KINEMATICS OF BIRDSONG - FUNCTIONAL CORRELATION OF CRANIAL MOVEMENTS AND ACOUSTIC FEATURES IN SPARROWS

The movements of the head and beak of songbirds may play a functional role in vocal production by influencing the acoustic properties of son gs. We investigated this possibility by synchronously measuring the ac oustic 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 a re closely related emberizine sparrows, but their songs differ radical ly in frequency and amplitude structure. We found that the acoustic fr equencies of notes in a song have a consistent, positive correlation w ith 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 3 kHz corresponding to a gape of 0.4 cm (a 15-degrees gape angle) and a mean frequency for note 2 of 4 kHz corresponding to a gape of 0.7 cm (a 30-degrees gape angle). The r elationship between gape and frequency for the upswept third note in Z . albicollis also was significant. In M. georgiana, low frequencies of 3 kHz corresponded to beak gapes of 0.2-0.3 cm (a 10-15-degrees beak angle), whereas frequencies of 7-8 kHz were associated with flaring of the beak to over 1 cm (a beak angle greater than 50-degrees). Beak ga pe and song amplitude are poorly correlated in both species. We conclu de that cranial kinematics, particularly beak movements, influence the resonance properties of the vocal tract by varying its physical dimen sions and thus play an active role in the production of birdsong.