Like humans, songbirds learn the sounds used by their species to communicate during a sensitive period of early development. In many songbird species only males learn to sing and a striking consequence of this behavioral sex difference is that the brain regions for song control are prominent in adult males and diminished in size (or absent) in adult females. In the zebra finch, some vocal control regions initially develop in both sexes. However, during the juvenile phase of song learning these regions show substantial neuron death in females, and growth with little neuron death in males. Our working hypothesis is that the quality of song learning and the morphological development of song regions are influenced by how often juvenile birds practice their vocal pattern. A role for use-dependent mechanisms in motor learning and neural growth and plasticity has ample precedent in other animals, including humans. However, the contribution of use-dependent mechanisms in vocal learning (human or songbird) has never been evaluated, partly because of the difficulty of measuring a behavior that is voluntary and that occurs at high levels over the course of development. However, with the use of an event-triggered recording system (which allows continuous monitoring of the vocal output of individual birds), we have shown that daily song production is significantly greater in juvenile birds that are learning to sing than in adults. Individual differences in song production are also greatest in juveniles, and birds that sing more as juveniles have a higher quality song as adults. Thus, songbird vocal learning appears to involve a 'practice-driven' component, which may interact with maturational factors to influence neural development and determine the quality of the adult vocal pattern. Several gene products will be examined in this context to establish links between singing and 1.) storage of the effects of practice, 2.) mechanisms that drive the growth of song regions, and 3.) motivational systems that influence rehearsal of song by juveniles. All of the gene systems that we will study in zebra finches show a high degree of homology to their human counterparts, so what we learn about their role in songbirds may have functional implications for human brain development and vocal learning.
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