Window collisions kill millions of bird each year. David Sibley has been experimenting with using a highlighter to mark windows in his home that are prone to being struck by flying birds. Highlighter markings on the windows are not visible in white light, but are in UV light. Since birds can see in UV, they can see the markings.
The use of the highlighters has proven to have some limitations, but it is an interesting idea.
You can read his most current comments and the original idea and testing on his web site.
http://sibleyguides.blogspot.com/2007/1 ... ndows.html