Acoustic generators 6 Complex sounds Delay differential equations 6 Differential equations 6 Sound source 6 Bird species 4 Birds 4 Complex solution 4 Mathematical models 4 Motor commands 4 Oscillatory solutions 4 Parameter spaces 4 Periodic solution 4 Sound production 4 Vocal-tracts 4 Acoustic effects 2 Acoustic features 2 Acoustic interaction 2 Acoustic signature 2 Acoustics 2 Behavioral research 2 Bioacoustics 2 Biomechanics 2 Close-in 2 Complex behavior 2 Coupled systems 2 Fundamental frequencies 2 Hysteresis phenomenon 2 Ideal model 2 Limit cycle 2