Short-term and long-term effects of vocal distortion on song maintenance in zebra finches.

Adult zebra finch song is irreversibly altered when birds are deprived of correct feedback by deafening or denervation of the syrinx. To clarify the role of feedback in song maintenance, we developed a reversible technique to distort vocal output without damaging the auditory or vocal systems. We implanted flexible beads adjacent to the syrinx to alter its biomechanics. Immediate song aberrations included low volume, frequency shifts, missing harmonics, and production of click-like syllables. After a few weeks, seven of nine birds stopped producing some syllables. In six of these birds, the gaps left by the silenced syllables gradually shortened, and the lost syllables did not return when beads were removed 16 weeks after treatment began. The nondeleted syllables of all birds regained their preimplant morphology, insofar as could be detected, within 9 d after bead removal. In four other birds, we removed the beads as soon as syllables were deleted, when the silent intervals were still full length. In these birds, all deleted syllables returned within 1 week. Our results indicate that both silenced syllables and syllable morphology can recover as long as the song's temporal structure is maintained, but once altered, changes in the song sequence can be permanent. A hierarchical organization of the song production system has recently been described (Margoliash, 1997). Reversible disruption of song production by our method appears to permanently alter the higher levels of the system that encode song sequence, but not the lower levels that encode individual syllable structure.