Hummingbirds can be instantly recognized by their sound of the same name, but the cause of this characteristic has always been a mystery.
Now, the researchers say they solved the puzzle, finally taking the “hmm?” of hummingbirds.
David Lentink, the assistant professor of mechanical engineering at Stanford University and co-author of the research, says that while tinnitus was known to be related to wing movement, it was unclear what exactly was behind the sound. Pressure changes generated by the beats, vortexes in the air flow and hissing sounds from the feathers themselves were among the possibilities.
Now, it seems that the answer is predominantly in the aerodynamic forces and, therefore, in the pressure changes, produced as the wings move.
“That alone is enough to really understand what the main source of tinnitus is,” said Lentink.
Writing in the elife journal, scientists at Eindhoven University of Technology, spin-out company Sorama and Stanford University report how they came to the conclusion after carrying out experiments on a species known as Anna’s hummingbird.
In one configuration, the team organized more than 2,000 microphones, as well as high-speed cameras, around a cage in which six hummingbirds fed on an artificial flower, one at a time. This allowed them to capture the sounds produced by the birds to create a 3D acoustic map visually linked to the movement of the wings.
To explore what was driving the sounds, the team sought to measure the lift and drag produced by the flapping of 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. This took the size of the pressure forces produced and how they changed over time.
When the researchers gathered the information about the forces together with the movement of the birds’ wings, they were able to predict the sounds that would be created only by these factors. They then compared them to the 3D acoustic map produced from the microphone configuration.
The results reveal that the aerodynamic forces produced by the movement of the wings, together with the speed and direction of the movements of the wings, are sufficiently sufficient to explain the humming of hummingbirds.
The team notes that a crucial factor is the movement of a hummingbird’s wings. While most birds only create lift on the descent – considered by the team as the primary sound source – hummingbirds do so on the descent and climb as a result of their unusual wing movement, which follows a path similar to a U-shaped smile. What’s more, these strikes occur much faster for hummingbirds – about 40 times per second. As a result, the team says, the hummingbird’s wing movement generates sounds at 40 Hz and 80 Hz – sounds that are well within our hearing range and which were considered to be the dominant components of birds’ tinnitus.
But variations in forces within the blows, along with the additional influence of U-shaped wing movement, generate higher frequency overtones of these sounds.
“What’s lovely about the complex humming of the hummingbirds’ wings is that these two primary pulses also cause even higher harmonics,” said Lentink, adding that such tones increase the timbre of the overall sound.
“It is really the specific way in which the forces fluctuate that create the sound we hear,” he said.
The team applied a simplified version of their theory to data for flying creatures, from mosquitoes to birds like pigeons, to reveal why their movement makes different sounds.
“It’s the way they generate strength that is different,” said Lentink. “And it makes them whoosh versus hum, versus buzz, versus whine.”