Development and learning in the birdsong system: Are there shared mechanisms?

The vocal control system of songbirds provides a useful system for studying both development and learning because it undergoes dramatic, sexually dimorphic, hormone-dependent development in young birds and is essential both for learning and production of song. Song system development and song learning may prove to share many cellular mechanisms, although much remains to be learned. For instance, the innervation of the song motor nucleus RA by the AF output nucleus LMAN initially promotes neuronal survival in RA, and later is critical for sensorimotor learning, perhaps using similar mechanisms to guide synaptic rearrangement in the motor pathway as song develops. Testosterone also appears to influence both maturation of the song system and vocal plasticity Some developmental mechanisms, however, are clearly different from those of learning. For example, auditory experience is essential for normal learning, but many properties of the song system develop normally in the complete absence of auditory input. Estrogen is crucial for the normal emergence of a song system, but its role in learning is unclear. A complication of comparing development and learning in the song system is that normal neural development is a prerequisite for learning. Thus, disruption of mechanisms critical for the development of the song system inevitably interferes with normal learning, but does not necessarily imply that those mechanisms were directly responsible for learning. In the song system it is possible to disentangle development and learning, however, by experimentally manipulating behavior, for instance by altering the phases of learning or by disrupting auditory inputs or vocal outputs. Such behavioral experiments provide a powerful tool to reveal which neural modifications are causally related to song learning.