Up to one billion birds are estimated to collide with buildings and other human made structures in the U.S. every year, and billions of additional birds probably collide with buildings each year in other parts of the world. These collisions occur because artificial light at night and/or glass cause birds to fly into buildings and other human made structures. Unfortunately, most of these collisions are fatal.
Reflective and transparent glass exteriors, indoor plants, and greenery close to buildings can all create confusing situations for birds who often mistake the reflections they see on windows and other glass surfaces for real scenes, or mistake transparent glass surfaces for clear passage ways to fly through. Birds do this because most wild birds, unlike most household pets, have never had an opportunity to learn what glass is, so they need strong clues on or around glass to warn them that it’s there.
Most species of birds migrate at night using a variety of cues to find their way, including the earth’s magnetic field and the position of the stars. Because birds are particularly sensitive to light when they are migrating at night, artificial lights at night can disorient them, particularly during cloudy or rainy weather when the celestial cues they use to navigate may be not be visible. Under these circumstances birds may congregate around artificial lights causing them to collide with various parts of the buildings associated with lights, much like a deer in headlights.