Designed to prevent birds hitting windows. Although some birds will collide in the heat of a chase, most do so because they see a reflection of the sky and trees in the glass, or because there is another window or mirror in the room making the bird think there is a way through.
Double glazed windows tend to pose a greater risk than single glazed, since they produce clearer reflections. This hazard can be reduced by making the window pane more obvious to the bird. Fixing to the outside of the glass an object which will indicate that an obstacle is present should help.