Social performance reveals unexpected vocal competency in young songbirds

Vocal ontogeny in songbirds provides a good model for understanding how complex motor behavior, including speech, is learned. For birdsong, as for other motor learning, it has generally been assumed that a subject's motor output at any point during learning represents what the subject has learned to produce by that time. Here, we show, however, that juvenile zebra finches partway through song learning, singing immature song, are capable of producing song with much more mature properties, depending on the behavioral context. In these birds, we were able to elicit courtship (female-directed) song, which young birds normally sing infrequently, and to compare it with the alone or "undirected" song (Undir) predominantly produced during learning as well as with the same bird's subsequent adult song. We found that the juvenile courtship song was much less variable than the immature Undir and as stereotyped as the adult song produced after a further month of practice. More strikingly, the juvenile courtship song was also acoustically much more similar than Undir to the adult song. This finding demonstrates that the Undir that juvenile birds usually produce underestimates the extent of learning and that song structure is learned faster than previously thought. Moreover, the rapid improvement in song quality in response to external social cues supports the idea that courtship singing is a state of motor "performance," in which the bird selects the best variants of the song learned during singing alone, and suggests that such performance states can reveal unappreciated progression of learning.