Matters of life and death in the songbird forebrain.

Male zebra finches learn a specific vocal pattern during a restricted period of development. They produce that song in stereotyped form throughout adulthood, and are unable to learn new song patterns. Development of the neural substrate for song learning and behavior is delayed relative to other brain regions, and neural song-control circuits undergo dramatic changes during the period of vocal learning due to both loss of neurons as well as incorporation of newly generated neurons. In contrast, canaries do learn new song patterns in adulthood and modify their vocal repertoires each breeding season. Adult canaries also maintain a large population of dividing cells in the ependymal zone of the telencephalon, and vast numbers of newly generated neurons migrate out to become incorporated into functional circuits and replace older neurons. We review the relationships between cellular and behavioral aspects of song learning in both zebra finches and canaries, as well as the role of gonadal hormones in regulating diverse aspects of the song-control system.