Transitions and tricks: nonlinear phenomena in the avian voice

Abstract

Birds evolved a novel vocal organ, the syrinx, that exhibits a highanatomical diversity. In the few species investigated, the syrinx cancontain up to three pairs of functional syringeal vocal folds, acting asindependent sound sources, and eight pairs of muscles. This rich varietyin vocal structures and motor control results in a wide range of nonlinearphenomena (NLPs) and interactions that are distinct to avian vocalphysiology, with many fascinating mechanisms yet to be discovered. Here,we review the occurrence of classical signatures of nonlinear dynamics,such as NLPs, including frequency jumps and transitions to chaos in birds.However, birds employ several additional unique tricks and transitionsof inherent nonlinear dynamical nature that further enrich their vocaldynamics and are relevant for understanding the motor control of theirvocalizations. Particularly, saddle-node in limit cycle (SNILC) bifurcationscan switch sounds from tonal to harmonically rich and change thephysiological control of fundamental frequency. In mammalian phonation,these bifurcations are mostly explored in the context of register transitionsbut could be equally relevant to altering vocal fold dynamical behaviour.Due to their diverse anatomy compared to mammals, birds provide uniqueopportunities to explore rich nonlinear dynamics in vocal production.This article is part of the theme issue ‘Nonlinear phenomena invertebrate vocalizations: mechanisms and communicative functions’.