Jan. 30, 2008 -- Given their tiny anatomy, very small birds would only seem to produce quiet sounds, but a notable exception is the male Anna's hummingbird, which researchers have just determined produces a loud chirp with its unusually shaped tail feathers.
The noises fill the western U.S. skies during the mating season when males of this common species dive from heights of 100 feet or more and then spread their tapered, narrow-tipped outer tail feathers at the swoop's nadir. In that instant, the feathers act like a clarinet reed, with the wind "playing" the single note.
"The sound that the males make at the bottom of their display dive is brief and loud with a frequency of around 4 kilohertz, which is roughly four octaves above middle C, or the highest key on a piano, and has been described as a beep, chirp or whistle by various people," co-author Teresa Feo told Discovery News.
Feo, a researcher at the University of California at Berkeley's Museum of Vertebrate Zoology, and colleague Christopher Clark further describe the sound, and their findings, in a paper published in this week's online version of the Proceedings of the Royal Society B.
The study represents the first time that this feather-vibrating sound mechanism in birds has been documented. Other birds, however, such as sparrow-sized manakins, are known to snap or clap their wings or to rub their feathers together to produce noise.
Feo and Clark first made the tail feather/sound connection after they recorded, over the course of two spring mating seasons, the hummingbird's dive-bomber displays at a San Francisco Bay shoreline park.
Using a high-speed video camera, they captured the males as the colorful birds tried to impress a caged female or a stuffed female Anna's hummingbird, which the scientists stuck to a bush. Still images from the video established that the male's chirp coincided with a 60-millisecond tail flare of the male's relatively stiff, barbed tail feathers.
The researchers next removed the outer tail feathers from several captured males. These feathers normally grow back in 5 weeks. They transported the feathers to a wind tunnel at Stanford University's Hopkins Marine Station. When the wind blew at the same speed that the diving birds traveled-approximately 50 miles per hour-the tail feathers fluttered at the high C note frequency.
Feo, a clarinetist in UC Berkeley's marching band, said, "You can essentially play the feathers like an instrument."
She and Clark suspect that sexual selection drove either, or both, the evolution of the male Anna's hummingbird tail feathers and the sound.
It's not clear, though, which came first: the female's desire for the feathers or their attraction to the beep.
The researchers believe that the world's smallest hummingbirds, like the ruby-throated and black-chinned hummers, might also be tail feather chirpers. In the future, they hope to study these, and other bird species to see if they too might chirp from their back ends.
Jimmy McGuire, an assistant professor in the Department of Integrative Biology at UC Berkeley, who did not work on the latest research, told Discovery News that the data is "overwhelming" and that he is now "100 percent convinced that the sound is all about the tail."
Douglas Altshuler, an assistant professor of biology at UC Riverside, echoed that sentiment.
"The research is flawless and the results are novel," Altshuler said, adding that he predicts the findings "will soon be incorporated into textbooks in ornithology, animal behavior and biomechanics."
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