Abstract: Birdsong provides neuroscientists with a uniquely powerful model for studying imitative vocal learning in a system where the brain structures responsible for song learning and production are well known. The 4,500+ species of songbirds provide a remarkable diversity of songs with a variety of tonal, structural, and learning characteristics, but most studies of the neural bases of learning have concentrated on two domesticated species, the canary and the zebra finch. Important differences in the songs of these two species provide useful properties for comparative studies, which could be expanded by using other species that demonstrate mimicry or action-based learning. Although the primary goal of most studies of the neural bases of song has been to define the mechanisms responsible for imitative learning during development, studies of adult crystallized song are important for two reasons. First, they define the endpoint of learning, and second, adult song shows interesting forms of variability in its performance. The degree of adult song variability itself varies among individuals and is influenced by the sources from which the song was learned, how the song was assembled during learning, behavioral responses of adult listeners, and levels of circulating sex steroids. In addition, song may be associated with coordinated visual displays, which also contribute to its communicative function. Thus the study of crystallized adult song is likely to provide insights into the neural control of facultative behavior as well as into the important question of how imitative learning takes place.