Examples of the subsong, plastic song and adult song, which are produced at different ages by zebra finches can be found here.
"Subsong" - thought to be equivalent to a human baby's babbling - produced by a normal 38 day old juvenile bird.
This early vocal behavior does not rely on area HVC (the "high vocal center"), which contains the neural circuit for adult song production. Thus, bilateral HVC deactivation does not alter subsong produced by the same zebra finch in the above example.
The next phase of song production, called "plastic song" is characterized by the presence of variable syllables. The normal plastic song produced by a 50 day old juvenile bird is shown below.
Song produced by the same 50 day old bird after bilateral lesions of HVC. Although the bird still vocalizes, the song has lost its structure and resembles the more primitive subsong.
Normal undirected singing produced by an adult bird.
Singing produced by the same adult bird after bilateral lesions of HVC. The distinct syllables that characterize normal adult songs are lost.