Many birds produce mechanical sounds—familiar examples include the low-pitched booms of a drumming Ruffed Grouse and the twittering sounds made by the wings of American Woodcocks during their springtime display flights. These nonvocal sounds are termed “sonations,” a neologism coined by Cornell scientist Kim Bostwick to distinguish birds’ mechanical sounds from the more ubiquitous forms of song that birds make using their vocal tracts.

Birds have such astounding abilities to produce sounds vocally that it is often hard to prove that one is producing a particular sound some other way. One long-standing acoustic mystery involves the high-pitched squeak that male Anna’s Hummingbirds emit during the last part of their aerial diving display. This squeak resembles part of the vocal song these hummingbirds sing when perched, but it is much louder. Since the 1940s, ornithologists have debated whether this squeak is simply a louder version of the species’ regular vocal song or a different sound produced by some unknown mechanical structure.

The mystery of the hummingbird squeak has now been solved by two students from the University of California. Using ultra-high-speed video cameras, they first filmed and recorded several wild Anna’s Hummingbirds performing their dive display. When they looked at and listened closely to their tapes, the researchers discovered that the squeak was only made during the later part of the dive, coincident with the hummingbird suddenly spreading its tail. This finding focused their attention on the hummingbird’s tail feathers.

As you can see in the photograph, the outermost tail feathers of Anna’s Hummingbirds are shaped differently from the feathers in the interior part of the tail. By temporarily manipulating the outermost tail feathers on a series of wild hummingbirds, these researchers were able to show that the birds with modified feathers were no longer able to make the squeak sound during their dive display. In a final confirmation, they also showed that a similar tone could be generated artificially in the laboratory by blowing an intense air stream over a single outer tail feather.

How is the squeak sound produced by a hummingbird tail feather? When the bird opens its tail during a dive, the rear (trailing) part of the feather vane encounters high air turbulence, which causes it to flutter at its resonant frequency, thereby producing the squeak. This mechanical sound production is akin to the much lower tone produced by a flag fluttering extremely rapidly in a very strong wind, or (in a somewhat less technically analogous way) to the vibrations of a reed in a wind instrument.

Related species of hummingbirds produce a variety of different sounds during their display dives. It seems likely that most, if not all, of these hummingbird sonations are produced by these birds’ equally impressive variety of tail feather shapes, each tuned to its own species’ unique squeak tone.

Clark, C. J., and T. J. Feo. 2008. The Anna’s hummingbird chirps with its tail: a new mechanism of sonation in birds. Proceeding of the Royal Society of London B 275: 955-962.