Glass Collisions

About ABC's Threat Factor Rating System

Vassar Bridge Science Building, N.Y. ©Christine Sheppard
What is a Material Threat Factor?

It is difficult to objectively define what makes glass “bird-friendly.” Although used frequently, the term ‘bird-friendly' itself provides no specific guidance. Architects interested in designing a building that does not kill birds can select products for their insulation value, breaking strength, or a host of other characteristics, but until recently there was no system for specifying bird-friendly materials. Making analysis more difficult, the quality of being bird-friendly is more complex than a quality like insulation value because birds respond to the appearance of glass and this can vary dramatically for a single product as seen in the changing light of day, as well as under artificial light at night. Additional complexities such as IGU build, façade or freestanding installation, differing reflected environments and other factors make each situation unique. Contributing factors vary so much that an absolute measurement cannot be provided.

In 2010, experts from American Bird Conservancy and a team of architects interested in advancing the field of bird-friendly design developed the concept of Material Threat Factor (commonly Threat Factor or TF). This is a way to assign scores that provide a relative measure of how well materials with patterns of visual markers cause avoidance by birds in a standard, controlled test environment. These scores allow architects to use collision deterrence as a factor when designing buildings. The system also permits evaluation of products that can be applied to existing glass (retrofits). TFs also made it possible to create a credit for reducing bird collisions in the LEED rating system and have now begun to be included in legislation, like New York City's Local Law 15 of 2020.

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Where Do Threat Factors Come From?

Ideally, Material Threat Factors could be derived from monitoring collisions on glass at a collection of diverse, existing buildings, replacing that glass with a product under review, continuing to monitor to see whether collisions are reduced and calculating the reduction. Unfortunately, this type of data is very rare and this type of test would be very costly. It would also take years to acquire enough monitoring data to reliably detect a trend for a single product and, importantly, it would kill birds. ABC realized the need to create practical ways to evaluate glass. Initially, TFs were derived only from a tunnel test (described below). More recently, data from field trials and other types of testing have been factored in.

Assigning Material Threat Factors Using Tunnel Testing

In 2003, Martin Rössler created the first “tunnel” at the Hohenau-Ringelsdorf Biological Station (Austria), to test proposed solutions to bird collisions on freestanding noise barriers. ABC's tunnels, using a second-generation design, followed in 2009 and 2021. Tunnel testing is a non-injurious, standardized binary choice technique that uses wild songbirds to determine the relative effectiveness of patterns at deterring bird collisions. In the U.S., tunnel testing takes place at American Bird Conservancy's tunnel at the Carnegie Museum of Natural History's Powdermill Avian Research Center and Washington College's Foreman's Branch Bird Observatory.

In a test flight, a bird flies down a completely dark space, the ‘tunnel' (Figure 1), toward light at the far end, where side-by-side panels of glass appear to offer exit routes (Figure 2). One of these panes of glass is clear, unmarked glass (the control, invisible to the bird) and the other has the pattern of visual markers under evaluation (the test pane). A net ensures that the bird is safely stopped before it can hit the glass. After one trial, the bird is immediately released. When few birds fly to the patterned glass, we believe that they see and are avoiding the visual markers.

Figure 1: The Powdermill tunnel in operation. Birds are placed into the tunnel through a sleeve, at the right end, where video camera and computer are also located. The release door can be see at the left end of the facing side of the tunnel. Glass is mounted outside the right end of the tunnel.

Figure 2: A bird's view from inside ABC's Powdermill Test Tunnel, with clear glass on the left, glass with vertical stripes on the right. Note the net, which prevents birds from actually contacting the glass.

Specimen Performance Index (SPI) of a material is calculated by flying at least 80 individual birds down the tunnel and recording the percent that fly toward the test pane. For example, suppose 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 have SPI of 25. If there is no additional information, for example, from a field trial, then the TF is equal to the SPI.

Important Considerations:

  • Threat Factors do not equal the percent reduction in collisions expected when a glass is installed on a building. In fact, the same glass may perform differently on each facade of a building, depending on angle to the sun, habitat reflected, etc. Threat factors are an index, reflecting the relative response to different patterns by songbirds flown in the tunnel. However, where monitoring data on tunnel-tested products are available, they confirm that products with lower threat factors have yielded fewer collisions. ABC defines “bird-friendly” materials conservatively, as having a threat factor ≤30, corresponding to a reduction of collisions of at least 50% under real world conditions.
  • The lower the TF, the more effective the test pattern will be at reducing collisions. This has been confirmed when data is available. However, the relationship is not linear, so one cannot say that a pattern with TF=15 is twice as effective as a pattern with TF=30.
  • Strong reflections can reduce pattern effectiveness by obscuring the pattern from view during part or all of the day, especially if the pattern is on an inside surface of the glass. Reflections vary with insolation, and with the angle of view, from the strongest reflections at acute angles to the weakest reflections when glass is viewed head on (Figure 3).
    • Figure 3: A single piece of glass viewed head on and from the side: The reflection is much stronger at an acute angle than it is head on. This glass has a surface reflectivity <14%. However, for a bird to view this glass at an angle where there is a dangerous reflection, it would be flying almost parallel to the glass surface, where a collision would be unlikely to cause injury.

      • ABC's tunnel test cannot replicate the range of possible reflections on a glass sample, so our tests model situations where the pattern must be visible against a competing view. In the tunnel, the pattern is always seen against a complex background (seen in figure 2), modeling situations where either the glass has a strong reflection or when birds can see through the glass to a landscape beyond. While it might seem at first that “reflection” and “see-through” conditions are quite different, in this context they are similar. In figure 4a, below, we cannot determine whether the image is a photo of a tree, a tree seen through a window, or the reflection of a tree. With the information in 4b, showing mullions, a crack, and right angles, we know glass is involved, but only with the information in figure 4c can we confirm that the image is a reflection and that we are not looking out the window at trees.
        • 4a



          • ABC does not test materials with greater than 15% reflection at the outside surface of the glass, unless the pattern itself is on the outside surface, because internal patterns would not be sufficiently visible, overwhelmed by the reflection on the surface.
            • Because the amount and type of reflection on glass is both site and product specific, reflections should be considered early in the design process. Recommended design phase analysis ideally involves samples of glass viewed on the project site from a variety of angles, at different times of day, and with all potential site-specific reflections taken into consideration.

              Spacing of Visual Markers

              Where early recommendations, based on research by Klem (1990) and Rössler et al., showed that horizontal lines spaced 2” (5cm) apart and vertical lines spaced 4” (10cm) apart significantly reduced collisions, subsequent monitoring showed that the 4” spacing does not stop collisions by the smallest birds, especially hummingbirds. The spacing guideline for the Prescriptive Rating Option assigns a lower score when there is 2” (5 cm) spacing between the pattern's visual markers vertically and horizontally and a higher score for patterns with 4” (10 cm) horizontal spacing. Two inch (5 cm) spacing for two-dimensional patterns on glass or window film is now typically recommended in Canada (Canadian Standard A460) and increasingly in the United States. Thus, what was once the ‘2×4 rule' is now the ‘2×2 rule'. However, 2×4” spacing has been demonstrated to reduce collisions significantly and materials based on that guideline can still receive a ‘bird-friendly' TF rating.

              Visual Acuity

              For a pattern of visual markers on glass or other materials to successfully deter bird collisions, birds must be able to perceive it in time to change course. The pattern of visual markers must be of an appropriate size and spacing and must be distinguishable from the both glass and any reflections or the view through the glass (e.g., habitat, sky, water, a potential flight path, etc.). We have adopted 9'-9' ½” (3 meters) as the standard viewing distance from which birds must be able to detect the pattern, following Ros et al.

              Our instinct is to use human vision as a proxy for what birds can see. However, birds can see more colors and even perceive the earth's magnetic field, while humans have much better visual acuity. ‘Visual acuity', the ability to resolve spatial features depends on the size of the eye and the number of photoreceptors in the retina. Songbird eyes are much smaller than human eyes. Because of this, birds, particularly small birds like songbirds, have very poor visual acuity in comparison to humans. Therefore, a pattern that appears quite visible to us may not be apparent at all to birds.

              Assigning Material Threat Factors Without a Tunnel Test

              Not all materials need to be tunnel tested, specifically materials that are similar to others previously tested. ABC partners with the Bird-Safe Buildings Alliance (BSBA), a group of architects experienced in bird-friendly design, conservation biologists, and others with collisions expertise, to provide TFs for materials similar to those already rated. These ratings are very conservative: the rating assigned is the maximum expected TF; lower ratings might be obtainable through actual tunnel testing.

              TFs can be assigned by review, instead of by tunnel test, to products that: 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) comply with ABC's Prescriptive Rating Option (Appendix III). The Prescriptive Rating Option describes only a subset of materials that can be rated by other methods.

              These guidelines can change as new information becomes available. For example, where early guidelines (Klem, 2009) showed that horizontal lines spaced 2” (5cm) apart and vertical lines spaced 4” (10cm) apart significantly reduced collisions, monitoring showed that this vertical spacing does not stop collisions by the smallest birds, especially hummingbirds. ABC's current recommended spacing guideline is therefore 2” (5 cm), vertically and horizontally. This spacing, for two dimensional patterns on glass or window film, is now typically recommended in Canada and increasingly in the United States.


Why Birds Hit Glass

Birds, unlike humans, are unable to understand or learn the concept of ‘glass' as an invisible barrier that can also be a mirror. Birds take what they see literally – and glass can appear to be habitat they can fly into, whether the habitat is reflected, or seen through a pane of glass.


Photo of hummingbird


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