(Related video: "First Footage of Rare Hummingbird Courtship" [April 19, 2007].)
So he and Feo caught one of the birds, plucked its two outermost tail feathers, and let it go.
Later they set out a stuffed female bird in the plucked male's territory. When the male saw the decoy, he dove—silently.
"It was astonishing when it didn't make the sound," Clark said.
Feo then attached one of the tail feathers to a long stick and found that when she spun it around fast enough, the feather began to hum.
Further tests showed that the plucked feathers sounded a tone when exposed to wind speeds of about 44 miles (70 kilometers) an hour.
But hummingbirds flying horizontally don't reach those speeds.
"They have to dive in order to get going fast enough to produce the sound," Clark said.
To find out how fast the animals were diving, the researchers filmed wild birds as they performed for stuffed decoys or live females in cages.
The biologists could predict the directions of the dives, because diving males face the sun—the better to flash their iridescent throats.
Filming from the side, the team clocked birds swooping at a little over 50 miles (80 kilometers) an hour.
"You expect the bird to be doing something with it's tail at the same time as the sound, and in fact that's what the video shows," Clark said.
The birds flick their tails open for a split second at the bottom of the dive. The researchers noticed that the flick was perfectly timed with the chirp.
Clark and Feo then plucked different feathers from hummingbirds' tails to confirm that it is the outermost pair that sings in the wind.
As a final step, the researchers held feathers in a wind tunnel while filming with a high-speed camera. The footage revealed that the trailing edge fluttered like a flag, waving about 4,000 times a second.
But why the birds started using the diving beep as part of the mating ritual is unknown.
"It's possible the birds initially spread their tails to control fast aerial maneuvers, and the sound was initially incidental," Clark said.
If females found that attractive, he said, selection may have favored males with the best moves and tails that chirped most distinctively.
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