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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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.
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.
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.
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.
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.
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