Window strikes are estimated to kill up to 1 billion birds annually in North America alone, making glass a leading human-caused mortality source for birds.
Birds that strike windows die from brain hemorrhage and skull fracture. Those that are stunned but survive face secondary risks from predation while incapacitated and internal bleeding that causes delayed death. Window strikes are concentrated at buildings during migration periods but occur year-round at residential properties with garden habitat. The problem affects all bird species but is particularly significant for migratory species passing through urban areas. Building design changes including external shading, angled glass, and visual markers can prevent strikes. The American Bird Conservancy rates building materials and window treatments for bird safety. Retrofitting existing windows with external deterrents is effective and affordable.