Window collisions are a significant problem impacting bird populations globally. They occur wherever birds and windows coexist, leading to widespread bird mortality.
How Birds Interpret Glass
Birds possess highly developed visual systems, often surpassing human capabilities in acuity and color perception. Despite this, they frequently collide with windows because glass lacks the natural cues birds use to identify solid objects. This creates optical illusions birds cannot easily perceive as barriers.
Glass often appears transparent to birds, allowing them to see through to habitat or sky. This creates the illusion of an unobstructed flight path, making birds believe they can fly directly through. In other instances, windows act like mirrors, reflecting the surrounding environment, such as trees or sky. Birds mistake these reflections for open space or perching areas, attempting to fly into what they perceive as a continuation of their natural surroundings. Glass near vegetation increases collision likelihood, as reflections confuse birds.
The Reality of Collisions
Window collisions severely injure or kill birds. Upon impact, birds frequently suffer concussions, internal bleeding, and broken bones. Many birds temporarily stunned and able to fly away may still die later from unseen internal injuries. Research indicates less than half of birds stunned by collisions survive, even with rehabilitative care.
Window collisions are a leading human-caused source of bird mortality. Estimates suggest over one billion birds die annually in the United States from striking buildings. This issue intensifies during migration periods when large numbers of birds navigate unfamiliar landscapes.
Practical Solutions to Protect Birds
Making windows visible to birds is the most effective way to prevent collisions. Applying visual deterrents to the outside surface of glass breaks up reflections and eliminates the illusion of open space. Decals, films, or patterns should be spaced no more than two inches apart horizontally and vertically to be effective for small birds. High-contrast designs, such as white or light-colored materials, stand out best.
External screens or netting can provide a visual and physical barrier, reducing reflections and cushioning potential impacts. Hanging parachute cords, spaced a few inches apart, in front of windows also creates a visible curtain. These solutions should cover the entire glass surface, as even small untreated sections pose a risk.
Adjusting the placement of bird attractants like feeders and baths can reduce collision risk. Positioning feeders very close to windows, within three feet, prevents birds from gaining speed for a fatal impact. Alternatively, placing feeders more than 25-30 feet away from windows keeps birds out of the collision danger zone.
Reducing indoor lighting at night is another important step, particularly during spring and fall migration seasons. Artificial light can disorient migrating birds, drawing them into urban areas and increasing their risk of striking illuminated buildings. Turning off non-essential lights, using motion sensors, and directing exterior lighting downward can help. Closing curtains or blinds at night also reduces light escaping through windows.