Acoustic generators 2 Acoustic interaction 2 Acoustic signature 2 Behavioral research 2 Biomechanics 2 Bird species 2 Birds 2 Complex behavior 2 Complex sounds 2 Coupled systems 2 Delay differential equations 2 Differential equations 2 Fundamental frequencies 2 Mathematical models 2 Nonlinear effect 2 Nonlinear features 2 Oscillatory solutions 2 Periodic solution 2 Peripheral devices 2 Sound production 2 Sound source 2 Speech processing 2 Underlying mechanism 2 Vocal-tracts 2