Kinematics of birdsong in sparrows: Functional correlates of beak and head motion

Birds often display exaggerated movements of the head, throat and beak while singing, but the functional significance of these motions for song production remains largely unexplored. The kinematics of the head and beak during song were analyzed in two bird species, the swamp sparrow (Melospiza georgiana ) and the white-throated sparrow (Zonotrichica albicollis ). Kinematic data were correlated with acoustic features of the songs produced. The kinematic profile of a song includes changes in angle of the head to the body, beak gape, and angle of the jaws. In both species frequency upsweeps and downsweeps correlate with increase and decrease in beak gape, respectively. Kinematic differences between the two species are associated with functional differences in producing repetitive frequency sweeps (swamp sparrow) or constant frequency pure tones (white-throated sparrow). We suggest that the degree of beak flair influences the effective tube length of the birds vocal tract, and thus affects acoustic properties of song production.