The 2×4 rule
Tunnel testing has consistently shown birds will not attempt to fly through horizontal spaces less than 51 mm (2 in.) high or through vertical spaces 102 mm (4 in.) wide or less. This is referred to as the ‘2×4 rule’ and is applicable regardless of the type of visual noise being proposed. Whether lines, dots, or squiggles, elements creating the spaces must be big enough to be visible to birds—about 12.7 mm (½ in.) in diameter for dots, and 3.17 mm (1/8 in.) wide for lines (Figure 6.)
More is more for patterns
Even if a pattern meeting the 2×4 rule cannot be achieved, more pattern is better than less. The goal is to reduce the intervals of clear glass as much as possible so birds are aware of a barrier. The pattern need not obstruct building occupants’ view—tests have identified patterns that deter most bird collisions while only covering five to seven per cent of the glass.
Less is more if it is glass
Bird mortality is directly related to the percentage of transparent glass on a façade. All-glass buildings are becoming more prevalent due to marketing perceptions, life-style changes, and more energy-efficient glass products. Thus, more buildings are now hazardous to birds. Given this trend is not likely to be reversed in the foreseeable future, the glass industry needs to step up and seriously address the bird-kill issue.
Eliminating the mirror effect
Highly reflective glass is particularly hazardous and should be avoided, especially where it reflects sky or vegetation. While high reflectivity used to be synonymous with high performance, this is no longer the case as the glass industry continues to develop more energy-efficient coatings allowing greater transparency with lower reflectivity. Reflective glass is now specified more for uniformity of appearance between vision glass and opaque glass than it is to meet energy codes and such. Tests have shown glass reflectivity of less than 15 per cent reduces bird collisions.
Apply treatments to the exterior glass
The most effective solutions are installed on the outside surface of glass or in front of glass, because reflections can be strong enough to make internal treatments invisible from the outside. Some treatments that reduce transparency, such as interior shades, can even increase reflectivity in certain situations, so it is important to analyze each building to determine the nature of the problem before choosing material.
Reduce unnecessary lighting
The key to all bird-friendly design is the importance of reduced night lighting as illumination attracts birds to a building in the first place. Special efforts should be made to reduce landscape, parking lot, tower, and ‘vanity’ lighting, such as for façades and signage. Instead, motion detectors or computerized systems that respond to actual building use should be employed, so lights are not on when there are no occupants in the space.
Shield interior landscape
Interior plants—especially trees—attract migratory birds looking for food and refuge after a long flight. Unfortunately, many of these interior landscapes are lit at night for the enjoyment of people using the building. If they are visible from the outside through glass, they pose a particular hazard to birds. Screening these landscapes, or even just turning off the accent lights, can reduce the threat.
