Fig. 5.

A model integrating previous behavioral work with the present results. Song crystallization occurs at ∼90 d after hatching. Before this age, song structure and amplitude are highly variable. After crystallization, song structure remains highly stereotyped throughout adulthood. However, even after song structure becomes stable, song stereotypy relies on the bird's ability to compare vocal output with song memories and make motor adjustments accordingly. Therefore, song stereotypy requires sensorimotor flexibility. With increasing adult age, the motor program for song becomes increasingly stable and independent of auditory feedback. Our results indicate that over the same interval, neuronal incorporation decreases, and the population of replaceable HVC neurons becomes more stable.