Age-dependent effects of song exposure: song crystallization sets a boundary between fast and delayed vocal imitation

Song crystallization is a prominent developmental phase of oscine birds in which there is a transition from a production of plastic vocal material to a performance of elaborated song patterns that are typical for adult birds. Here we show that crystallization can be related to a marked change in memory properties involved in supplementary learning of song occurring during this phase. We studied nightingales, Luscinia megarhynchos, a species renowned for its large repertoire of song types. After a period of early tutoring as fledglings, hand-reared subjects (N=8) were exposed to a set of temporally distributed training experiences as juveniles and young adults. Analyses of the birds' singing yielded clear evidence for late song learning and also a striking phase-related shift in their memory properties. New songs heard shortly before crystallization (at an age of 40-42 weeks) were imitated within a few days and their structural and syntactical ‘quality’ seemed not to be inferior to imitations developed from songs heard earlier. In contrast, none of the songs experienced soon after song crystallization (at 45-47 weeks) appeared as imitations in the repertoires of the young adults (at 48-55 weeks). However, birds had clearly memorized these songs, as they produced imitations of them in their second spring. Our findings show that auditory song acquisition in nightingales extends well into the phase of vocal production. At the same time, however, the boundary between an immediate and a delayed form of vocal imitation suggests that song crystallization reflects a marked change in memory properties. That is, song crystallization seems to constrain the ability to develop motor programs for song patterns that are heard even though these are committed to memory. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.