Window collisions kill up to one billion birds annually in North America, making glass buildings one of the largest human-caused sources of bird mortality and welfare harm.
Birds that collide with windows experience traumatic brain injury, neck injury, and thoracic trauma that causes immediate death or prolonged suffering before dying hours to days later. Survivors of non-fatal collisions may be dazed and vulnerable to predation during recovery. The scale of window strike mortality - comparable in magnitude to the number of birds killed by outdoor cats - means this is a major unaddressed source of chronic wildlife welfare harm. Prevention is achievable through building design modifications that make glass visible to birds. Individual homeowners can apply effective deterrent patterns to problem windows at low cost.