How birds sing
When a bird breathes it takes in air through its bill, draws it in past its throat and into its windpipe (or trachea). The windpipe forks to carry some air to each of the lungs. These dual passages are called bronchial tubes (singular-bronchus, plural-bronchi). Air is then processed in the lungs and exhaled back along the same route. This system of air passage has become adapted for the secondary purpose of sound production (as it has in humans too).
Illustration of the respiratory system of a songbird and its trachea, bronchial tubes, syrinx and lungs.
At the point where the windpipe divides is situated the bird's sound producing organ, a “voice box” called the syrinx. Humans have no syrinx but a larynx instead. The larynx is a cavity in the throat and contains our vocal chords. The avian syrinx is called a lower larynx, being differently situated at the other end of the windpipe. It is as if the human larynx were positioned in the chest.
The syrinx is double-barrelled - as if a man had twin instruments between his lips and was playing one with the outflow of one lung, and the other with air from his second lung. Theoretically, a bird with one collapsed lung would then still be able to produce sound. But if it were a species that is known to produce two different sounds simultaneously, then clearly it would not be able to offer a normal contribution to nature's symphony.
Each half of the bird's voice box is in the mirror-position of the other at the top of its own bronchus (so the set up is literally double-barrelled) and each consists of a pair of organs opposite each other on the wall of its bronchus. Thus if you were to slit open a bronchial tube, you would see on one side a little tympanum, a circular elastic membrane. This is the vocal chord, and opposite it would be a little bump of erectile tissue.
The stage is set
The bird is about to take its cue. It forces air along the bronchial tube, past these two organs, both of which can be adjusted. The diameter of the tympanum can be changed and the degree that it protrudes into the bronchial tube can also be varied.
As air passes, so an acoustic disturbance is set up, the tympanum vibrates and sound is created. The pitch (frequency) of that sound and the loudness (amplitude) of it can be modulated. So far as the tympanum is concerned, these two effects are usually coupled. The function of the extendable little bump is believed to be to change the loudness without having to change the pitch. Bear in mind that the bird may be playing at the same time a second tune on his other half.
The song of a skylark, recorded in the Outer Hebrides, Scotland, by Alan Burbidge.
Some birds sing continuously
Birds do not sing only when exhaling. A grasshopper warbler may “reel” for over two minutes, a nightjar may chur continuously for eight minutes, and a skylark may “pour forth its full heart” in completely unbroken song for 18 minutes. To replenish their oxygen these birds must breath in, and must do so while singing. In the case of the nightjar, the bird discernably alternates soft short trills with loud long ones and these short trills are believed to be when the bird inhales. Less continuous singers may also use air travelling in either direction. The white-rumped shama, for example, is believed so to do.