Modulation in Bird Sounds

Abstract

The 1st part of this paper shows the kinds of variation in the 2 forms of modulation in the song phrases of birds. Portions of these phrases are used to demonstrate the variation in the parameters of amplitude modulation and frequency modulation. Evidence supporting the dual-oscillator hypothesis, i.e. that oscine birds have 2 independent oscillator systems that may be operated independently or simultaneously, is presented. Three possible explanations for the presence of amplitude changes in FM [frequency-modulated] signals are presented. The 2nd part hypothesizes how certain details of sound structure may be related to specific structures in the vocal tract, particularly the internal and external tympaniform membranes, the internal and external labia, the pessulus and semilunar membrane, and certain intrinsic muscles. In passerines, sound and syringeal anatomy have been utilized in behavioral, anatomical, and systematic studies. The usefulness and applicability of these 2 can only be strengthened by establishing their interrelationship more precisely.