Bird-window collisions are a widespread phenomenon affecting avian populations. Each year, hundreds of millions to over a billion birds in the United States die from striking windows, making it a significant human-caused threat to avian life. This issue occurs across various environments, from urban centers to suburban homes. Understanding the reasons behind these collisions and their consequences is important for mitigating their impact.
Why Birds Collide With Windows
Birds frequently collide with windows due to visual misinterpretations. Windows often reflect the surrounding environment, like sky or trees, creating an illusion of open space. Birds perceive these reflections as safe flight paths, flying directly into the glass. Glass transparency can also trick birds into believing there is a clear passage through a building.
Artificial light pollution, especially during night migrations, also contributes to collisions. Many birds migrate at night, navigating by stars, and bright city lights can disorient them, drawing them into urban landscapes. Collisions are more frequent during spring and fall migration due to increased bird movement.
Some birds may repeatedly strike windows due to territorial aggression, mistaking their own reflection for a rival. Their vision, which includes the ability to see ultraviolet light, means they may not recognize their own image or the glass as a barrier.
The Immediate Aftermath of an Impact
When a bird strikes a window, the physical impact can result in a range of severe injuries. Common injuries include concussions, internal bleeding, broken bones, and shock. Brain injuries, such as swelling or bleeding, are frequent and can have lasting consequences. Even if a bird appears to fly away immediately after a collision, it may have sustained internal trauma or neurological damage that can prove fatal. Studies show that a substantial portion of birds that initially seem stunned and recover enough to fly off still succumb to their injuries, often within hours or days.
A bird’s survival chances are influenced by multiple factors. The velocity and angle of impact contribute to the extent of injury. Species and size also play a role, with smaller, more delicate species are more vulnerable to severe harm.
The design of the window affects how birds perceive it and the nature of the collision. Birds that remain stunned on the ground are also at increased risk of predation, further reducing their survival odds. Data from wildlife rehabilitators indicates that fewer than half of injured birds brought in for care after a collision ultimately survive, underscoring the hidden impact of these incidents.
What to Do After a Window Strike
If you discover a bird that has collided with a window, approach it cautiously to avoid causing further stress or injury. If the bird appears only stunned, it may recover on its own. You can gently place the bird in a small, dark cardboard box with air holes, lined with a soft cloth. This provides a safe, calm environment for the bird to recover from shock and potential concussion, away from predators.
Place the closed box in a warm, quiet location, away from pets and human activity. Check on the bird periodically, but avoid excessive handling or opening the box, as this can increase stress. Do not attempt to force-feed the bird or give it water, as this can cause aspiration or worsen its condition.
If the bird recovers and becomes active, take the box outdoors to a sheltered spot and open it, allowing the bird to fly away. If the bird does not recover within a few hours, or shows obvious signs of injury like bleeding or broken limbs, contact a local wildlife rehabilitator or agency for professional assistance.
Strategies to Prevent Collisions
Implementing various strategies can reduce bird-window collisions. One effective method involves making windows visible by applying patterns or markings to the exterior glass surface. Decals, tapes, or films should be spaced closely, no more than two inches apart, to ensure birds perceive the glass as a barrier. These markings disrupt reflections and transparency, alerting birds to the solid surface.
External screens or netting installed a few inches from the window also create a physical barrier and break up reflections, cushioning any impact. For new construction, specialized “bird-friendly” glass with patterns or UV-reflective coatings can be used, visible to birds but often transparent to humans.
Simple actions like closing blinds or curtains, especially when reflections are strong, can reduce the illusion of open space. Turning off unnecessary indoor lights at night, particularly during migratory seasons, helps prevent disorienting birds and drawing them toward buildings. Placing bird feeders very close (within 3 feet) to windows can also reduce injury risk, as birds do not build up high speeds over short distances.