Water-like timbral quality in birdsong arises from complex motor control of two sound sources

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.