Models of Syringeal Mechanisms

Abstract

There are presently two hypotheses concerning the modulation of avian vocalizations, one holding that resonance phenomena play an important role, the other that modulations are source generated. A wide variety of data tend to question the former and support the latter. The Greenewalt model is examined in the light of recent findings, many of which support its major contentions and implications. On the other hand, the model seems to have certain weaknesses in describing the interactions of pressure, tension, and membrane movement, hence the linkage of AM and FM. Mathematical expressions of these relationships are developed. They show that the amplitude of oscillation will vary with the magnitude of the driving pressure perturbation and with the interaction of the frequencies of the perturbation and the resonant frequency of the membrane. This suggests an alternative mechanism for the AM-FM linkage. Caution is advised in the development of generalized models. Although all birds may exploit certain fundamental principles, the possibility remains that a variety of techniques may be employed. A major difference between the techniques utilized by oscines and non-oscines is especially probable.