Billions of migratory birds die each year from collisions with glass windows and illuminated buildings during seasonal migrations, representing one of the largest human-caused sources of bird mortality worldwide.
The suffering involved in bird-building collisions can be severe. Many birds die immediately on impact from broken necks or internal injuries. Others are stunned and left vulnerable to predators, exposure, and secondary injury. Some linger for hours in pain before dying. The scale — potentially a billion deaths per year in North America alone — makes this a massive welfare concern that often goes unnoticed because collisions happen silently and victims are quickly removed by scavengers or cleaning crews. Migratory birds are already under stress from long flights, food scarcity, and habitat loss, making collision mortality an additional burden on already-stressed populations.