The general principles underlying the acoustic performance of the avian voc
al tract are examined both theoretically and experimentally. The formant re
sonances produced both by the total vocal tract and by the branchial tubes
are evaluated quantitatively, and their dependence upon anatomical paramete
rs is investigated. A simplified cylindrical model for the beak is examined
theoretically, and experimental results are presented that confirm the pre
dictions of the theory for this model. A similar theoretical and experiment
al investigation using a more realistic conical beak model is also reported
, and behaves as predicted. Finally a theoretical study of the effect of mo
uth volume, as influenced by tongue position, is integrated with these othe
r studies to produce a complete analysis. The implications of these studies
for understanding the acoustical behavior of avian vocal tracts are discus
sed. (C) 1999 Acoustical Society of America. [S0001-4966(98)00912-6].