After building the visitors' center at the Lehigh Gap Nature Center, director Dan Kunkle found birds were flying into the windows and dying. His solution was a type of film called CollidEscape found online. He said more commonly available is WindowPerf. For the visitors' center, the film was cut into the shapes of raptors. It comes in rolls. The back is peeled off and it sticks to the glass. The material is see-through from the inside and white on the outside. Kunkle says birds flying into windows is the No. 1 human-caused bird killer.