About Bird Song

The distinction between birds songs and calls is based upon complexity, length, and context. In non-technical use, bird songs are the bird sounds that are melodious to the human ear. Songs are longer and more complex and are associated with courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact.
Howell and Webb (1995) make the distinction based on function, so that short vocalizations such as those of pigeons and even non-vocal sounds such as the drumming of woodpeckers and the "winnowing" of snipes' wings in display flight are considered songs.
It is generally agreed upon in birding and ornithology which sounds are songs and which are calls, and a good field guide will differentiate between the two.The songs of different species of birds vary, and are more or less characteristic of the species.
In modern-day biology, bird song is typically analyzed using acoustic spectroscopy. Species vary greatly in the complexity of their songs and in the number of distinct kinds of song they sing (up to 3000 in the Brown Thrasher); in some species, individuals vary in the same way.The specificity of bird calls has been used extensively for species identification.
Some  musicologists believe that birdsong has had a large influence on the development of music. There seem to be three general ways musicians or composers can be affected by birdsong: they can be influenced or inspired (consciously or unconsciously) by birdsong, they can include intentional imitations of bird song in a composition, or they can incorporate recordings of birds into their works.
Bird song is a popular subject in poetry.
Source:
http://en.wikipedia.org/wiki/Bird_vocalization

Read more about birds:
http://www.stanford.edu/~kendric/birds/BirdSite.html
http://www.stanfordalumni.org/news/magazine/2001/marapr/features/cardinal.html
http://birds.audubon.org/faq/birds
http://birds.audubon.org/common-birds-decline
http://www.birds.cornell.edu/
http://imnh.isu.edu/digitalatlas/bio/birds/main/ident/bvk6sb.htm