I am fortunate in that there are a couple of hummingbirds that live near my home and I frequently get to see them hovering just outside my window, giving me a close-up view. This article looks at what we have learned about how the hummingbirds get their name.
The results reveal 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.
The team note a crucial factor is the motion of a hummingbird’s wings. While most birds only create lift on the downstroke – found by the team to be the primary sound source – hummingbirds do so on the down and upstroke as a result of their unusual wing motion, which follows a path akin to a U-shaped smile. What’s more, these strokes occur much faster for 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,” said Lentink, adding that such tones added to the timbre of the overall sound.
“It truly is the specific way that the forces fluctuate that creates the sound that 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 motion produces different sounds.
“It’s the way they generate forces that is different,” said Lentink. “And that causes why they whoosh versus hum, versus buzz, versus whine.”