Nonlinear phenomena in northern mockingbird (Mimus polyglottos) vocalizations: Acoustics and physiology

Abstract

The bird sound source (syrinx), like the mammalian larynx, has been discovered to behave as a nonlinear system of coupled oscillators. Modeling approaches as well as excised syrinx experiments show that the syrinx can be considered a nonlinear sound source. Until now, only few acoustic data were available showing to what extent a birdsong species’ vocal repertoire actually contains nonlinear phenomena, like subharmonics, frequency jumps, biphonation and deterministic chaos. The passerine syrinx, in contrast to the mammalian larynx, consists of two sound sources, one situated in each of the bronchi that are independently controlled. Bronchial airflow measurements showed that individual mockingbirds use each side of the syrinx independently. However, there are interactions of both sound sources, which have not yet been investigated acoustically/spectrographically. The present study shows how many of the spectrographically observed nonlinear phenomena are attributed to the interaction of the two sound sources, i.e., nonlinear phenomena, while both sound sources show positive airflow values, compared to single sound source utterances, i.e., positive flow through one sound source only. Nonlinear phenomena could be attributed to the interaction of the two sound sources (earlier referred to as ‘‘two-voice-phenomenon’’) but were also found to be produced by a single sound source.