A young songbird knows only simple note combinations, avian equivalent of infant babbling: plastic song. It takes more complex singing ability to relate the loneliness of the heart in isolation, or the location of particularly suitable nest-building material: stable song.
To facilitate flight these birds have tiny air-tight brains inside a two-ply shell of skull.
The brain of a young songbird (chaffinch or canary, say) is programmed for only plastic song. How does the necessary reprogramming occur?
When a young, but sexually mature, songbird with the knowledge and capacity of plastic song comes within earshot of an older bird's stable song dramatic neural transformations occur. Obsolete neural connections separate and redundant neurons fall away. Neurons of increased complexity are hatched at the base of the skull and migrate, swim through complex nets of existing neural connections, to their rightful place. In this manner is the brain rewired for stable song.
To discover this scientists slaughtered dozens, hundreds, of birds, little yellow canaries. At the moment they were about to open their mouths in chirping. Instead of hearing the song, they see it laid out in the split-open brain.
This is called neurogenesis.