Collisions with glass buildings, windows, and other transparent or reflective structures kill an estimated 600 million to 1 billion birds annually in the United States alone, making building collisions one of the leading human-caused sources of bird mortality. Understanding and preventing these collisions is a significant bird welfare priority.
Birds cannot perceive glass as a barrier. When glass reflects vegetation or sky, birds see habitat, not an obstacle. When transparent glass allows views through buildings to visible vegetation beyond, birds attempt to fly through. Both types of situations — reflective and transparent glass — cause collisions throughout the year but particularly during spring and autumn migration when billions of birds navigate unfamiliar landscapes.
Collisions cause a spectrum of outcomes from immediate death to varying degrees of injury. Immediate death from broken necks and brain trauma is common. Stunned birds that appear to recover often die from internal injuries within 24-48 hours. Birds that survive face ongoing vulnerability while stunned — predation, trampling, and exposure while disoriented significantly worsen outcomes.
Collision injuries cause acute pain and distress. Raptors and other heavier species often suffer severe chest trauma. Many collision survivors are permanently injured and cannot survive in the wild.
Contrary to popular assumption, large skyscrapers are not the primary killers. Low-rise buildings (1-3 stories) and residential homes cause the majority of collision fatalities because of their sheer number. However, individual high-rise buildings can kill hundreds of birds on a single night during peak migration. Glass bridges, bus shelters, and sound barriers also cause significant mortality.
Bird-safe glass and retrofit treatments are effective and widely available:
Artificial light at night attracts and disorients migratory birds, increasing collision risk in cities. "Lights Out" programs in major cities including New York, Chicago, and Toronto encourage building managers to reduce unnecessary lighting during peak migration periods. Documented reductions in collision mortality of 60-70% have been reported in participating buildings.
Bird-safe building standards are now required in several US cities (New York, San Francisco, Toronto). FLAP Canada and the American Bird Conservancy have developed guidelines that are increasingly incorporated into building codes. Advocacy for widespread adoption of bird-safe building standards is an important welfare priority.