Bird Voices

photograph of a male Mallard by Rohan Kamath
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.