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.