From National Geographic:
A time-lapse image shows a male Anna's hummingbird performing a rapid dive that is part of the birds' courtship display.
At the bottom of the dive, a new study shows, the birds flick their tails open for a split second, producing a loud chirp. The find is the first demonstration of "a new way for a bird to make a sound," researchers say.
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 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.
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.
You can read the rest here.
Here's a video of it from New Scientist: