Hummingbirds may be instantly recognizable by their eponymous sound, but the cause of the trait has long been a mystery.
Now researchers say they’ve broken the riddle and finally got the “hmm?” from hummingbirds.
David Lentink, the assistant professor of mechanical engineering at Stanford University and co-author of the study, says that while the buzz was known to be related to the movement of the wings, it was previously unclear what exactly was behind the sound. Pressure changes generated by the chatter, eddies in the airflow, and whistling noises from the springs themselves were all possible.
Now the answer seems to lie mainly in the aerodynamic forces, and thus pressure changes, that are produced when the wings move.
“This alone is enough to really understand what the main source of the buzz is,” said Lentink.
Scientists from Eindhoven University of Technology, the spin-out company Sorama and Stanford University write in the magazine elife how they came to the conclusion after experiments with a species known as Anna’s hummingbird.
In one setup, the team placed more than 2,000 microphones and high-speed cameras around a cage in which six hummingbirds fed an artificial flower one by one. This allowed them to pick up the sounds of the birds to create an acoustic 3D map that was visually linked to the movement of the wings.
To investigate what was causing the noises, the team attempted to measure the lift and drag forces produced by the flapping of the wings. To do this, they created another experiment where the birds were surrounded by pressure plates, as well as high-speed cameras, and monitored while floating. This picked up on the magnitude of the compressive forces produced and how they changed over time.
When the researchers put together the information about the forces along with the movement of the birds’ wings, they were able to predict the sounds that would be caused by these factors alone. They then compared them to the 3D acoustic map produced with the microphone array.
The results show that aerodynamic forces produced while the wings are moving, along with the speed and direction of the wing movements, are largely sufficient to explain the hummingbirds’ hum.
The team notes that a critical factor is the movement of a hummingbird’s wings. While most birds only create lift on the downstroke – considered by the team as the primary sound source – hummingbirds do so on the downstroke and upstroke due to their unusual wing movement, which follows a path that resembles a U-shaped smile. In addition, these beats occur much more quickly in hummingbirds – about 40 times per second. As a result, the team says, the hummingbird wing movement generates sounds at both 40Hz and 80Hz – sounds well within our hearing range that have been found to be the dominant components of the birds’ hum.
But variations of the forces within the beats, along with further influence of the U-shaped wing movement, generate higher frequency overtones from these sounds.
“The great thing about the hummingbirds’ complex wing beat is that those two primary pulses also produce even higher harmonics,” said Lentink, adding that such notes added to the timbre of the overall sound.
“It’s really the specific way that the forces fluctuate that creates the sound we hear,” he said.
The team applied a simplified version of their theory to data from flying creatures, from mosquitoes to birds like pigeons, to reveal why their movement produces different sounds.
“It’s the way they generate forces that is different,” says Lentink. “And that’s why they buzz versus hum, versus buzz, versus whine.”