Contains a component which brilliantly reflects ultraviolet sunlight Invisible to humans, but glows like a stoplight for birds Decals help birds see windows and avoid striking the glass Four low-tack modern square decals 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 Modern Square decals measure approximately 3.5" 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. Reapply decals every 6-9 months. CONTENTS: 4 Decals per Envelope. Each decal is approximately 3.5 inches across. TYPE: Static cling.