It’s a Bird - itsa Plain
Musicians spend good money to add the right harmonic overtones to their instruments' sounds. Birds go the other direction -- filtering them out to yield completely pure tones. Now we know how they do it.

It's the overtones that make an instrument or voice sound rich, and a distinct set of overtones helps define an instrumentalist's or vocalist's unique sound. Professional recording engineers use an array of electronics to add harmonic overtones, enriching the sound and adding presence.

Birds want nothing to do with that. Their songs are pure, unadorned with harmonics, and scientists have long wondered how they create such fundamental, plain tones.

According to a recently published paper in the Proceedings of the National Academy of Sciences, it turns out to be a neat trick. One theory had been that birds' syrinxes (like our human larynxes or voice boxes) just created pure tones; now this has been shown to be incorrect.

The truth is more complicated. Scientists measuring the sound quality at various positions inside a bird (tiny, surgically-implanted acoustic sensors, i.e. what we might think of as microphones) have discovered that the syrinx works like a larynx, that is, the sounds its creates contain overtones.

The neat trick is that birds have a second stage of processing -- probably using the trachea and the air sac surrounding the syrinx -- that filters OUT the overtones. So the overtones are actually there at first, but the bird's body is precisely tuned to throw them away.

Precisely how and where (and why!) this happens is the subject of further research.

Browse the Proceedings of the National Academy of Sciences

Here's the abstract of this particular article:

Pure-tone birdsong by resonance filtering of harmonic overtones
Gabriël J. L. Beckers * , Roderick A. Suthers and Carel ten Cate *
*Behavioural Biology, Inst. of Evolutionary & Ecological Sciences
Leiden University, P.O. Box 9516
2300 RA Leiden, The Netherlands;
and
School of Medicine, Dept. of Biology and Program for Neuroscience,
Jordan Hall, Indiana University, Bloomington, IN 47405

Pure-tone song is a common and widespread phenomenon in birds. The mechanistic origin of this type of phonation has been the subject of long-standing discussion. Currently, there are three hypotheses. (i) A vibrating valve in the avian vocal organ, the syrinx, generates a multifrequency harmonic source sound, which is filtered to a pure tone by a vocal tract filter ("source-filter" model, analogous to human speech production). (ii) Vocal tract resonances couple with a vibrating valve source, suppressing the normal production of harmonic overtones at this source ("soprano" model, analogous to human soprano singing). (iii) Pure-tone sound is produced as such by a sound-generating mechanism that is fundamentally different from a vibrating valve.

Here we present direct evidence of a source-filter mechanism in the production of pure-tone birdsong. Using tracheal thermistors and air sac pressure cannulae, we recorded sound signals close to the syringeal sound source during spontaneous, pure-tone vocalizations of two species of turtledove. The results show that pure-tone dove vocalizations originate through filtering of a multifrequency harmonic sound source.