Bird-window collisions are common and can have serious consequences. Understanding why birds collide with windows is the first step toward implementing effective measures to protect them. This article explores the reasons behind these collisions, provides practical solutions to prevent them, and offers guidance on how to respond if a collision occurs.
Understanding Why Birds Collide
Birds collide with windows because glass creates an optical illusion, reflecting the sky, trees, or other foliage. This leads birds to perceive an open pathway or a continuation of their habitat. This reflection can be particularly deceiving, as birds may attempt to fly into what appears to be open space.
Glass can also be transparent, allowing birds to see through a building to vegetation or another window. They interpret this as a clear fly-through passage, especially in structures like glass walkways or homes with corner windows. Birds, with their monocular vision, often have difficulty perceiving stationary objects directly in front of them, making glass an invisible barrier.
Some birds hit windows due to territorial aggression, especially during breeding season. They perceive their own reflection as a rival bird and attack the perceived intruder. This behavior is common in species that nest near homes, such as American Robins or Northern Cardinals, and can persist for days or weeks.
Artificial light poses a threat to nocturnal migratory birds. City lights disorient these birds, drawing them off migration paths into urban areas. Once disoriented, they become more vulnerable to colliding with illuminated buildings and windows.
The Impact of Window Collisions
Window collisions have severe consequences for birds, ranging from immediate death to delayed mortality. Birds often suffer concussions, internal bleeding, broken bones, or eye damage upon impact. Even if a bird appears unharmed, it may have sustained internal injuries leading to its demise.
Up to one billion birds die annually in the United States from window collisions. This makes window strikes a leading cause of bird mortality. This high number of fatalities underscores the widespread nature of the problem.
Preventing Future Collisions
Making windows visible to birds is the most effective way to prevent collisions. Decals, stickers, or adhesive films applied to the outside of the glass can break up reflections. For optimal effectiveness, these markers should be spaced closely, ideally in a grid pattern with no more than two inches between markings for small birds, or four inches for larger birds.
External window screens or netting are highly effective, as they obscure reflections and provide a physical barrier that cushions any potential impact. Vertical cords or “Zen curtains” hung a few inches in front of the window, spaced two to four inches apart, are another option. For new construction or window replacements, specialized “fritted” glass, which has patterns fused onto its surface, is designed to be visible to birds while maintaining human visibility.
Simple, temporary solutions include drawing patterns on windows using inexpensive, easily removable tempera paint or soap. Reducing reflections from the inside by closing blinds or curtains can help, especially if the issue is transparency through the glass. However, this method is less effective for strong external reflections.
To address territorial aggression, temporarily cover the outside of the window with non-reflective material. This behavior typically subsides after the breeding season concludes. Minimizing artificial light at night, particularly during spring and fall migration, can significantly reduce collisions. Turning off unnecessary lights, using motion sensors, or directing lights downward can help prevent disoriented birds from being drawn to windows.
Responding to a Window Collision
If a bird collides with a window, observe it from a distance to assess its condition. If the bird appears stunned but is still alive, gently place it in a safe, dark, and quiet space. A shoebox with small air holes and a soft lining, placed in a warm area away from pets and disturbances, works well.
Do not provide the bird with food or water, as this can be harmful. Allow the bird to recover undisturbed for 15 to 30 minutes. If the bird recovers and flies off, it is safe to leave it.
For birds that do not recover quickly or show obvious signs of injury, contacting a local wildlife rehabilitator is recommended. Many birds that appear fine may have internal injuries requiring professional care. Wildlife rehabilitators possess the expertise to provide appropriate treatment and improve the bird’s chances of survival.