BIRDS WITH THEIR VOICES
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.
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