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 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 things more difficult, the quality of being bird-friendly is more complex than a quality like insulation value because glass varies so much in appearance – from one time of day or season to another, with different reflected environments, etc. – so an absolute measurement cannot be provided.

In 2010, 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) as a way to assign scores that provide a relative measure of birds' ability to see and avoid patterned glass and other materials. These scores allow architects to design buildings using rated glass and also permit evaluation of products that can be applied to existing glass (retrofits) to reduce collisions. TFs also made it possible to create a credit for reducing bird collisions in the LEED rating system.

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Where Do Threat Factors Come From?

Ideally, Threat Factors could be derived from monitoring collisions on glass at a diversity of existing buildings, replacing that glass with a product under review, and then continuing to monitor to see whether collisions are reduced. 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 and, importantly, it would kill birds.

ABC realized the need to create practical ways to evaluate glass. 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 tunnel, using the same design, followed in 2009. Tunnel testing is a non-injurious, standardized binomial 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.

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 glass (the control, invisible to the bird) and the other has the pattern under evaluation (the test). A net ensures that birds are safely stopped before they hit the glass. After a trial, birds are immediately released. When few birds fly to the patterned glass, we believe that they see and are avoiding the pattern. For a detailed description of the Powdermill and Hohenau tunnels, see Sheppard (2019) and Roessler et al. (2007).

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.

The threat factor of a material is based on flying at least 80 individual birds down the tunnel and recording whether they fly toward the control or to the patterned 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 therefore have TF=25.

It is important to note two things:

  • 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.


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What About Reflections?

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.




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.

Can Threat Factors Be Assigned Without a Tunnel Test?

In order to keep tunnel testing from creating a bottleneck and to make a range of options available quickly, ABC partners with the Bird-Safe Building Alliance (BSBA), a group of architects experienced in bird-friendly design, conservation biologists, and other collision experts. In a simplistic example, this allows us to assign a score of TF=1 to a brick wall without having to conduct a tunnel test.

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) meet ABC's Prescriptive Standards, created using principles derived from tunnel scores, data from collisions monitoring, and information from the literature on avian perception.

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.

The Prescriptive Standards are not an attempt to describe all bird-friendly glass, although that is a long-term goal. In the short term, the Standards can be used to identify groups of materials that meet a set of criteria that allows them to be considered bird friendly without needing a tunnel test. The Standards will continue to evolve over time as more data become available.

ABC's Prescriptive Standards are currently under development and will be posted here in the near future.


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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