Each year, glass windows contribute to the deaths of hundreds of millions, possibly over a billion, birds in the United States alone. This issue highlights a significant challenge birds face in human environments. Understanding the causes is crucial for addressing this problem and protecting birds.
Understanding the Causes
Birds frequently collide with windows because they cannot perceive glass as a solid barrier. Window reflections are a primary reason for these strikes. Glass surfaces mirror the surrounding environment, such as trees or sky. Birds interpret these reflections as open space, attempting to fly through what appears to be an unobstructed path.
Transparency is another factor, where birds see through a building to vegetation or open sky. If plants are visible through the glass, birds may mistake it for a clear passage. This “fly-through” effect can also occur in glass corners or when interior plants are positioned near windows. During breeding season, birds may intentionally strike windows if they see their own reflection, mistaking it for a rival.
Disorientation, especially during migration, also contributes to collisions. Many birds migrate at night, navigating by stars. Artificial light from buildings can attract and disorient nocturnal migrants, drawing them off course into urban areas. This can lead to collisions and exhaustion.
Birds may also collide with windows when pursuing food or evading predators. They might chase insects on or near the glass, or suddenly swerve to escape a predator, inadvertently flying into the window.
Assessing the Bird’s Condition
After a window collision, a bird’s condition varies; some are stunned, while others suffer serious injuries. Many birds are temporarily disoriented and recover quickly, flying away within minutes or hours. Even if a bird flies off, it may have sustained internal injuries like concussions, internal bleeding, or bone fractures that are not immediately apparent. These internal injuries can lead to death hours or days after impact.
Visible signs of serious injury include an inability to fly, unusual posture, drooping wings, visible wounds, or labored breathing. A bird that remains still when approached or appears dazed is likely injured. Observe the bird from a distance to assess its state. Keep pets, such as cats or dogs, away from the area, as they might predate on the vulnerable bird.
If the bird is stunned but otherwise appears unharmed, provide a quiet, safe space for it to recover. Gently place a cardboard box with air holes over the bird, or place the bird inside a box lined with a soft cloth. Check on the bird every 20-30 minutes; if it recovers and flies away within hours, it was likely just stunned.
If the bird shows clear signs of injury, does not recover, or if you are unsure, contact a local wildlife rehabilitator or animal control immediately for professional guidance. Do not attempt to force-feed the bird or give it water. Avoid handling it directly unless necessary, and only with extreme caution, as even small birds are fragile.
Effective Prevention Strategies
Making windows more visible to birds is the most effective approach to preventing collisions. Applying decals or stickers to the exterior surface of the glass is a common method. For these to be effective, they should be closely spaced, no more than two to four inches apart, to create a pattern birds recognize as a barrier. Some specialized decals reflect ultraviolet light, visible to birds but largely transparent to human eyes.
External screens or nets provide a physical buffer and reduce reflections. Installing insect screens on the outside of windows significantly minimizes reflections and cushions impact, reducing the likelihood of injury. Tensioned netting or hanging cords, placed a few inches in front of the window, also create a visible barrier. These cords should be spaced no more than four inches apart to prevent birds from attempting to fly through the gaps.
Creating patterns on the glass with tape or tempera paint can also deter birds. Weatherproof tape can be applied in horizontal stripes no more than two inches apart or in vertical stripes up to four inches apart. For a temporary solution, soapy water can be used to create patterns that obscure the glass. Consistent, close spacing across the entire window surface is key for all these visual markers.
Adjusting the surrounding environment and lighting can further reduce collision risks. The placement of bird feeders is important: feeders should be either very close to the window (within three feet) to prevent serious injury, or at least 30 feet away. Turning off indoor lights at night, particularly during migration seasons, helps prevent nocturnal migrants from becoming disoriented and attracted to buildings. Closing blinds or curtains at night can also reduce the transparency of windows, making them less inviting for birds.