Acoustic generators 4 Bird species 4 Birds 4 Complex sounds 4 Delay differential equations 4 Differential equations 4 Mathematical models 4 Oscillatory solutions 4 Sound production 4 Sound source 4 Vocal-tracts Acoustic effects 2 Acoustic features 2 Acoustic interaction 2 Acoustic signature 2 Acoustics 2 Behavioral research 2 Bioacoustics 2 Biomechanics 2 Complex behavior 2 Complex solution 2 Coupled systems 2 Fundamental frequencies 2 Ideal model 2 Motor commands 2 Nonlinear effect 2 Nonlinear features 2 Parameter spaces 2 Periodic solution 2 Peripheral devices 2