WindowAlert decals help prevent wild birds from accidentally striking windows. Each decal has a special coating that reflects ultraviolet sunlight. This ultraviolet light is invisible to humans, but birds see it as a brilliant glow much like a stoplight. To the human eye, the decals appear as frosted or etched glass. The decals measure approximately 4 inches across, and are designed to be placed on the outside of a window. As a rough guide, we recommend one decal for every 3 ft by 3 ft section of glass. Contents: 4 Decals per Envelope. Type: Static cling.