Window and glass collisions kill an estimated 100-988 million birds annually in the United States and 16-42 million in Canada — making it the second leading human-caused source of bird mortality after outdoor cats. The welfare impact of these collisions is enormous, and many are preventable with readily available solutions.
Birds cannot perceive glass as a barrier. When windows reflect sky, trees, or vegetation, birds see habitat continuity. When glass is transparent, birds attempt to fly through apparent flight paths. Migratory birds navigating unfamiliar landscapes during spring and autumn migration are most vulnerable — they encounter many unfamiliar windows while moving rapidly through landscapes they have not had opportunity to learn.
Collision outcomes range on a spectrum from instant death to delayed mortality from internal injuries, to full recovery. The welfare impact at each point is significant:
Not all windows kill equally. Corner windows that create apparent fly-throughs, large picture windows reflecting vegetation, and glass connecting indoor plants to outdoor views are highest risk. Residential windows kill more birds in aggregate than office buildings simply because there are more of them. However, identified high-kill buildings — particularly those in migratory flight paths — represent priority intervention targets.
Multiple interventions effectively reduce collision mortality:
Single raptor silhouettes do not work — birds avoid the silhouette area but fly freely around it. Hawk decoys are similarly ineffective. Methods that treat a small proportion of window area while leaving most reflective are ineffective. Coverage and pattern density are the critical variables in collision prevention.