Why Do Birds Fly Into Windows?

The bird: majestic. Soaring. A creature of song. Muse of Alfred Hitchcock. And occasionally, apt to plunk its tiny head into your double-paned window.

More than 100 million birds are estimated to die each year as a result of flying directly into glass; even a bird that scoots away may suffer internal injuries. Despite being widely thought of as intelligent, most avian species are still vulnerable to this ignoble demise. Are they fooled by reflections of trees? Are they attacking their own reflection?

It’s a little bit of both, but that’s not the whole story. “Birds may be attempting a rapid escape when disturbed by people or predators,” says Graham Martin, B.Sc., Ph.D., D.Sc., Emeritus Professor, School of Biosciences at the University of Birmingham. “[They] probably perceive the reflections of the vegetation behind them as a safe haven.”

A bird anxious to flee isn’t likely to be able to process a lack of actual sky. In other cases, birds might interpret their own image as a rival during breeding season, but Martin believes these kinds of existential crises aren’t usually fatal. “They more likely walk up to their reflections,” he says. “Parrots, especially budgerigars, can spend a long time chattering to their reflection in a mirror hung in their cage, but it is not clear whether the bird perceives its reflection as another bird or whether the mirror is just a bright object that they take an interest in.”

Plus, bird turf wars in the air usually don’t result in high-speed impact. After a brief aerial tussle, Martin says, they take it to the ground, “When one bird can pin the other down and attack with bill and feet.”

Window collisions can be reduced if window decals or other signals (like branches) help give them some visual cues. Birds, like humans, can be guilty of “looking without seeing,” or relying on information that isn’t yet present visually. Martin believes birds are somewhat like automobile drivers in this regard: data on car collisions has shown an accident is more likely to happen when an unexpected obstacle is present, interrupting the pathway in memory.

So, if a bird flies into your window, it’s not because it’s stupid. It’s because it was scared. Or disoriented. And remember: we’re really not so different.

Additional Sources:

“Understanding Bird Collisions with Manmade Objects: A Sensory Ecology Approach,” The International Journal of Avian Science, 2011 [PDF].