Causes of bird/glass collisions
Glass appears differently to birds depending on numerous factors, including how it is fabricated, the angle at which it is viewed, and the difference between exterior and interior light levels. Combinations of these factors can cause glass to look like a mirror or dark passageway, or to be completely transparent.
Birds strike transparent windows, attempting to reach potential perches, plants, food, water sources, or other lures seen through the glass (Figure 1). Common architectural features that are especially dangerous include:
- glazed skywalks joining buildings overhead;
- glass walls in front of lobbies with interior plants;
- corner windows with glass installed perpendicularly on both sides; and
- exterior glass handrails.
Viewed from the outside, transparent glass can be highly reflective. Under the right conditions, almost every type of architectural glass, reflects the sky, clouds, or nearby habitat familiar and attractive to birds. When birds try to fly to the reflection, they hit the glass. Reflected vegetation is the most dangerous, but birds also attempt to fly past reflected buildings or through passageways that are not there. Mirrored glass is particularly hazardous because it is reflective at all times of day and presents undistorted images of sky, trees, and other habitat features birds mistake for reality (Figure 2).
Fatal light attraction
The problem of bird collisions with glass is greatly exacerbated by artificial light. Light escaping from building interiors or from exterior fixtures can attract birds, particularly during migration on foggy nights or when the cloud base is low. Strong beams of light can cause birds to circle in confusion and collide with structures, each other, or even the ground (Figure 3). Others may simply land in lighted areas and must then navigate an urban environment rife with dangers, including glass.
Trends toward bird-friendly construction
In the last several years, the problem of bird/glass collisions has received considerable public attention. Scientists began researching the problem intensively around 1979, and their estimates spurred birding groups to get involved. The Toronto non-profit group, Fatal Light Awareness Program Canada (FLAP Canada), led the way.2 In 1993, its volunteers began documenting thousands of ‘kills’ in downtown Toronto by walking the streets in the early morning hours and picking up dead and injured birds at the base of glass buildings. The powerful photographs of hundreds of dead songbirds have motivated groups in other cities to mount similar efforts (Figure 4). Today, there are volunteer monitoring programs in many major Canadian and U.S. cities, and their findings are an important basis of scientists’ estimates of bird mortality.
It quickly became apparent the building industry could play a lead role in reducing these deaths by designing and constructing buildings that would not kill birds. The U.S. Green Building Council (USGBC) now offers a pilot credit for reducing bird collisions in the Leadership in Energy and Environmental Design (LEED) program. Pilot Credit 55 has been the most popular LEED pilot credit ever offered.
Currently, there are no mandatory prerequisites or optional credits for preventing bird collisions for buildings targeting LEED certification in Canada. However, the issue is mentioned in the Canada Green Building Council (CaGBC) 2009 LEED Canada New Construction (NC) reference guides, under the Sustainable Sites (SS) category. Specifically, the guide suggests birds’ migratory patterns be considered in building designs.