Acoustic generators 8 Birds 6 Complex sounds 6 Delay differential equations 6 Differential equations 6 Sound production 6 Sound source 6 Acoustics 4 Bird species 4 Complex solution 4 Mathematical models 4 Motor commands 4 Oscillatory solutions 4 Parameter spaces 4 Periodic solution 4 Sound sources 4 Vocal-tracts 4 Acoustic effects 2 Acoustic features 2 Acoustic interaction 2 Acoustic signature 2 American Physical Society (APS) 2 Arsenic compounds 2 Behavioral research 2 Bioacoustics 2 Biocommunications 2 Biological organs 2 Biomechanics 2 Brain 2 Close-in 2