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A songbird is a bird belonging to the order 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. There is evidence to suggest that songbirds evolved about 50 million years ago in the western part of Gondwana that later became Australia, New Zealand, New Guinea and Antarctica, before spreading 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, and are especially important in birds that feed or migrate in flocks.
Other birds have songs to attract mates or hold territory, but these are usually simple and repetitive, lacking the variety of many passerine songs. The monotonous repetition of the Common Cuckoo or Little Crake can be contrasted with the variety of a Nightingale or Marsh Warbler.
Although many songbirds have songs which are pleasant to the human ear, this is not invariably the case. Many members of the crow family make croaks or screeches which sound harsh to humans.