Birds hitting windows is a common and distressing event for homeowners. In the United States alone, estimates suggest between 365 million and 1 billion birds die annually from colliding with windows. Understanding the reasons behind these collisions can help protect avian wildlife.
Territorial Displays
One frequent reason for a bird repeatedly hitting a window is a territorial display. During breeding season, male birds establish and defend their nesting territories. When a bird sees its own reflection, it perceives this image as a rival. The bird then attempts to drive away this perceived competitor by flying at the glass, pecking, or flapping its wings against the window.
This behavior is particularly common in species that nest near human homes, such as American Robins and Northern Cardinals. Birds have excellent visual acuity but often cannot discern that a reflection is their own image. This perceptual limitation leads them to believe they are confronting a real rival, causing them to expend energy and time. This aggressive behavior usually subsides after the breeding season concludes.
Perception and Transparency
Collisions also occur because birds do not recognize glass as a solid barrier. Glass can appear invisible or reflect the surrounding environment, creating the illusion of a clear flight path. Windows often reflect trees, sky, or other natural elements, which birds perceive as extensions of their habitat. They may attempt to fly towards these reflections, mistaking them for open space or vegetation.
Birds might also try to fly through transparent glass, seeing vegetation or an open space on the other side. Disoriented birds, particularly during migration, may also strike windows. Artificial lights, especially at night, can attract and disorient migrating birds, drawing them into urban environments where glass poses a greater threat. Birds fleeing from predators may also hit windows if they do not perceive the glass barrier while attempting to escape.
Attraction to Indoor Elements
Birds can also be drawn to elements inside a building, leading them to collide with windows in an attempt to reach perceived attractants. Indoor plants, for instance, can appear as inviting resting spots or sources of food or shelter. Birds may try to fly towards these plants, not realizing the glass creates an impassable barrier.
Other indoor elements like lights or patterns on curtains can attract a bird’s attention. This type of collision is less about reflections and more about a direct visual appeal of something beyond the glass. While less common for repeated striking compared to territorial displays, it still contributes to bird-window collisions.
Preventing Window Collisions
Homeowners can take several steps to reduce bird-window collisions. Making windows more visible is an effective strategy. Applying decals, tape strips, or patterns directly to the outside surface of the glass can help birds recognize the barrier. These markings should be spaced no more than two inches apart, both horizontally and vertically, to be effective even for small birds like hummingbirds. External screens or netting can also obscure reflections and provide a visual barrier.
Reducing reflections is another measure. Closing blinds or curtains can minimize the reflective properties of windows. For bird feeders, placing them either very close (within three feet) or far away (more than 30 feet) from windows can reduce collision risk, as close feeders prevent serious injury. During migration periods, especially at night, turning off non-essential indoor and outdoor lights can prevent disorienting birds. Apply any deterrents to the outside of the window, as internal applications are often ineffective.