Growing up in Sri Lanka, it was an article of faith with my mother (and the mothers of many of my friends) that after a shower, we should not go outside while our hair was still wet because that increased the risk of us catching a cold or getting a chill. If we happened to get caught in the rain, we were told to quickly dry ourselves thoroughly so that we did not catch a cold. But Sri Lanka is a tropical country where what is considered ‘cold’ would be like a balmy summer day in the US. Furthermore houses have very open architecture with open windows and doors so that air is freely circulating and hence there is little difference between ‘inside’ and ‘outside’ except that inside one had a roof over one’s head. Hence these restrictions made no sense. But given the power of confirmation bias, it was always easy to find such a cause to blame whenever one caught a cold.
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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.”

In the February 15/22, 2021 issue of The New Yorker there is a photo essay by Brendan George Ko of the annual migration of monarch butterflies, with accompanying text by Carolyn Kormann.

The butterflies have never seen the forest before, but they know—perhaps through an inner compass—that this is where they belong. an inner compass—that this is where they belong. They leave Canada and the northeastern United States in late summer and fly for two months, as far as three thousand miles south and west across the continent. The migration is accomplished in a single generation that lives eight months, whereas the return journey north will occur over some four generations, each living four to five weeks. This is the most evolutionarily advanced migration of any known butterfly, perhaps of any known insect.

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