Window Alert Decals Nothing good happens when a bird flies into a windowpane. Usually something breaks... most of the time, it's the bird's neck, but windows also can crack from repeated hits. Ordinary decals are not so attractive but these are nearly invisible to humans. (They fluoresce in the ultraviolet band birds are highly sensitive to.) Multiple styles are available. The decals stay in place by electrostatic cling, not adhesive. A 4-Pack is $7.99 at Doctors Foster and Smith.