The ABC focuses on how many times birds approach the glass samples in the tunnels. Following testing, ABC provides manufacturers with a bird-friendly rating, called a threat factor. Developed in 2010, a threat factor is a way to assign scores measuring a bird’s ability to see and avoid patterned glass and other materials. This rating makes it easier for direct product comparisons but also allows governments to enact bird-friendly policies and architects to design buildings bird-safe structures. A full list of products approved by the ABC, plus associated threat factors, is available on the ABC website.
Unlike a traditional test lab, ABC only tests products which advance the science in some new way. Based on past research, there are certain parameters which are proven to be effective. Leading researchers on the topic have already identified the required spacing, length, width, opacity, color, and orientation of elements marked on glass. These determined parameters become the basis of prescriptive or permitted products and substitutions.
For example, certain maximum distances (e.g. 100 mm [3.04 in.] for vertical stripes, 80 mm [3.55 in.] for dotted grids, and 50 mm [1.97 in.] for horizontal lines) have been determined as effective in preventing collisions. Patterns covering as little as one to five percent of the total glass surface can deter most strikes under experimental conditions. As a rule, most birds will not attempt to fly through horizontal spaces less than 50.8 mm (2 in.) high or through vertical spaces 101 mm (4 in.) wide or less. This has become known as the “2 x 4 rule,” and it relates to the size and shape of birds in flight.
Given a choice between translucent or white stripes, white stripes perform better because light is more visible against the background reflections. The stripes should be at least 3.175 mm (0.125 in.) wide. While humans see patterns, birds focus on the spaces within patterns. Dot patterns have been found to work well if the dot diameter is at least 6 mm (0.25 in.) in size. Generally, products meeting these criteria do not need to be tested.
The key benefit to tunnel testing is it produces rapid reproducible results. Tests can be usually completed in a day. The two North American testing facilities can issue a threat factor, and based on the score, architects design buildings using rated glass. The test also permits the evaluation of products which can be applied to existing glass (retrofits) to reduce collisions.5
A lower threat factor score means the product is more effective at reducing collisions. ABC uses a threat factor of less than 30 to correspond to at least a 50 percent reduction under real-world conditions. Glazing product manufacturers strive for a threat factor of 30 or under.
