Hundreds of millions of birds die every year hitting the windows in our homes. We have all seen the photos of dead birds that hit urban buildings. Ten times as many die hitting our home windows.
Humans know that a dark space on the side of a building is a window. Birds do not. They see in it a reflection of the safe space they have just been flying through, and crash into the window and die – though some fly off, to die later from internal injuries or stunned, eaten by predators.