Abstract
Sound coloration (timbre) plays an important role in music and speech as well as in non-human animal vocalizations. Although many physical attributes of sound can be reliably quantified, the description of multidimensional timbre has remained difficult.(1)(,)(2)(,)(3)(,)(4)(,)(5) Upper vocal tract modification of sound gives rise to many timbral features, but some, such as the profile of rising amplitude, are generated by the sound source.(2)(,)(5) Here, we show that the liquid-sounding elements of cowbird song are source generated and arise from a dynamic interaction of two sound sources whose oscillations spring from an unusual oscillatory regime. Sounds with "watery timbre" are characterized by a highly asymmetric amplitude profile, and the different frequencies from alternating contributions of the two sources mimic sounds emitted by a sequence of droplets of differing sizes hitting a water surface. The generation of these sounds requires exquisite motor control of two sound sources. Starlings imitating cowbird song use the same unusual source control, thus confirming a functional connection between this motor control and the timbral features. These findings illustrate how an unusual timbral feature of birdsong arises from the interaction of physiology, morphology, and aerodynamics.