The
organ that birds use to produce vocalizations (songs and
calls) is very different in location and structure from our
own. The mammalian larynx is located at the top of the
"windpipe" (trachea), and contains hard membranes (vocal
cords) whose vibration as air passes is controlled by a
complex of muscles and cartilage. The vocal organ of birds,
in contrast, is a unique bony structure called a syrinx,
which lies at the lower end of the trachea, is surrounded by
an air sac, and may be deep in the breast cavity. Thus
situated, the syrinx becomes a resonating chamber (the air
sac may resonate also) in conjunction with highly elastic
vibrating membranes. Specialized sets of syringeal muscles
control the movement of the syrinx, including the tension on
the membranes (which can be adjusted like the skin of a
drum). Birds can vary both the intensity (loudness) and
frequency (pitch) of sounds by altering the air pressure
passing from the lungs to the syrinx and by varying the
tension exerted by the syringeal muscles on the membranes.
The attributes of song that characterize individual species
appear to result mostly from differences in the learning
process rather than from differences in the structure of the
vocal apparatus. Neurobiologist Fernando
Nottebohm has shown that the two sides of the syrinx are
independently controlled, which explains the "two-voice"
phenomenon seen in sonograms of some species: simultaneous
double tones that are nonharmonically related and therefore
must be derived from two independent acoustic sources. Our
understanding of how the syrinx works is based on studies of
only a very few species (including the domestic chicken and
Mallard, which hardly typify birds in general), and many of
our ideas about how the passerine syrinx functions are based
on "informed guesswork." Recent work on the neural
basis of song in passerines by Nottebohm and his colleagues
not only identified the specific regions in the brain that
control song production but also demonstrated differences
between the sexes in the size of these regions. The
substantially smaller size of these areas in female Canaries
and Zebra Finches suggests an explanation for their
inability to sing. The assertion that singing ability is
dependent on the amount of brain space allocated to it is
further supported by Nottebohm's demonstration that superior
singers among male Canaries, Zebra Finches, and Marsh Wrens
have larger song control regions in their brains. In fact,
Pacific Coast Marsh Wrens, which have song repertoires that
are three times larger than Atlantic Coast birds, have 30-40
percent larger song control areas in their
brains. SEE: Vocal
Development;
Adaptations
for Flight Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.