Birds may be killed or injured by flying into windows, although most will recover with no ill effects. If you find a bird lying below a window, put it in a darkened box in a quiet place for an hour or two, then let it go.
Birds fly into windows because they do not see them, or because reflections of trees and the sky suggest there is a way through. Some aggressive birds may attack their own reflections.
Stickers or other objects attached to the glass will help the bird to realize that there is something solid ahead of them. Any shape will do, but the most effective is thought to be the silhouette of a hawk or falcon.