Modulation in Bird Sounds

Yet these complexities are useful not only in the study of the patterns themselves, but also for interpreting the functional anatomy of the sound producing structures, the nature of the message, and the relationships among the birds involved. This paper analyzes several bird vocalizations and interprets their patterns as the product of individual and interacting oscillators and sound modifying structures; it also suggests a model for the operation of the syrinx during sound production. To determine some of the variation and complexity in bird vocalizations I made sound spectrograms and oscillograms of song phrases sampled from the records produced by Kellogg and Allen (1959, 1962). These spectrograms indicated that the most complex fundamentals were similar to electronically produced modulations. Such modulations are easily recognized by their audible buzzy quality. On the spectrograms time is indicated on the horizontal axis, frequency on the vertical one. The horizontal line on some spectrograms indicates that portion of the signal shown on the corresponding oscillogram. On oscillograms time is indicated on the horizontal axis and the instantaneous relative amplitude of the sound wave on the vertical one. Some bird vocalizations are simple, in that their fundamentals are analogous to signals produced electronically by a single sine-wave oscillator. Up-slurs and down-slurs are simple changes in frequency. Other sounds are mixtures of two such independent signals. The most complex sounds discussed in this paper are amplitude-modulated (AM) and frequency-modulated (FM) signals, particular interactions between two independent oscillations and analogous to radio broadcasting waves. Explanations of the nature of such waves may be found in the Radio amateur's kandbook (Amateur Radio Relay League, 1960). In essence, AM and FM are complex waves, the shapes of which are