Birds sing to communicate. Male birds use song to advertise their territories and
attract females. Each bird species has a unique song or set of songs. Song conveys
both species and individual identity. In most species, young birds learn some features
of adult song. Song develops gradually from amorphous to fixed patterns of
vocalization as if crystals form out of liquid. Learning of a song proceeds in two
steps; birds commit the song to memory in the first stage and then they vocally
reproduce it in the second stage. The two stages overlap each other in some species,
while they are separated by several months in other species. The ability of a bird to
commit a song to memory is restricted to a period known as the sensitive phase. Vocal
reproduction of the memorized song requires auditory feedback. Birds deafened
before the second stage cannot reproduce the memorized song. Birds change vocal
output until it matches with the memorized song, which thus serves as a template.
Birds use a built-in template when a tutor model is not available. Exposure to a
tutor model modifies this innate template
