How hummingbirds chirp:
It's all in the tail
Friday, February 8, 2008
Christopher Clark went to Strawberry Canyon in Berkeley and got a bad case of poison oak. Then he tried a shoreline park in Albany, where his camera was stolen and sopping-wet dogs covered his field notes with muddy paw prints.
Those were a few of the hurdles that Clark and colleague Teresa Feo overcame to produce a paper, just published in a prestigious British journal, exploring the physics of how birds make sound.
The title of their UC Berkeley study sums it up: "The Anna's hummingbird chirps with its tail: a new mechanism of sonation in birds."
Clark and Feo filmed the birds' plunges and recorded the sound they made at the end of their roughly 50 mph descent from a height of 100 feet or more. High-speed video, at 500 frames per second, showed that the birds started their dives with their tails shut and suddenly spread them at the bottom, for one-twentieth of a second - quicker than a blinking eye.
"Now we have a greater understanding of what's actually going on in really sophisticated behavior by one of our residents," said Robert Dudley, a professor of integrative biology at UC Berkeley. "It's a pretty amazing sort of experiment. It took a lot of initiative, and they put in a huge amount of field time."
Clark and Feo concluded that the squeaks and beeps made by the dive-bombing birds are not vocal - as some research has asserted - but instead are created by their tail feathers.
"I found it really interesting just because these birds were basically doing mechanical sounds," said Feo, 22, who played clarinet in the Cal Band for four years. "It sort of speaks to the musician in me."
Clark, who is finishing his Ph.D. in the department of integrative biology, began the project more than three years ago. He eventually acquired a collaborator in Feo, who graduated in May and is working at Cal's Museum of Vertebrate Zoology. They had to obtain an array of permits from agencies ranging from the state Department of Fish & Game to the university's Animal Care and Use Committee, which ensures that experiments are done ethically.
In the course of the research, published in the Proceedings of the Royal Society B, Clark and Feo encountered many visitors to the Albany Bulb, a former dump that is part of Eastshore State Park.
"We had a lot of people ask what we were doing," said the 28-year-old Clark, who also met up with any number of curious canines.
Before acquiring a car, he and Feo would take a bus to the Albany Bulb, hauling a duffel bag that held a stuffed hummingbird mounted on a stick and a cage they had made from netting and tent poles.
During the November-to-May breeding season, the Cal students devoted up to four hours at a time, two or three days a week, on the male Anna's hummingbird - a magenta-splashed creature that looks like something you'd see in New Orleans during Mardi Gras or in San Francisco's Castro district on Halloween.
Clark and Feo, aided over time by a dozen research assistants, lured the birds into traps, banded them and plucked or trimmed one of their tail feathers, which grow back after about five weeks and are not needed for flight. Then they captured their dives with audio and video equipment.
"It's a great example of aerial acrobatics," said Dudley, who is Clark's graduate adviser. "And what's really interesting is not only the mechanism of the chirp but also the timing. Everything is so beautifully synchronized. The males are using it to advertise to females."
He said it's an occasion where sound, color and movement come together.
"Everyone in the Bay Area can see this in our natural areas and parks," Dudley said. "We see this on campus, remarkably enough."
Clark said that people have known birds make sounds with their feathers since before the time of Charles Darwin, who wrote about it in his 1871 book, "The Descent of Man." However, the physics of how non-vocal sounds are created hasn't received much attention until recently, he said.
"The first year, I got basically no data," Clark said. "I was figuring out how to do it."
After he contracted poison oak in the thick brush of Strawberry Canyon, he relocated to the Albany Bulb because the trees and bushes are short, the birds perch at eye level, and they stand out against the blue sky, making them easier to photograph.
"There were other issues with the Bulb," said Clark, recalling how he fell flat on his face chasing someone who had purloined his camera. It was all recorded.
"You can hear the crash of me hitting the bush," said the researcher, who succeeded in catching the thief.
Clark and Feo employed dead birds from the Lindsay Museum in Walnut Creek and live caged birds to lure the male Anna's hummingbird. The work was slowed by wind, rain and avian confusion.
"A big part of the project was sitting and waiting," Clark said. "There were short moments when it was exciting."
The researchers relied on several cameras, including a $50,000 model. They produced sounds from the feathers they'd collected by placing them in front of a jet of air or inside a wind tunnel. They decided, after painstakingly analyzing the male bird's tail feathers - 10 total, five on each side - that the chirping sound comes from the fluttering of part of the inside edge of the outer feathers.
Clark, a bird-watcher since high school, said he likes to work with hummingbirds because they are common and easy to catch.
"If I didn't study birds, I'd probably study something else that flies," he said. "Flight fascinates me. I'm jealous - I wish I could fly."
Local ornithologists and bird-watchers have long debated the source of the sound made by the diving male Anna's hummingbird.
In the 1940s, a UC Berkeley graduate student stated in a published paper that he could produce sound by attaching one of the creature's feathers to a strip of bamboo and whipping it through the air. However, a 1979 paper by the curator of birds at the California Academy of Sciences declared that the sound was vocal. Almost 30 years later, Clark and Feo have countered that argument.
"They're fascinating little creatures," Clark said. "And even though the landscape at the Albany Bulb seems kind of devastated, there's interesting research to do in your own backyard."