Hummingbirds may be immediately recognizable by their nickname sounds, but the cause of the characteristic has long been a mystery.
Now researchers say they’ve solved the riddle, finally “Hmm?” From hummingbirds
David Lentinck, an assistant professor of mechanical engineering and co-author of the research at Stanford University, says that when hum was thought to be associated with wing motion, it was not clear beforehand what exactly was behind the sound. Changes in the pressure produced by the fl pping ping, steam in the air flow, and whistling sounds from the feathers were all possibilities.
Now it seems that the answer lies mainly in the aerodynamic forces, and hence the change in pressure, which moves the wings forward.
“This alone is enough to really understand what our main source is,” Lentinck said.
Writing in the journal Elif, scientists at Eindhoven University of Technology, spin-out company Sorama and Stanford University explain how they came to this conclusion after experimenting with a species known as Anna’s hummingbird.
In one setup, the team set up more than 3,000 microphones around the cage, as well as high-speed cameras, feeding six hummingbirds at a time on one artificial flower. This allowed them to create a 3D acoustic map that visually connected the sounds produced by birds with the movement of the wings.
To explore what sounds were playing, the team attempted to measure the lift and drag forces produced by fluttering wings. To do this, they created another experiment in which birds were surrounded by pressure plates, as well as high-speed cameras, and they moved around. Was kept under surveillance. He chose the size of the pressure forces he produced and how they changed over time.
When the researchers put together information about the forces associated with the movement of birds’ wings, they were able to predict sounds that would only arise from these factors. They then compared them to a 3D acoustic map generated from a microphone setup.
The results show that with the speed and direction of wing movements, aerodynamic forces are generated as the wings move, large enough to explain the humming of hummingbirds.
The team notes that the deciding factor is the speed of the hummingbird’s wings. While most birds only make an uplift on a downstroke – the team has found to be the primary sound source – hummingbirds perform down and upstrokes as a result of their unusual wing movements, following a similar path to U-shaped smiles. What’s more, for Hummingbirds, these strokes happen very quickly – about 40 times a second. As a result, the hummingbird wing movement produces sound at both 40 Hz and 80 Hz – sounds that are good in our hearing range and have been found to be dominant components of the hum of birds, the team says.
But with the greater influence of U-shaped wing motion, the variation of the forces within the stroke, the higher the frequency of these sounds.
“The great joy of the Hummingbirds’ intricate wing is that they cause even more harmonics than the two primary beans,” Lentinck said.
“It’s really the exact way that the pressure fluctuates that makes the sound we hear.”
The team applied a simple version of their theory to data on flying creatures, from mosquitoes to pigeons, to why their motion produces different sounds.
“This is the way they generate forces that are different,” Lentink said. “And that’s why they babble vs. hum vs. booze.”
