Take a close look at this picture. It's a composite image compiled from high-speed video of a diving male Anna's Hummingbird, and it represents a new piece of evidence in a long-standing ornithological debate.
During mating season, males of the West Coast species climb about 100 feet in the air before dropping headfirst and swooping over a female. At the bottom of the dive, the bird makes a very brief but loud chirp. (Turn up your speakers to hear it.)
Scientists have argued whether the sound is vocal or mechanical, that is, produced by feathers. In a short 1940 paper in The Condor, UC-Berkeley's Thomas Rodgers described an experiment in which he attached a plucked outer tail feather to a slender strip of bamboo. "By whipping this through the air a note was produced, which was almost identical with that produced by the bird," Rodgers wrote. "No other feathers produced this note."
In 1979, noted ornithologist Luis Baptista and colleague Margaret Matsui recorded the dive note and suggested that it was "mostly, if not entirely, vocal in origin." Three years later, F. Gary Stiles (who would go on to co-author A Guide to the Birds of Costa Rica) published a detailed description of the male Anna's courtship displays in which he disagreed with Baptista and Matsui: "The note seems too loud in relation to the size of the syrinx and airsac system of a hummingbird."
Into this field of heavyweights wade two students from, fittingly, UC-Berkeley. Ph.D. student Christopher J. Clark and recent graduate Teresa J. Feo, working under biology professor Robert Dudley, show in this week's online edition of the Proceedings of the Royal Society B that Rodgers had it right: The bird's outer tail feathers produce its signature sound.
Clark and Feo videotaped wild birds and placed tail feathers in a jet of air and in a wind tunnel. The videos showed that the males began the dive with their tail shut, then abruptly spread it for 60 milliseconds at the bottom of the dive. The chirp occurs at the same time the tail spreads. In the composite image above, the bird's tail is spread as it levels out its dive.
This silent two-second video from Clark and Feo shows an Anna's opening its tail.
The students also plucked inner and outer tail feathers to test whether birds continued to make the sound. Birds failed to chirp only when their outer tail feathers (known as R5 feathers) were fully or partially removed.
In the lab, tests in a jet of air and in a wind tunnel confirmed that the outer tail feathers vibrate like a reed in a clarinet, producing the chirp. High-speed video revealed that the tone originated in the trailing edge or vane of the R5 feather, meaning that Clark and Feo appear to have isolated the sound's true origin.
The authors note that Costa's Hummingbird produces a similar dive note and suggest that it may originate in the tail. And the characteristic may exist in other birds such as snipe.
Summing up the finding, Clark's advisor, Robert Dudley, said, "this phenomenon nicely illustrates the strength of the evolutionary process, and sexual selection, in particular, to derive novel functions from pre-existing structures." — M.M.