Biomedical Engineering Reference
In-Depth Information
5 The Instructions for the Syrinx
The diversity of the basic acoustic elements that constitute a bird's song is
amazing. In order to create these elemental vocalizations, the brain of the
bird has to send a set of precise instructions to the vocal organ. In the case
of songbirds, the neural motor pathway involved is composed of four neural
nuclei (each with thousands of neurons), which innervates some 20 muscles
(syringeal and respiratory). Now that we have some understanding of the
mechanics of the avian vocal organ, some questions naturally arise. How
does the bird achieve the coordination of so many signals? How complex are
the instructions sent by the avian brain to the syrinx in order to create a
song?
5.1 The Structure of a Song
5.1.1 Syllables
A bird's song is one of the richest acoustic phenomena found in nature. Typ-
ically, it has a complex and varied structure, built out of a series of blocks,
each one made from the repetition of brief, continuous vocalizations that we
call syllables. In the case of the canary, for example, the syllables last between
15 and 300 ms (i.e., the bird executes several syllables per second). Each bird
has several dozen syllables, which are combined in order to create the songs in
its repertory. In Fig. 5.1, we show sonograms of two song fragments: one vo-
calized by an ashy-tailed swift (
Chaetura andrei
) [Straneck 1990b], and the
other by a greenish manakin (
Schiffornis virescens
) [Straneck 1990c]. The
songs of these two species are globally very different, yet at the level of the
syllables, they show important similarities. Although the structure of the
harmonics the timbre, the durations or frequency ranges might be different,
in both cases the sonograms show a sequence of small continuous “curves”.
These are typically repeated several times, and are separated by silences.
Each continuous curve sweeps a certain frequency range. As we can see, the
frequency typically evolves within a syllable, either upwards, or downwards
or “
n
” shaped.
