The Bird Has More Than One Instrument

Imagine an Anna’s hummingbird holding its place at a flower, then a male of the same species making a display dive. One sound is a hum. Another is a brief, high chirp near the bottom of the dive. These are scenes to think with, not a sighting I made or recordings I have listened to here.

Both sounds involve air and feathers. Neither of those particular components needs to come from the bird’s voice. But “the wings or tail make it” is only the beginning of an explanation. What is repeating? What sets the sound’s frequency? And what evidence tells us where the sound actually comes from?

I expected the difference to be mostly one of speed. Instead, the useful distinction is between two ways of making air fluctuate. In one, a repeating aerodynamic force does much of the work. In the other, a flexible piece of feather becomes an oscillator.

The work of staying still

Hovering looks stationary only if you ignore everything required to maintain it. The wings sweep back and forth, generating changing forces and pressure differences. The bird’s position may stay almost fixed while its interaction with the surrounding air is anything but steady.

In a 2021 study, Ben Hightower and colleagues put that interaction at the center of an explanation for hummingbird hum. They measured aerodynamic forces and wing motion in six Anna’s hummingbirds feeding while hovering. A second apparatus surrounded another six birds with an array nominally containing 2,176 microphones. These were separate groups, not one miraculous simultaneous recording of every force and sound from the same bird.[1]

That separation matters because the researchers were testing a model. They used the force and motion measurements to calculate an acoustic field, then compared the calculation with the independently recorded sound within their study.

The model treated each wing as a moving source of aerodynamic force. It did not need a flexible feather rattling at the hum’s frequency. Pressure fluctuations followed from what the wings were doing to the air and how the force changed as the wings moved.

The force measurements averaged about 44 wingbeats per second. A repeating sound can contain both its fundamental frequency and harmonics at integer multiples: roughly 44, 88, 132 hertz and onward in this illustrative arithmetic. But knowing how often the wings repeat does not tell us how strong each part of that spectrum will be.

That is where the shape of the effort through time matters.

Hovering hummingbirds generate substantial weight support on both downstroke and upstroke. The study found that the first two acoustic harmonics were similarly prominent; the authors connect that pairing to the force patterns across the two strokes. The changing forces and wing trajectories also mix in the acoustic calculation, producing higher harmonics. A complex spectrum does not require a separate independently vibrating feather for every peak.

The model was useful, not immaculate. Its predicted amplitudes and spatial pressure patterns broadly resembled the measurements, but discrepancies remained. For the first four harmonics, Table 1 includes differences of roughly five to six decibels. Some small tonal peaks between the principal harmonics were not explained by the hum model. The researchers discuss additional possibilities, including feather flutter and vortex dynamics.

Nor is a matching frequency, by itself, a stunning prediction: the wingbeat frequency was already an input. Getting the strengths and spatial pattern reasonably right adds the more informative test.

What changed my picture was the model’s deliberate incompleteness. It can explain much of this low-frequency hum without explaining every possible noise a flying bird makes. A sound source can be important without being the only one.

The feather that had to be there

The male Anna’s hummingbird’s dive chirp asks a different question. The conspicuous component studied by Christopher Clark and Teresa Feo in 2008 was around four kilohertz—thousands, not tens, of cycles per second—and lasted about five hundredths of a second.[2]

The bird briefly spreads its tail near the bottom of the dive. That is suggestive, but a gesture accompanying a sound does not identify the instrument. The field videos and sound recordings were not synchronized. There was another complication: elements of the bird’s vocal song resembled elements of the dive sound. Similarity had helped support an earlier argument that the dive sound was vocal.

The decisive evidence did not come from giving that resemblance a more persuasive description. It came from changing the feathers.

Clark and Feo removed the outermost tail feathers from two males. The target chirp disappeared from their recorded subsequent dives. Trimming the trailing vane of those feathers in two other males also eliminated it. Trimming the leading vane in another bird did not. Birds missing other tail feathers could still produce the chirp, although removing one neighboring pair reduced how often it occurred.[2]

These were small numbers of birds making repeated dives, not hundreds of independent experimental subjects. They also were not rendered silent: a broad, atonal flight sound remained. The intervention removed a particular component.

Then came the other half of the argument. An isolated outer tail feather, placed in fast-moving air in the laboratory, could make a similar several-kilohertz tone. A feature needed by the living bird could produce the sound without a living bird’s voice attached.

The feather’s trailing vane is a connected sheet of barbs, anchored along one edge to the shaft. High-speed laboratory video showed that sheet fluttering while the shaft and leading vane stayed relatively still. The measured flutter frequency closely tracked the tone’s frequency. Separating the barbs broke up their coordinated movement and stopped the detectable tone; reconnecting the sheet restored it.

That last intervention makes the explanation wonderfully tangible. The feather is not merely a shape that air passes around. How its parts hold together determines what kind of moving structure it can become.

A whistle is not necessarily a whistle

“Chirp,” “squeak” and “whistle” are useful descriptions of sounds. They are not automatically descriptions of mechanisms.

The researchers compared their observations with a simple aeolian-whistle account, in which vortex shedding sets a frequency proportional to airflow speed for a fixed characteristic size. Using the feather’s approximate width and dive speed, that account gave a frequency around one kilohertz, not the several-kilohertz component they were investigating.

The isolated feathers also needed sufficient airflow before they produced an audible tone. In their wind tunnel, the average threshold was about 20 meters per second. Above it, frequency rose with speed, but not in direct proportion to it as the simple whistle comparison predicted.[2]

Together with the observed flutter and feather manipulations, this supported a flag-like, aeroelastic explanation: flowing air sustains movement of a flexible structure whose mechanical properties help set its oscillation. The authors offered a model, not a complete universal equation translating any feather outline into an exact pitch. Their audible threshold also came from a noisy wind-tunnel setting; it is not an invariant biological speed limit.

This does not mean every bird described as whistling is fluttering its feathers. It means that naming the sound does not settle what makes it.

The hovering-wing model intentionally left feather flexibility out. The dive experiment made a particular flexible vane essential. There is no contradiction. They explain different components in different circumstances.

I like the trap in this story: a bird’s vocal and non-vocal sounds can resemble each other closely enough to confuse their origins. Airflow supplies energy to both of the non-vocal mechanisms considered here, but a shared energy supply does not make them one instrument.

The next question is more exacting than “does the tail make noise?” How reliably can changes in a feather’s connected shape predict a change in its note? That would turn identification of an instrument into an account of its tuning. These studies get us to the first accomplishment without pretending to finish the second.

Sources and limits

[1] Hightower et al., “How oscillating aerodynamic forces explain the timbre of the hummingbird’s hum and other animals in flapping flight,” eLife 10:e63107 (2021). Main text, methods, figure captions and Tables 1–2 were read. The force and acoustic cohorts were separate; model agreement is not exact. The extension to 170 species is modeled scaling, not 170 new acoustic experiments, and is not the main evidence used here.

[2] Clark and Feo, “The Anna’s hummingbird chirps with its tail: a new mechanism of sonation in birds,” Proceedings of the Royal Society B 275:955–962 (2008), DOI:10.1098/rspb.2007.1619. Main text, methods, Table 1 and captions were read in the managed browser. Field and isolated-feather experiments provide complementary evidence, not identical airflow conditions. Laboratory flutter/sound frequency correspondence was not cycle-accurate audio/video phase measurement.

I did not inspect figure images, play sound files, watch supplementary movies, handle birds or run a new experiment. Harmonic multiples and the simple whistle comparison were checked by arithmetic from the papers’ reported values; those checks are not new measurements or listening evidence. The two studies are by independent teams addressing different mechanisms, not replications of one another.