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But how do birds produce such a complex variety of notes? How do they sing non-stop for minutes on end without pausing to catch their breath?The vocal skill of birds derive from the unusual structure of their powerful vocal equipment. The syrinx is the sound-producing organ in birds. It is the equivalent of the human sound box. The syrinx contains membranes which vibrate and generate sound waves when air from the lungs is passed over them. The muscles of the syrinx control the details of song production; birds with more elaborate system of vocal muscles produce more complex songs.But unlike our soundbox, which is situated at the top of the trachea, the bird's syrinx is set much lower down, at the junction of the two bronchi or air tubes leading to the lungs.This means that the syrinx has two potential sound sources, one in each bronchus. The separate membranes on each bronchus produce separate sounds, which are then mixed when fed into the higher vocal tract. This complex design means that birds can produce a far greater variety of sounds than humans can.Birds give the impression of singing in long bursts for minutes on end without catching their breath. But they actually do this by taking a series of shallow mini-breaths, which are synchronized with each syllable they sing.
The complexity and dependence on learning of many bird sounds have suggested parallels between birdsong and human speech1–4, but the mechanisms by which each is produced have been supposed to differ markedly. In human speech, resonances of the vocal tract are thought to modulate in complex ways the sound produced by vibration of the vocal folds5–7. The current theory of birdsong production holds that all variation in sound quality arises from the primary sound-producing organ, the syrinx, and that resonances of the vocal tract play no part8,9. Here I present evidence, obtained from acoustic analyses of birdsongs recorded in a helium atmosphere, which contradicts this hypothesis. Not only does the songbird's vocal tract act as an acoustic filter, but its filter characteristics are actively coordinated with the output of the syrinx. Songbird and human phonation are thus more analogous than previously thought, in that both require coordination of an array of diverse motor systems.
X-ray movies of singing cardinals (Cardinalis cardinalis) reveal that songbirds adjust their song’s tonal qualities by actively changing the shape of their upper vocal tract. Humans control their speech through movements in their upper vocal tract, but less is known about how birds control their song. Current hypotheses consider the songbird’s vocal tract as a rigid tube that can alter the song’s tonality only if the bird opens or closes its beak, in much the same way that a flute’s sound can change if a user manipulates fingerholes along the instrument’s length. Tobias Riede and colleagues used x-ray cinematography of male northern cardinals as they sang notes common in cardinal songs. The movies showed that the birds expand their upper vocal tract in a cyclical manner. These movements change the shape of the pharynx and esophagus so that the resonance of those cavities is “tuned” to the song’s lowest harmonic, making this vocal system highly efficient.“Songbirds, who learn their vocal patterns just as humans do, demonstrate here yet another similarity to humans—that is, in using precise motor control to create complex patterns in the upper vocal tract,” says Riede.