Kinematics of Birdsong: Functional Correlation of Cranial Movements and Acoustic Features in Sparrows

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Abstract
The movements of the head and beak of songbirds may play a functional role in vocal production by influencing the acoustic properties of songs. We investigated this possibility by synchronously measuring the acoustic frequency and amplitude and the kinematics (beak gape and head angle) of singing behavior in the white-throated sparrow (Zonotrichia albicollis) and the swamp sparrow (Melospiza georgiana). These birds are closely related emberizine sparrows, but their songs differ radically in frequency and amplitude structure. We found that the acoustic frequencies of notes in a song have a consistent, positive correlation with beak gape in both species. Beak gape increased significantly with increasing frequency during the first two notes in Z. albicollis song, with a mean frequency for note 1 of 3kHz corresponding to a gape of 0.4cm (a 15° gape angle) and a mean frequency for note 2 of 4kHz corresponding to a gape of 0.7cm (a 30° gape angle). The relationship between gape and frequency for the upswept third note in Z. albicollis also was significant. In M. georgiana, low frequencies of 3kHz corresponded to beak gapes of 0.2–0.3 cm (a 10–15° beak angle), whereas frequencies of 7–8kHz were associated with flaring of the beak to over 1cm (a beak angle greater than 50°). Beak gape and song amplitude are poorly correlated in both species. We conclude that cranial kinematics, particularly beak movements, influence the resonance properties of the vocal tract by varying its physical dimensions and thus play an active role in the production of birdsong.

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