How Birds Produce Vocal Sounds

Birds produce an extremely wide range of vocal sounds. Some birds sing extended melodious songs, others make brief identifying calls, and still others just blurt out harsh noises. Many birds can vary their sounds from loud to soft, and from high-pitched to low-pitched. How do birds produce their vocal sounds?

Like humans, most birds begin the process of making vocal sounds by exhaling air from the lungs. However, after that starting point, there are two big differences between humans and birds in vocal production. One difference is the role of the mouth. Humans rely heavily on variable mouth shapes to produce variable sounds. But in birds the mouth plays a minor role; birds often sing with their bills closed or full of food.

The other main difference is that humans and birds have dissimilar voice boxes. The human voice box, called the larynx, is located in the upper part of the windpipe, contains vocal cords, and produces sound when exhaled air activates the cords. A bird, too, has a larynx in the upper part of the windpipe, but it lacks vocal cords and serves mainly as an air-flow valve. The bird's voice box, called the syrinx, is located in the lower part of the windpipe, just above the two bronchi that lead into the lungs.

The syrinx, a boxlike organ associated with elastic vibrating membranes, functions as a resonating chamber for producing the bird's sounds. Specialized muscles, called syringeal muscles, control the syrinx. A bird can vary its sounds in two ways. It can manipulate its syringeal muscles to alter the tension and position of the membranes, thus raising or lowering the pitch (frequency of vibration) of a sound. A bird can also increase or decrease the flow of air pressure from its lungs, thus making a sound louder or softer, and sometimes affecting the pitch as well. (Resonance within the bird's windpipe, which varies in size and shape in different types of birds, also influences volume.)