Hummingbirds may be immediately recognizable by their sound of the same name, but the cause of the feature has long been a mystery.
Now researchers say they cracked the mystery and finally the “hmm?” from hummingbirds.
David Lentink, the assistant professor of mechanical engineering at Stanford University and co-author of the research, says although it was known that the hum was linked to the movement of the wings, it was previously unclear what exactly was behind the sound. Pressure changes generated by the clapping, vortices in the airflow and whistling sounds from the springs themselves were all one of the possibilities.
It seems that the answer lies mainly in the aerodynamic forces, and thus causes pressure changes when the wings move.
“That alone is enough to really understand what the main source of the moth is,” Lentink said.
In the journal elife, scientists from Eindhoven University of Technology, the cutting company Sorama and Stanford University write about how they came to the conclusion after conducting experiments with a species known as Anna’s hummingbird.
In one setup, the team arranged more than 2,000 microphones, as well as high-speed cameras, around a cage in which six hummingbirds fed on an artificial flower one by one. This enabled them to pick up the sounds produced by the birds to create a 3D acoustic map that was visually linked to the movement of the wings.
To examine the sounds, the team tried to measure the lifting and towing force slapped by the wings. To do this, they created another experiment in which the birds were surrounded by pressure plates, as well as high-speed cameras, and monitored while hovering. It noticed the magnitude of the compressive forces produced and how it changed over time.
When the researchers compiled the information about the forces along with the movement of the birds’ wings, they were able to predict the sounds that would arise only from these factors. They then compare it to the 3D acoustic map produced by the microphone setup.
The results show that aerodynamic forces produced while the wings are moving, coupled with the speed and direction of the wing movements, are largely sufficient to explain the hummingbirds’ hum.
The team notes that an important factor is the movement of a hummingbird’s wings. While most birds only create an elevator on the descent – which is found by the team as the primary sound source – hummingbirds do so on the descent and ascent due to their unusual wing movement, which follows a path like a U- shaped smile. What’s more, these strokes occur much faster in hummingbirds – about 40 times per second. As a result, according to the team, the hummingbird wing movement generates sounds at both 40Hz and 80Hz – sounds that are very much within our hearing range and which are the dominant components of the birds’ humming.
But variations of the forces within the strokes, together with the further influence of the U-shaped wing motion, generate a higher frequency of these sounds.
“The lovely thing about the hummingbirds’ intricate flying beat is that these two primary pulses also cause even higher harmonics,” Lentink said, adding that such tones contribute to the sound of the overall sound.
“It’s really the specific way the forces vary, that creates the sound we hear,” he said.
The team applied a simplified version of their theory to data for flying creatures from mosquitoes to birds such as pigeons to explain why their movement produces different sounds.
“It’s the way they generate forces, which is different,” Lentink said. “And that’s why they whine about humming, about buzzing, about whining.”