If you want to learn more about acoustic communication in birds, check out these....
The Cornell Lab of Ornithology , http://www.birds.cornell.edu/netcommunity/Page.aspx?pid=1478
Macaulay Library of Natural Sound: http://macaulaylibrary.org/index.do
Book: Nature’s Music: The Science of Bird Song, Marler & Slabbekoorn
Book: The Singing Life of Birds, Donald Kroodsma
Book: Bird Song: Biological Themes and Variations