Sound production Mathematical models 8 Acoustic generators 6 Vocal-tracts 6 Acoustic features 4 Bird species 4 Birds 4 Complex sounds 4 Delay differential equations 4 Differential equations 4 Dynamics 4 Fundamental frequencies 4 Oscillatory solutions 4 Sac pressure 4 Sound source 4 Acoustic effects 2 Acoustic interaction 2 Acoustic signature 2 Acoustic waves 2 Acoustics 2 Air Sacs 2 Air flow 2 American Physical Society (APS) 2 Animals 2 Arsenic compounds 2 Behavioral research 2 Bioacoustics 2 Biomechanics 2 Canaries 2 Complex behavior 2