Now you can help reduce the huge number of birds killed each year flying into windows windowalert is a static cling decal thats easily applied to windows at home at the office on greenhouses conservatories or anywhere that glass may creat an invisible and fatal obstacle the decals contain a component which reflects ultraviolet sunlight undetectable to humans it glows brightly for birds to spell out danger ahead pack of 4 decals each about 102cm 4 diameter 1 decal is sufficient