What is the superstition involving a bird hitting a window?

It is said that if a bird flies into a window, that it portends a death. The Cornell Ornithology Lab, however, has a different explanation for why birds collide with windows. Quite simply, they mistake reflections in windows for open space.

Birds are not always able to tell that windows are barriers between them and the destination to which they think they are flying. Sometimes, birds are simply being territorial and mistake their own reflection for that of another bird, and attack to try to drive the other bird away. Unfortunately, most injuries incurred by birds as the result of colliding with a window are fatal.