DetailsThe Bird Saver Decals contain a component which reflects ultraviolet sunlight to prevent birds from hitting your windows.This ultraviolet light is invisible to humans, but birds see it as a brilliant glow, keeping them from colliding with your windows. To the human eye, the decals appear as frosted or etched glass. The decals measure approximately 4" across. Place the decals on the outside of a window. One decal is recommended for every 3' x 3' section of glass. 4 decals per pack.