A songbird or oscine is a bird belonging to the suborder Passeri of Passeriformes (ca. 4000 species), in which the vocal organ is developed in such a way as to produce various sound notes, commonly known as bird song. Songbirds evolved about 50 million years ago in the western part of Gondwana that later became Australia, New Zealand and Antarctica and later spread around the world. This 'bird song' is essentially territorial in that it communicates the identity and whereabouts of an individual to other birds and also signals sexual intentions. It is not to be confused with bird calls which are used for alarms and contact. They are especially important in birds that feed or migrate in flocks.

Note: Above information from http://en.wikipedia.org/wiki/Songbird