The voice-box of birds is
not the larynx with its vocal cords, as in mammals, but the syrinx, a bony
structure that is unique to birds. Birds have a larynx, but it lacks vocal
chords.
In song birds, the syrinx is located at the lower end of the trachea, or
windpipe, where the two bronchi join, surrounded by an air sac.
Specialized sets
of syringeal muscles control the movement of the syrinx, including the tension
on the membranes.
How do Birds Sing Different Songs?
Just as different sounds are produced
on a guitar by vibrating strings of varying length and tension, the bird uses different degrees of tension
produced by the syringeal muscles to achieve variety in their song.
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.
A few species have no syrinx at all, such as vultures and
some storks.?You will never hear melodic singing coming from these birds!
In general, the more primitive birds have fewer syringeal
muscles. Mimics and others that produce a wide variety of bird song sounds, like the crow,
mockingbird, and starling have a greater number of syringeal muscles.
The Lyre Bird demonstrates his complex song ability:
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