Why does the hummingbird hum?

Hummingbirds might be instantly recognizable from the sound that gives them their name, but the cause of this characteristic has long been a mystery. Now, researchers say they have finally taken the “hmm” out of hummingbirds.

David Lentink, assistant professor of mechanical engineering at Stanford University in California and a co-author of the research, says that while the hum was known to be connected to the movement of the wings, it was unclear what exactly was behind the sound. Pressure changes generated by the flapping, vortices in the air flow and whistling sounds from the feathers themselves were all among the possibilities.

Aerodynamic forces

Now, it seems the answer mainly lies in the aerodynamic forces, and therefore pressure changes, produced as the wings move. “This alone is enough to really understand what the main source of the hum is,” says Lentink.

Writing in the journal eLife, scientists from Eindhoven University of Technology, the spin-off company Sorama and Stanford University report how they came to the conclusion after carrying out experiments with a species known as Anna’s hummingbird.

In one set-up, the team arranged more than 2,000 microphones and high-speed cameras around a cage in which six hummingbirds fed on an artificial flower, one at a time. This allowed them to pick up the sounds produced by the birds to create a 3D acoustic map linked visually with the movement of the wings.

To explore what was driving the sounds, the team sought to measure the lift and drag forces produced by the flapping of the wings. To do this, they created another experiment in which the birds were surrounded by pressure plates and high-speed cameras, and monitored them as they hovered. This picked up the size of the pressure forces produced and how they changed over time.

When the researchers combined the information about the forces with the information about the motion of the birds’ wings, they were able to predict the sounds that would be created from these factors alone. They then compared them with the 3D acoustic map produced from the microphone set-up.

The results show that aerodynamic forces produced as the wings move, together with the speed and direction of the wing movements, are largely enough to explain the hummingbirds’ hum.

Wing motion

The team determined that the motion of a hummingbird’s wings is an important factor. While most birds create lift only on the downstroke — found by the team to be the primary sound source — hummingbirds do so both on the downstroke and the upstroke as a result of their unusual wing motion, which follows a path like a U-shaped smile. These strokes are also much faster among hummingbirds — about 40 times a second.

As a result, the team say, the hummingbird wing movement generates sounds at both 40Hz and 80Hz — sounds that are well within our hearing range and which were found to be the dominant components of the birds’ hum. But variations of the forces within the strokes, together with further influence of the U-shaped wing motion, generate higher frequency overtones of these sounds.

“The lovely thing about the hummingbirds’ complex wingstroke is that those two primary pulses also cause even higher harmonics,” says Lentink, adding that such tones contributed to the timbre of the overall sound. “It truly is the specific way that the forces fluctuate that creates the sound that we hear.”

The team applied a simplified version of their theory to data for flying creatures ranging from mosquitoes to birds such as pigeons to show why their motion produces different sounds. “It’s the way they generate forces that is different,” says Lentink. “And that is the reason why they whoosh rather than hum, buzz or whine.”

© Guardian News & Media 2021