why birds sing?

http://www.whybirdssing.com/

http://www.pbs.org/lifeofbirds/songs/index.html

http://news.bbc.co.uk/1/hi/sci/tech/992538.stm

http://environment.newscientist.com/article/dn11714-urban-birds-sing-at-night-to-be-heard.html

http://petcaretips.net/why-bird-sings.html

http://petcaretips.net/bird-language.html