Barriers to Birds Not only windows pose a threat to birds. Any reflective or transparent surface is a potential bird killer. Several central European countries have built transparent, three-metre-high noise barriers along major highways. Authorities now recognize that these barriers, made of plastic-glass or what the Austrians refer to as "float glass", are a serious cause of bird mortality. In an effort to minimize the number of deaths, the transportation departments have used raptor silhouettes. Unfortunately, the wide spacing of the silhouettes has made them all but useless. It's one point in our favour, that noise barriers in Ontario at least are built out of opaque materials, making them clearly visible to birds.