Vocal tract function in birdsong production: Experimental manipulation of beak movements

Kinematic analyses have demonstrated that the extent to which a songbird's beak is open when singing correlates with the acoustic frequencies of the s ounds produced, suggesting that beak movements function to modulate the aco ustic properties of the vocal tract during song production. If motions of t he beak are necessary for normal song production, then disrupting the abili ty of a bird to perform these movements should alter the acoustic propertie s of its song. We tested this prediction by comparing songs produced normal ly by white-throated sparrows and swamp sparrows with songs produced when t he beak was temporarily immobilized. We also observed how temporarily loadi ng the beak of canaries with extra mass affected vocal tract movements and song production. Disruption of vocal tract movements resulted in the predic ted frequency-dependent amplitude changes in the songs of both white-throat ed sparrows and swamp sparrows. Canaries with mass added to their beak sang with their beak open more widely than normal and produced notes with great er harmonic content than those without weights. Both manipulations resulted in acoustic changes consistent with a model in which beak motions affect v ocal tract resonances, thus supporting the hypothesis that dynamic vocal tr act motions and post-production modulation of sound are necessary features of normal song production.