Anatomy of the Vocal Organ - The Physics of Birdsong - page 39

The first sonogram shows three consecutive syllables of song of the chingolo
sparrow ( Zonotrichia capensis ) [Straneck 1990a], while the second one corre-
sponds to the utterance of the word “taxi” by a human. We notice that, in
the second diagram, the fundamental frequency of each vowel remains con-
stant at approximately 120 Hz (this being the frequency at which the vocal
folds vibrate). The main difference between the spectra of different vowels is
in the quantity and position of the harmonics, which are shown by the traces
parallel to the trace of the fundamental frequency. The change in the spectral
content, as we discussed in Chap. 2, is due to a change in the morphology
of the passive filter (i.e., the position of the tongue in the mouth, the labial
configuration, etc.). In contrast, the sonogram associated with the chingolo
vocalization shows rich dynamics in the fundamental frequency (moreover,
the harmonics are quite weak in this vocalization). The third syllable, for ex-
ample, begins at 6 kHz and then, displaying an amazing amplitude of register,
decreases to 3 kHz.
This shows that it is at the level of the sound sources that the bird gives
the song some of its main features. To achieve this, the bird requires a delicate
control system that involves the coordinated action of three sets of muscles:
those that control the syrinx, those that control the respiratory system, and
those in charge of the control of the vocal tract, even if, in principle, they
have a less important role [Wild 1993, Wild et al. 1998]. Let us analyze the
vocal organ in detail.
3.2 The Oscine Syrinx
Oscine songbirds are capable of vocal learning. This has led to a detailed
study of the sophisticated set of neural nuclei responsible for the central
control of the song. Oscines also show a high degree of complexity at the level
of the vocal organ. For this reason, we dedicate a section to the description
of the oscine syrinx.
3.2.1 The Source of Sound
A ventral section of a typical oscine syrinx is displayed in Fig. 3.1a. This
illustration is based on a particular species, the brown thrasher ( Toxostoma
rufum ) [Goller and Suthers 1996a]. In contrast to the sound-producing organ
in humans (the larynx), the syrinx in oscines is a double structure located at
the junction between the bronchi and the trachea. All along these pipes we
can observe highly modified tracheal and bronchial cartilaginous rings, some
surrounded by muscles, and membranes [Goller and Suthers 1996b].
The vocal valves in oscines involve some of the membranes found in the
syrinx. These membranes are called the medial and lateral labia (ML and
LL). The labia are membranous connective-tissue masses capable of estab-
lishing, under appropriate conditions, oscillations in a way analogous to those