1. http://science.howstuffworks.com/zoology/birds/owl-fly-silently1.htm
2. http://blogs.bu.edu/bioaerial2012/2012/10/08/owl-silent-and-swift/
3. http://www.asknature.org/product/f6fa7aad8c550b605c8e864f613e4f87
4. http://www.m2woman.co.nz/read.php/article/feathers-inspire-quieter-planes
5. http://ziehl-abegg.com