THE vocal organ, the syrinx, of some songbirds has been hypothesized to contain two sound sources that can be operated independently. The syrinx of songbirds (Oscines) is a bipartite structure1,2 whose two sides are potentially capable of acting either together or independently to produce sound3-9. Sound production is lateralized in some species such that one side produces most of the song9-11. I have now directly measured the acoustic output and motor dynamics of the left and right sides of the syrinx during song in catbirds and thrashers. In these birds, sound may be produced by either side of the syrinx alone, by both sides acting together, or by switching from side to side. When both sides of the syrinx contribute simultaneously to a note or syllable, both may generate the same sound or each side may produce a different sound. A given syllable type is generated by a similar motor pattern each time it is produced.
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