Song-learning behavior: the interface with neuroethology.

Behavioral studies of song learning in birds continue to raise new problems for neuroethological investigation. Evidence is emerging for a new form of vocal plasticity, called 'action-based learning'. Motor patterns are overproduced during a particular phase of ontogeny, and are then subjected to attrition and selective reinforcement by various kinds of social stimulation as the young bird matures. This form of learning, analogous to operant conditioning, can occur at phases of development when the more traditional form of 'memory-based learning' is no longer possible. There is evidence that different physiological mechanisms are involved in the development and the maintenance of mature singing, with a transition occurring at the time of song crystallization. Neural events associated with the developmental stabilization of motor patterns are worthy of more study.