Window Alert 4 Butterfly Decals Protect Wild Birds Window Alert is a static-cling or low-tack decal that may be applied to home and office windows. The decals contain a component which brilliantly reflects ultraviolet sunlight. This ultraviolet light is invisible to humans but glows like a stoplight for birds. Birds have vision that is up to 12 times better than that of humans. WindowAlert decals help birds see windows and avoid striking the glass. 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"; across, and are designed to be placed on the OUTSIDE of a window. As a rough guide, we recommend one decal for every 3'; x 3'; section of glass.