WindowAlert Hummingbird UV Decal helps prevent wild birds from accidentally flying into your windows by making the glass visible to them. This decal features a special coating that reflects ultraviolet sunlight, which is invisible to humans but appears as a bright glow to birds, similar to a stoplight. To use, simply place the decal on the outside of your window, concentrating decals in the center and then working outward. It is to place decals at least every 2 feet across the glass. For protection, scientific studies suggest spacing decals 2 inches horizontally and 4 inches vertically from one another, effectively reducing bird collisions.