Bird-window collisions are a widespread issue causing significant avian mortality in urban and rural areas. Understanding the reasons is crucial for effective prevention.
Why Birds Strike Windows
Birds collide with windows due to misleading optical illusions. Glass can appear as transparent passageways or reflective extensions of the environment. Reflections of sky, trees, or habitat are perceived as open space, leading birds to fly into invisible barriers. Similarly, visible interior plants or outdoor scenery through a window can prompt birds to attempt flying through.
Avian vision contributes to these misperceptions. Birds are tetrachromats, seeing ultraviolet (UV) light invisible to humans. Their side-positioned eyes provide a wide field of view but limit frontal depth perception. This makes judging distances and identifying transparent obstacles challenging.
Environmental and behavioral factors exacerbate collisions. Reflection intensity changes with light conditions. Artificial lights at night, especially in urban areas, disorient migrating birds, making them vulnerable. Vegetation near windows attracts birds, and its reflection compounds the illusion of continuous habitat. Some birds, particularly during breeding season, strike windows due to territorial aggression, attacking their own reflection.
The Consequences for Birds
Window collisions often result in fatal outcomes. Annually, hundreds of millions to over a billion birds perish in the United States from these strikes. Many birds suffer critical internal injuries not immediately apparent, even if they fly away. Injuries include concussions, internal bleeding, brain trauma, fractured bones, and eye damage.
Studies show about 60% of birds brought to rehabilitation centers after a window strike do not survive, often due to brain injuries. Stunned birds are vulnerable to predators, like domestic cats, or other environmental dangers before recovery. Glass in human structures poses an indiscriminate threat to avian populations.
Effective Prevention Methods
To minimize bird collisions, make windows visible. Applying visual markers to exterior glass is effective. Decals, tape strips, or tempera paint can be used, adhering to specific spacing. The “2×2 rule” (patterns no more than two inches apart horizontally and vertically) ensures birds perceive the glass as a barrier.
External screens or netting provide a physical and visual barrier, breaking up reflections and cushioning impacts. These include full-window screens or taut netting installed a few inches from the glass. Position bird feeders either very close (within three feet) or far away (beyond ten to thirty feet). This prevents birds from gaining speed for severe injury if startled.
Managing lighting, especially at night, reduces collision risks for migratory birds. Turn off non-essential indoor and outdoor lights during migration seasons to prevent disorientation. Motion-sensor lights or downward-directed exterior lights also contribute to a bird-safe environment. Adjust landscaping by moving indoor plants away from windows and strategically placing outdoor vegetation to prevent deceptive reflections.
Assisting a Stunned or Injured Bird
If a bird strikes a window and appears stunned or injured, careful intervention increases its survival chances. First, observe from a distance to see if it recovers. If it remains on the ground, approach cautiously, ideally from behind. Gently place it inside a small, dark, quiet container like a cardboard box or paper bag. Line the container with a soft cloth and ensure adequate ventilation before securing the lid.
Place the covered container in a warm, dark, quiet location away from pets and children. Do not offer food or water, as this can lead to aspiration or complications. Immediately contact a local wildlife rehabilitator. These professionals assess internal injuries and provide specialized care. If advised, and the bird recovers within an hour or two, release it outdoors in a safe, open area.