision Basics
As predators, our eyes are placed close to each other on the front of our skulls, concentrating our attention directly in front of us and providing us with strong binocular vision. With exceptions such as raptors, bird eye placement is more often on the sides of their heads—an evolutionary trait that allows them to have an almost 360° field of vision to watch for predators. The downside of this impressive field of vision is that their eyesight—and therefore their attention—is not concentrated directly in front of them. In addition, it also means that the images that they see in front of them are in stereo, and likely more out of focus than what is in the center of their vision. Both of these issues make birds more susceptible to flying into glass surfaces that they are moving toward.
Credit: Guardian Glass
With exceptions such as raptors, most birds have eyes on the sides of their heads. This evolutionary trait provides them with an almost 360° field of vision. The disadvantage of this trait when it comes to bird-building collisions, however, is that their eyesight and, therefore, their attention, is not focused directly in front of them.
Vision and UV Light
Birds not only have a more extended field of vision than humans do, many species also see a wider spectrum of colors than humans, which extends into the spectrum of UV light. This means that patterned specular coated glass can create a contrast when a bird views it.
The 2 x 4 and 2 x 2 Rules
Even though bird eyesight is limited in specific ways, birds do use their vision to avoid hazards in front of them. Nearly all bird species have an incredible perception of their bodies in relation to the space around them and slow down when approaching spaces they wish to fly through that seem smaller than their body. It follows that glass equipped with stripes or patterns placed closer together than the size of a bird in flight will reduce bird strikes. The 2×4 rule describes the maximum distance between elements in bird-deterrent patterning. The recommended spacing for any pattern is 2x2 inches.
Bird-Friendly Glass Coverage and Contrast
Bird behavior and physiology teaches that reducing bird strikes requires adding contrast to glass as well as ensuring that the contrasting elements are still apparent even when there is interference from glare and reflection. Though many designs celebrate highly visible patterns as part of the desired aesthetic, in some cases, architects wish for minimal contrast to accentuate increased daylight, views, and occupant well-being.
Higher coverage does not mean higher protection. Rather, visual contrast and proper spacing increase effectiveness. Treatments that work best place contrasting elements two inches apart and keep them large enough to create a perceived barrier for birds. For example, small, dense patterns are less visible than a larger pattern at lower density, even if the same percentage of surface area is covered with a pattern element (small = 1/8-inch dot spaced 1/2 inch apart; large = 1/4-inch dot spaced 2 inches apart). At the same time, 50% coverage could be less effective than 12.5% coverage if the contrasting elements are too large and far apart. Less coverage with contrasting elements is also generally more aesthetically pleasing to building occupants and preserves more of the benefits and advantages of glass.
Design Treatment Methods
While testing has determined that etching, fritting, and patterned specular coatings (of which UV glazing is a subset) are effective in reducing bird collisions in new builds, each of these methods comes with advantages and disadvantages.
Etching
A common glass treatment, glass etching provides uniform patterns that are translucent rather than opaque. It is moderately priced between more economical frit on glass and less economical patterned specular coatings of which UV glazing is a subset. It’s also moderately visible—more visible than UV glazing but less visible than fritted glass, depending on the lighting context. For the best overall performance, etching is applied on surface #1 with low-E coatings on surface #2.
Fritting
Commonly used in solar management, frit on glass is a well-known, longstanding technology that is widely available in North America. It’s also an economical choice for bird-friendly solutions while also seen as aesthetically pleasing. Frit can be provided on the #1 or #2 surface depending on the project-specific requirements related to codes, test score needed, performance, or design, though treatment on the #1 surface creates optimal contrast and greatest deterrence performance.
Patterned UV Glazing: A Subset of Patterned Specular Coatings
The most recent advancement in bird-friendly treatments is patterned specular coatings, of which, as mentioned, UV glazing is a subset. These coatings, like low-E coatings, act as a mirror and can change the reflectivity and color of the glass. When patterned, this can create a visual contrast that the animals can see. Some products in this category are designed to leverage the UV spectrum to reduce or void the impact the coating has on the spectrum that is visible to humans. In fact, under certain lighting conditions, some products may not be visible to humans at all. Not all UV glazings are the same or have the same level of visibility to birds or humans. In addition, they all have compromises and it is impossible to have a product that does not compromise human visibility and provides protection for birds. The more invisible a product is to humans, the less deterrence it will have for birds. That is the trade-off that needs to be managed. However, visibility to humans can lend to the design. Aesthetics of a pattern is the cheat code for making visibility of a product acceptable.
In some cases, the manufacturing process of these products involves applying the coating on surface #1 and requires a laminated outboard lite. The interlayer acts as a UV absorber, while the UV coating reflects, creating an even greater contrast/pattern for the birds to see and cue deterrence. Other surfaces of the glass can then be utilized with other coatings, such as a low-E coating.
However, not all products marketed as UV glass products have a UV coating on the first surface or a UV interlayer. “UV coatings are increasing in popularity because of the rapid influx of local ordinances, such as New York City’s Local Law 15 (LL15) of 2020, which requires materials that reduce bird strike fatalities be installed on newly constructed or altered buildings,” explains Greg Rogers, Lead Product Manager, Guardian Glass North America. “Owners, designers, and especially occupants do not want their views to be interrupted.”
And they are popular because some of them appear closer to untreated glass than any other code-compliant deterrents. The UV option also minimizes obstruction, so occupants enjoy all the natural light penetration a bird-friendly glass can offer, along with the maximum energy performance of the glazing that’s paired with the UV coating. In addition, there are two exceptions in LL15 that mandate the use of UV deterrents in some situations. Where ground-floor transparency is required by the New York City Zoning Resolution, transparent Bird Friendly Material (BFM) with a UV-reflective pattern and a maximum TF of 27 shall be provided. In areas of special food hazard and shaded X-Zones where flood-resistant glazing is proposed and ground floor transparency is required by the New York City Zoning Resolution, transparent BFM with a UV-reflective pattern and a maximum TF of 36 shall be provided.
Photo courtesy of Guardian Glass; Whitney Starbuck Boykin
The view from the interior of 160 Water in New York City illustrates the clarity provided by UV glass. The project uses this bird-friendly glass on the first six floors in compliance with Local Law 15 of 2020.
After-Market Products and Solutions
There is no shortage of after-market products that are marketed as “anti-collision” or “bird-friendly,” whether they have undergone testing to verify that fact or not. These products range from commercial films that may be applied to large buildings to small and inexpensive stickers (sometimes featuring bird or hawk silhouettes) that anyone can buy for residential buildings. These after-market bird products have some positive attributes. Assuming the deterrent meets all of the criteria outlined earlier, illustrations on the surface of the glass, or even prints on perforated vinyl films, can add a desirable aesthetic to the building, engage the community associated with the space, or refresh the look of the space regularly, unlike permanent deterrents. Applying the products correctly is key. For instance, the hawk silhouettes do not work unless they are spaces 2 inches apart across the whole facade.
By far the biggest positive aspect of after-market solutions is that they offer a solution for buildings already in existence that are difficult or expensive to retrofit with new glass altogether, especially buildings that have been designated as acutely dangerous to birds. But both after-market and do-it-yourself solutions have considerable drawbacks: they may not be tunnel tested, they may not be very effective, they may void your glass warranty, and they may damage your glass. They may also not be installed correctly. Many films and stickers are only effective if they include the correct 2-inch spacing requirements. In the same way, they may not be durable, long-lasting, or easy to maintain in the same way that bird-friendly glass is.
BEYOND BIRD-SAFE GLASS: BIRD-FRIENDLY DESIGN CONCEPTS AND CONSIDERATIONS
Architects can work with owners to avoid or reduce collision hazards, using a combination of bird-friendly glass treatments and general bird collision knowledge. For example, knowing that bird strikes most often happen up to the height of adjacent mature tree canopy, builders can integrate bird-friendly glass products into that part of the building, or mix with materials with lower Threat Factors as defined by the American Bird Conservatory throughout that area. Other factors to consider include lighting, reflections, window shades, and interior design, all of which can impact a building’s bird collision rate, and all of which should be given attention, along with materials and location, during the design process.
How to Design a Bird-Friendly Building
It can feel as if building a bird-friendly structure limits your design. However, while it may influence some choices or affect the building’s budget, bird-friendly choices improve outcomes for the environment, occupants, and society as a whole. In addition, bird-friendly strategies may be required by law or by the mandates of the project itself.
Keep up with glass manufacturers on the latest bird-friendly technology and product information. Selecting the correct bird-friendly glass for your project means balancing four different considerations: code, cost, performance, and aesthetics. Start bird-friendly considerations early in the building process to avoid issues down the road, control costs, and achieve the best results.
During the conceptual or schematic design phase, review your basic plans with a glass manufacturer, outlining the build’s vision, shape, orientation, location, and purpose. This information can help everyone on the project understand if and where reflection, lighting, or transmission may increase the likelihood of bird strikes and what design changes or types of bird-friendly glazing could help the predicted issues.
During the design development phase, it’s time to delve into deeper conversations with glass manufacturers about the specific products and sizes you’ll need, along with costs. Ask about any new product options or changes in cost. This is also the ideal phase to examine exterior and interior landscaping through a bird-friendly lens: What is the habitat around the building site and how will animals move around it? Is the structure in the line of flight for specific species or migrations? Also during this phase, make sure that your plans will earn the LEED Bird Collision Deterrence credit if you wish.
During the construction documents phase of building, finalize your bird-friendly products and verify all specs before bidding. Take a last overall look at your building from the standpoint of bird collision reduction. Remember that after-market bird-friendly solutions come with extra costs, less tested effectiveness, and other drawbacks.
CONCLUSION
Bird-friendly design is an architectural concern that is growing in importance as the industry recognizes it as an integral part of sustainable and healthy building practices. At the same time, glass is a major building material and will remain such. Finding bird-friendly treatment solutions for glass is one important key to reducing bird strikes while preserving the benefits of glass in design.
In most locations in the United States, bird-friendly design is voluntary, but legislation on the federal, state, and municipal level is changing quickly toward more bird-friendly regulations based on growing concerns about bird populations and the environment. It's also important to remember that bird collisions are not just a city problem. Suburban and rural areas with lots of habitat around buildings constructed with lots of glass are going to have more collisions over the building's life than the same building in a dense city.
By learning about the physiology and behavior of birds, researchers have determined the four major causes of bird collisions. Now the industry is developing best practices for manufacturing effective bird-friendly glass products. However, this is a relatively new area of research and scientific discovery, and one that is still evolving.
Today, there are three major bird-friendly glass treatments: etching, fritting, and patterned specular coatings, which includes UV glazing. Under certain lighting conditions, UV glazing provides an almost-invisible option for builders that does not compromise the overall design of the building or the benefits of designing with glass. With “patterned specular coated products," there will be a visible spectrum portion of the coating as well as an invisible-to-humans (UV spectrum) portion of the coating. Both will be present on every “UV glazing” product. For this reason, each product on the market looks and performs differently. How the manufacturers balance visible vs. invisible will impact the aesthetics and efficacy of everything marketed as UV. Architects can talk with owners and facilities about the benefits and rationale for including bird-friendly glass solutions into their building. They may also connect with manufacturers for the latest information on products, testing, and effectiveness that can be used in current or upcoming projects.
REFERENCES
https://www.terrapinbrightgreen.com/reports/the-economics-of-biophilia/ (benefits of birds/nature) https://www.lifehack.org/662393/the-price-of-distraction-is-hugely-beyond-your-imagination (refocus after distraction) https://www.audubon.org/news/more-half-us-birds-are-decline-warns-new-report#:~:text=The%202022%20U.S.%20State%20of,need%20for%20further%20conservation%20efforts. (bird populations) https://www.congress.gov/bill/117th-congress/senate-bill/791 (federal legislation) https://abcbirds.org/glass-collisions/existing-ordinances/ (state/city legislation) https://www.usgbc.org/credits/new-construction-core-and-shell-schools-new-construction-retail-new-construction-data-41 (LEED credit) https://abcbirds.org/wp-content/uploads/2015/05/Bird-friendly-Building-Guide_LINKS.pdf (tunnel test) https://abcbirds.org/glass-collisions/threat-factor-rating/ (threat factor rating system) https://flap.org/winding-path-advocating-bird-friendly-buildings/ (Western University retrofitting) https://audubonportland.org/press/new-report-shows-a-94-decrease-in-avian-window-collisions-at-city-building-after-retrofit/ (Columbia building retrofit) https://gothamist.com/news/dozens-migrating-birds-found-dead-outside-shiny-harlem-condo (deadly condo) https://www.guardianglass.com/us/en/projects/project-details/detroit-zoo (detroit zoo) https://www.allaboutbirds.org/analysis-the-economic-value-of-birds/ (why birds matter) https://www.sciencedirect.com/science/article/pii/S0272494421001353 (impact of multisensory approach on human well-being) https://www.sciencedirect.com/science/article/abs/pii/S0272494422001682 (influence of windows on cognitive performance) https://www.terrapinbrightgreen.com/reports/the-economics-of-biophilia/ (lower crime rates, higher profits)
Sarah Aswell is a journalist and editor living in Missoula, Montana. She contributes to continuing education courses and publications through Confluence Communications. www.confluencec.com
