RENEE MONTAGNE, host:
Every year, an enormous migration takes place in Western Europe. Millions of moths fly for days, riding wind currents towards South in the fall, and then the North in the spring. Scientists thought these insects were simply blown to their destinations. Now they've discovered something remarkable, the moths actually select the fastest wind currents and even change course to shorten their trip.
NPR's Christopher Joyce reports.
CHRISTOPHER JOYCE: Picture a massive cloud of moths flying in the darkness. They apparently know South from North by some internal compass tuned to the earth's magnetic field or perhaps to the position of the sun, and they can detect when the wind is blowing the way they want to go. But radar images of these migrations show that the moths aren't just blowing in the wind.
Here's entomologist Jason Chapman.
Mr. JASON CHAPMAN (Entomologist): The moths select the altitudes where the winds are fastest. And we don't know exactly how they do this. We believe that they can sense some cue related to the wind, like the strength of turbulence.
JOYCE: Turbulence, you see is an indication of wind speed, less turbulence means faster wind. That's what the moths want, a fast tailwind - really fast.
Mr. CHAPMAN: The maximum speeds we found were approaching, you know, approximately 100 kilometers an hour. They can go incredibly fast.
JOYCE: That's millions of moths flying at about 60 miles an hour.
Mr. CHAPMAN: The speeds at which the moths are regularly traveling is often faster than the birds travel speed.
JOYCE: Speed is important because wind is fickle. When it's going your way and you're a moth, you got to get when the getting's good. Chapman, who works at an agricultural institute in Great Britain, called Rothamsted Research, says the radar tracking revealed something else. The moths are like wind surfers - when the wind shifts, they adjust.
Mr. CHAPMAN: As it starts swinging around they will start trying to correct, and we find that, on average, the moths can correct for about 20 degrees of drift.
JOYCE: If the wind shifts too much, say, 90 degrees from where the moths want to go, they simply stop and wait for the wind to change. Chapman and his colleagues tracked these migrations for 10 years and published their results in the journal Science. They even ran a computer simulation that compared how dust particles and moths behaved under the same wind conditions. The moths, and one species of butterfly, clearly shifted their flight paths and speed with the wind.
Chapman says nightly moth migrations are everywhere, bats know all about them. Chapman cites reports of bats in the American Southwest suddenly swarming half a mile up into the sky.
Mr. CHAPMAN: The reason they're going up there is that there is a huge migration of moths coming in and out of Mexico into the Southern states. And the bats were taking advantage of this, so, you know, it's kind of a whole ecosystem based on this, you know, huge flow of insects through the atmosphere.
JOYCE: Insects that don't need a weatherman to know which way the wind blows.
Christopher Joyce, NPR News.
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