The foundations of bird friendly glass rating methods

By now, the building industry is familiar with the problem of bird-glass collisions. Governments, professional associations and other institutions across North America are establishing rules and guidelines to make bird safe windows the norm. But do these bird safe methodologies really work? How can one be certain that any given product will deter bird strikes? This article will address the predominant rating systems for bird friendly glass, their foundations, and their limitations. From this basis, it will move on to investigate how the results of each rating system correlate to real-world outcomes based upon on-site reporting. The goal of this analysis is to inform legislators, architects, building owners and conservationists as to the strengths, weaknesses and limitations of the different rating systems that are used to evaluate bird safe glass products.

In North America, bird friendly glass is evaluated based on two predominant methods: prescriptive and performance.

Prescriptive method

The Prescriptive method gives a recipe for bird friendly glazing design based on distinct variables: pattern spacing (following the 2×4 rule or 2×2 rule), visual marker size, surface coverage, contrast, and surface placement.

The Prescriptive method depends on its “recipe” being correct. Hence, the method has been adapted over time as scientists and the glass industry gain new information about bird behaviour. One notable development was a shift toward using the 2×2 rule, with tighter spaces between visual markers to deter smaller species like hummingbirds.1

Under this method, architects are free to use any product that meets the given requirements. This gives them the flexibility to design a bird friendly glass that works perfectly with a project’s concept and aesthetic.

Bird safe glazing models based on the Prescriptive Method include:

bird-safe glass 2x4 rule

A pattern following the 2×4 rule

Performance method

The Performance method grades individual bird friendly glass products based on one-off testing procedures. Products are tested in flight tunnels and graded based on the proportion of test subjects (captured birds) who fly toward the test sample. In North America, legislation using the Performance method typically depends on Threat Factors determined through American Bird Conservancy (ABC) tunnel testing. Birds are caught in mist nets, gathered by hand by ABC staff, carried in bags to the test flight tunnel, and then released one at a time into the tunnel. Staff record whether they fly toward the test pane, control pane, or neither.2 A net at the end of the tunnel prevents birds from hitting the glass, and birds are released after a single flight test.3

The Threat Factor is calculated as a percentage: the number of birds that fly toward the test sample, divided by the total number of test subjects used in the experiment. So, if 80 birds are released into the flight tunnel and 20 fly toward the test sample, the Threat Factor is calculated as 20/80 = 25%, and the product has a Threat Factor of 25.4

Under the Performance method, viable products are limited to what’s been tested and approved. This can cause a delay in bringing products to market, especially given that ABC testing is carried out only during spring and fall migration periods. This may make it more difficult for architects to incorporate bird friendly glass into their designs, since their aesthetic choices are limited to pre-approved options.

Please see for more information on tunnel testing.


Performance-based models for bird friendly glazing include:


The effectiveness of AviProtek® glass

Walker Glass has collected reports from building owners and managers across North America at sites glazed with AviProtek® bird friendly glass. Results are promising: 1 strike or less per year at every site. In the majority of cases, on-site contacts report zero bird strikes. Ready learn more about the effectiveness of AviProtek® glass?

Reflective conditions in field and tunnel tests

Tunnel tests

ABC recognizes the need to test reflective conditions and make efforts to incorporate this into their tunnel test method, using mirrors in the sides of the tunnel to deflect some sunlight toward the glass panels at the far end.5 Nonetheless, in separate analyses Seewagan, Rössler and Schmid observe that the Powdermill test tunnel does not create reflective conditions.6,7 Seewagan concludes that the tests “are therefore most representative of sound barrier collisions, and how well their results translate to prevention of collisions with traditional windows is uncertain.”8

Rössler and Schmid give more detail on the problem:

“However, the Powdermill tunnel is unable to provide a satisfactory testing situation for specular reflection (“window situation”) for two fundamental reasons. Firstly: the panes at the end of the tunnel are mounted at right angles to the birds’ flight path, and are thereby only able to “reflect” the dark inside of the tunnel. Secondly: plywood boards with “blue pattern” are mounted behind the panes … Since these boards are illuminated by the sun head-on, they lighten the background rather than reducing the amount of light there.”9

On tunnel tests

“With this experimental setting, it is impossible to achieve clearly visible reflections on the panes, such as they are characteristic for window situations in buildings. This means, in consequence: this experimental setting is actually a transparency setting, although it is intended to test for specular reflection.”

– Hans Schmid and Martin Rössler, “Critical examination of American Bird Conservancy’s (ABC) campaign to avoid bird collision”

Field tests

In his various field experiments, which were foundational to the Prescriptive method, researcher Daniel Klem tested glass samples installed in free-standing frames and in existing buildings.10 This allowed him to create either transparent or reflective conditions, depending on the position of glass samples and time of day, helping to identify the dangers of reflective glass surfaces to birds.11 As a result of these field tests and ensuing experiments, many proponents of the Prescriptive method recommend first-surface treatments for bird friendly glass.12 These tests also contributed to scientists’ understanding of ideal marker spacing, contrast, visual marker size, and surface coverage.

How reflective conditions occur, and why they matter for bird safe windows

In their “Critical examination of American Bird Conservancy’s (ABC) campaign to avoid bird collision”, Martin Rössler and Hans Schmid identify specular reflections as  “A characteristic trait of windows … as a consequence of the darker background of the room behind a window and the brightness of the foreground.”13 That is, the outside environment is often more brightly lit than the interiors of buildings, causing reflections on the surfaces of windows and obscuring treatments applied to interior surfaces.

Klem and Seewagen identify reflections on windows as a primary risk factor in bird-glass collisions, making them a necessary consideration for bird safe window design.14 In fact, it is because of reflectivity that bird friendly glass treatments are so often placed on the outermost surface of glazing. This first-surface placement prevents reflections from interfering with the appearance of bird friendly markers, ensuring that they remain visible to birds in all daylighting conditions.

The importance of in situ observations

The Prescriptive and Performance methods vary in many ways, but they have one key factor in common: the need for real-world validation. After all, the point of bird friendly glass isn’t to perform well in tests; it’s to stop birds from hitting windows.

An accidental experiment

Lovitch house in Durham Maine with bird safe windows

Late in 2022, biologists and bird conservationists Derek and Jeannette Lovitch unwittingly created an in situ experiment when they built a home using two different types of bird friendly glass. Their experiences, related here, shed light on the effectiveness of bird safe glass in a real-world setting.



Building a bird friendly home

Derek and Jeannette are both seasoned bird biologists with extensive knowledge of wild birds. In 2020, they set out to build a new house on bird-rich acreage near their birding supply store, Freeport Wild Bird Supply. They planned for the property to be a bird sanctuary and centre for wildlife tours, as well as their home.

As bird conservationists, Derek and Jeannette understood the importance of bird safe windows. They selected two types of bird friendly glass for the house: twenty-seven window units of AviProtek® 211 with acid-etched markers on the first surface, and five window units using a laminated glass with transparent UV-reflective markers on an inner surface. The five transparent units were installed at eye level for bird watching, with AviProtek® glass directly above them. These windows were a significant investment, but Derek and Jeannette believed that having a totally clear view through bird safe windows was worth the expense.

bird safe windows by area

In selecting their glass, Derek and Jeannette referred to the products’ Threat Factors. This was the most accessible information at the time, and they counted on Performance method metrics to be a good indicator of a product’s effectiveness. Both the AviProtek® glass and the UV-reflective glass had similar Threat Factors (23 and 26, respectively), and both were listed as “Approved” in the ABC Products & Solutions Database. Derek and Jeannette trusted that these two products would be effective against bird strikes.

Collisions against “bird friendly” glass

Just weeks after moving into their new home, Derek and Jeannette witnessed five bird strikes, one against each of their transparent UV windows. Four were immediately fatal. Derek and Jeannette were especially shocked when a Dark-eyed Junco flew directly into a corner window. Juncos can see in the ultraviolet spectrum, so the UV markers on the glass should have been visible to this bird. Furthermore, the bird hit a corner window where adjacent units created transparent conditions, with light behind the glass and little to no reflective effect. Based on these conditions and the product’s Threat Factor, the UV markers should have been visible to the junco. Nonetheless, it and four other birds flew straight into the UV-patterned windows as if they couldn’t see them.

Derek and Jeannette took immediate action, mounting cords over the transparent windows to prevent further strikes. First, red yarn was applied in 4” vertical spacing, which was replaced by Acopian BirdSavers a few months later. Although this solution stopped the collisions, it obstructed the view, which defeated the purpose of the transparent UV windows. Furthermore, as the yarn faded over time, it became less visible to birds. In late summer of 2023, Derek found two Mourning Doves dead at UV windows that had yarn in front of the glass, bringing the bird-glass casualties to seven. The yarn at these windows was soon replaced by Acopian BirdSavers.

bird safe windows by collisions

By comparison, Derek and Jeannette haven’t seen a single strike against the AviProtek® acid-etched glass. Even during two months of prolonged, daily observation in early 2023 when Derek was at home with an injury, he didn’t see any strikes against the etched glass. Nor has there been any evidence of strikes, even with inspection of the glass for impact marks.

bird safe windows composition

Interior view of bird safe windows at the Lovitch house
Top two rows: AviProtek® 211
Bottom row: transparent UV-reflective glass, retrofitted with yarn (left) and Feather Friendly markers (right).

Derek described his feelings about the situation, following the initial five strikes:

“As scientists by training, we realize one strike is better than 1.3 billion, but we weren’t here to kill a single junco. We put our hard-earned money into making sure that didn’t happen. We understand that it’s not 100% percent … But having five birds, three of which at least – two Dark-eyed Juncos and an American Goldfinch – have proven ultraviolet sight, hit a window? It’s been hard to take, especially with the first-hand observation of the direct-impact collision.”

Implications for the bird friendly building movement

The consequences of stories like this one go beyond individual experiences.  The failure of bird safe product recommendations could impact the building industry at large.

If other building owners and architects have similar experiences, will they be willing to invest in bird deterrent glass for future projects? Legislation may compel the use of bird friendly glass, but how long will it take before sub-optimal outcomes sap the movement’s momentum? In order to keep moving bird friendly building forward and establish it as a normal part of architecture, bird safe windows will need to work as they’re expected to.

Not all bad news

Fortunately, there’s also good news coming in about bird friendly glass. Many projects following the Prescriptive method as reflected in the National Glass Association’s Best Practices for Bird-Friendly Glazing Design Guide and the CSA A460:19 standard for Bird-friendly building design are showing zero or near-zero bird strikes in the years after installation. Some have seen a dramatic reduction of collisions following a retrofit with bird safe glass, such as the interpretation centre of a National Wildlife Area in Quebec. This building is located at the heart of a bird sanctuary, and used to see upwards of 24 bird strikes per year prior to bird friendly renovations in 2014. That year, the building was renovated with a variety of bird safe window treatments following the Prescriptive method, including AviProtek® patterns on 50% of the building’s glass. In the eight years following the renovations, there were only three recorded strikes against the AviProtek® windows.

The University of Saskatchewan (USask) presents another example of successful bird safe windows. In 2018, its Collaborative Science Research Building (CSRB) was constructed using AviProtek® pattern 217, a first-surface dot pattern with 2×2 spacing. Continued monitoring by the university’s Biology department has revealed zero bird strikes against the building’s windows in the years from 2018 to 2022. This success is especially meaningful when one considers a 2019 Master’s study that recorded bird strikes against four USask buildings, including the CSRB. The study recorded nine bird strikes over the course of three months against the Agriculture Building, just across the street from the CSRB.15 Interestingly, certain portions of the Agriculture Building incorporate bird friendly glass based on the Prescriptive method. There were no bird strikes against those portions of the building, and no bird strikes against the CSRB.16 You can read more about this study and the CSRB project in this article: Integrated Solutions and Bird-Friendly Glass at USask.

You can see post-installation outcomes on this and nine other projects in the Effectiveness of AviProtek® Bird Friendly Glass reference document.


A choice of futures for bird friendly glazing

The story of the Lovitchs’ house, combined with results from Prescriptive-based projects, reveals that not all “bird friendly” glass is equally effective. There are two trains of thought about legislation and guidelines on bird friendly building, and stories like these allow a comparison of their validity in real-life situations.

In the case discussed in this article, a homeowner referred to the Performance method to choose bird friendly glass for his house. Both products had similar Threat Factors, and both were labelled “Approved” on the ABC website. However, once installed, the outcomes of these two products were vastly different. One product successfully deterred bird strikes, while the other had five bird strikes in the first few weeks following construction.

In many other buildings, bird friendly glass based on the Prescriptive method has consistently reduced or eliminated bird strikes. Furthermore, the Prescriptive method allows for markers of any design, as long as they meet the method’s visibility requirements (spacing, surface, etc.). This freedom can make it easier for architects to incorporate bird friendly design into their projects. With the Performance method, designers and building owners are limited to a list of pre-approved designs.

Conservationists, building owners, and architects put a great deal of effort and funds into bird safe windows, and they count on legislators and bird conservation bodies for information about what works. As more bird friendly building standards are developed, legislators and industry leaders will have to decide whether they want to trust the Performance method for determining acceptable bird deterrent building products, or if they should turn to the Prescriptive method instead. Their choices will affect the lives of countless wild birds, along with the future of bird friendly glazing.


1. Daniel Klem, Solid Air: Invisible Killer – Saving Billions of Birds from Windows (Surrey: Hancock House, 2021), 133.

2. American Bird Conservancy, “Glass Collisions: About ABC’s Test Tunnel,” accessed October 4, 2023,

3. American Bird Conservancy, “Glass Collisions: About ABC’s Test Tunnel,” accessed October 4, 2023,

4. American Bird Conservancy, 2023. “About the ABC Rating System.” American Bird Conservancy, January 23.

5. American Bird Conservancy, “Glass Collisions: About ABC’s Test Tunnel,” accessed October 4, 2023,

6. Chad Seewagen, 2011. “A Review of Experimental Methods Used to Test the Effectiveness of Bird-Deterring Glass.” American Bird Conservancy, March 19.

7. Hans Schmid and Martin Rössler, 2016. “Critical examination of American Bird Conservancy’s (ABC) campaign to avoid bird collision.” Swiss Ornithological Institute, April.

8. Chad Seewagen, 2011. “A Review of Experimental Methods Used to Test the Effectiveness of Bird-Deterring Glass.” American Bird Conservancy, March 19.

9. Hans Schmid and Martin Rössler, 2016. “Critical examination of American Bird Conservancy’s (ABC) campaign to avoid bird collision.” Swiss Ornithological Institute, April.

10. Daniel Klem, Solid Air: Invisible Killer – Saving Billions of Birds from Windows (Surrey: Hancock House, 2021), 50,54.

11. Daniel Klem, Solid Air: Invisible Killer – Saving Billions of Birds from Windows (Surrey: Hancock House, 2021), 55.

12. Daniel Klem, 2019. “Technical Design of Bird Friendly Glass.” Walker White Papers. Walker Glass, April 29.

13. Hans Schmid and Martin Rössler, 2016. “Critical examination of American Bird Conservancy’s (ABC) campaign to avoid bird collision.” Swiss Ornithological Institute, April.

14. Chad Seewagen, 2011. “A Review of Experimental Methods Used to Test the Effectiveness of Bird-Deterring Glass.” American Bird Conservancy, March 19.

15. Anang Grace Yashim. “Risk and Mitigation Strategies for Bird-building Collisions on the University of Saskatchewan Campus”. MSEM. diss. University of Saskatchewan, 2019.

16. Anang Grace Yashim. “Risk and Mitigation Strategies for Bird-building Collisions on the University of Saskatchewan Campus”. MSEM. diss. University of Saskatchewan, 2019.

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