How Does a Bird Sing Two Notes at Once? It Has Two Voice Boxes

Mammals produce voice at the top of the windpipe, in a structure at the throat that vibrates a pair of folds as air passes. Birds have nothing equivalent there and instead make sound at the far end of the windpipe, at the point where it divides into the two tubes leading to the lungs. Membranes at that junction vibrate as air passes them, and muscles alter their tension and position. Placing the sound source at the bottom of a long tube means the whole windpipe acts as a resonator above it, which birds exploit by changing its length and by adjusting the throat and beak as the note is produced.

Having a sound source in each branch gives possibilities a single source does not:

• •Two different notes produced simultaneously by one bird
• •One side taking the high notes and the other the low, in some species
• •A continuous song while one side breathes, alternating between them
• •Rapid switching faster than a single source could retune
• •Harmonically unrelated pitches, which sound strange to human ears
• •Considerable variation between groups in how much each side is used

The number of muscles acting on the organ varies enormously between bird groups and tracks how elaborate their vocalisations are, which is one of the clearer anatomy and behaviour relationships in the group. Pigeons and many non-passerines have very few, and produce simple calls. Songbirds have several pairs on each side, giving fine independent control of tension, airflow and the membrane position, and they are the birds that produce elaborate learned song. Parrots achieve their imitative range with a comparatively simple version of the organ combined with unusually mobile tongues and throats, which is a reminder that the sound source is only part of the system.

The organ is only the instrument, and in songbirds what is played on it is learned rather than inherited, which is a further layer of complexity. A young bird hears adult song during a sensitive period, stores a memory of it, and then practises for weeks producing a rambling subsong that gradually converges on the memorised model, which is a sequence closely comparable to how human infants acquire speech. A bird raised in isolation produces abnormal song, and one that is deafened after learning deteriorates, showing that it monitors its own output continuously. Regional dialects arise because young birds copy local adults, and those dialects persist across generations.

Investigating an organ buried deep in the chest of a small animal that must be alive and singing is genuinely difficult, and the techniques are ingenious. Tiny thermistors placed in each bronchus measure airflow on each side separately, which reveals which side is producing which part of a song. Fine wires record the electrical activity of individual muscles during singing. Breathing air enriched with helium changes the speed of sound and therefore the resonances without changing the source, which separates the two contributions. Endoscopes have been threaded down to film the membranes moving. And computed scans of preserved specimens allow the anatomy of many species to be compared without dissection.

Birds make sound where the windpipe divides rather than at the throat, so they have a source in each branch and can produce two notes at once, alternate sides to sing continuously, and switch faster than retuning allows. Muscle counts vary from very few in pigeons to several pairs per side in songbirds, tracking how elaborate the vocalisations are. Helium, airflow probes and endoscopes are how it is studied.