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Hummingbirds Chirp With Tail Feathers When Diving

Susan Brown
for National Geographic News
February 1, 2008
 
Loud chirps during male hummingbirds' aerial displays come from the birds' tails, not their throats, a new study says.

Anna's hummingbirds make rapid, swooping dives during courtship rituals that are punctuated by high-frequency squeaks.

The sounds—roughly equivalent to four octaves above middle C on a piano—are created when the birds fan their tails as they pull out of fast dives, the study authors say.

Instead of acting like a whistle, which creates sound when air moves through a constriction, the birds' feathers vibrate like the reed in a clarinet.

"This is a new way for a bird to make a sound," said study co-author Christopher Clark of the Museum of Vertebrate Zoology in Berkeley, California.

Clark and colleague Teresa Feo report their findings this week in the journal Proceedings of the Royal Society B.

Richard Prum of Yale University has studied tropical birds called manakins, which strum their wings together to attract mates.

Other experts have proposed this type of resonance before as a way feathers might make sounds, Prum said. But Clark and Feo are the first to demonstrate it.

The pair, Prum said, has "done an elegant job of testing the idea."

Plucked Male

The Anna's hummingbird is fairly common in urban areas of the U.S. West Coast (see map), and the animals frequently perform their diving displays at the University of California, Berkeley, campus.

Clark was studying how hummingbirds control aerial maneuvers with their tails when he noticed that Anna's hummingbirds had unusually shaped tail feathers.

(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.

Fast Dive

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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