Demystifying a Hummingbird's Chirp

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UC Berkeley researchers have solved the mystery of a local hummingbird's loud chirp, revealing that the sound comes from the birds' tail feathers and not from the birds' vocal apparatus.

The Anna's hummingbird, the most common species in the Bay Area, produces the chirping sound by spreading its tail feathers during an elaborate dive thought to be a part of the birds' mating ritual, according to researchers.

The late California Academy of Sciences ornithologist Luis F. Baptista wrongly concluded in 1979 that the chirp was probably a vocal manifestation.

The case was reopened last year by UC Berkeley integrative biology graduate student Christopher Clark, who was studying the biomechanics of flight on hummingbirds at the Albany Bulb in the Albany Waterfront District.

Clark noticed that when he removed the outermost tail feathers of male hummingbirds, a process that does not inhibit flight and mimics the natural molting of feathers, the birds would no longer chirp while diving.

He added that only male hummingbirds dive, and do so in order to impress potential mates, he said.

"When it dove, it couldn't make the sound," Clark said. "This was the first line of evidence."

Clark then used high-speed cameras to capture the entire diving ritual of a group of birds who still had their tail feathers intact.

He noticed that at the bottom of the dive when the birds usually chirp, the birds spread their feathers for the same length of time as the sound can be heard, further suggesting that the sound was produced by the birds' tail feathers.

With the help of recent UC Berkeley graduate Teresa Feo, Clark set out to reproduce the sound in the lab with extracted tail feathers.

Feo was able to make the sound when she put the tail feathers into a wind tunnel, he noted.

"We set up a jet of air and got them to make a sound," he said.

After examining the feathers, which normally have a central shaft and barbs on either side, the researchers determined the sound was dependent on the stiffness of the barbs on one side.

"We also tested the hypothesis that (the chirp) is a whistle. In a whistle, the faster you blow, the higher the frequency," he said.

But when they varied the velocity of the jet stream, the frequency of the sound did not change. Instead, the feathers produce the chirping sound in a way similar to the reed of a clarinet, he said.

While other birds have been known to chirp with their tail feathers, this is the first in-depth examination of the process behind the sound, Clark said.

Clark and Feo are now developing a mathematical model of the tail feather chirp, which they believe may be a common method of sound production in birds.

"What we're trying to do is develop an idea of how birds evolved to make sounds with their feathers," Clark said.


Contact Tim Dunn at [email protected]

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