Why do birds fly into windows?

The most common reason for birds flying into windows is because the birds normally see a reflection in the window, not what is on the other side. The birds only see more trees and birds, which makes them think they have a clear path.

Although it is quite common to hear the "thud" of a bird hitting the window, it is also deadly to the birds flying into them. About half of all birds that accidentally fly into windows are flying at their normal travel speed, making the impact of the sudden window too extreme for the birds to live through.