Let me see you shake a tail feather!: Anna's hummingbird uses its tail feathers to produce a loud chirping sound when courting females.

Credit: Christopher Clark and Anand Varma

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SYDNEY: By selectively removing feathers, researchers have shown that a species of hummingbird produces loud chirps non-vocally by rapidly fluttering its tail.

Though ornithologists have suspected that a diverse array of birds make mechanicals sounds called 'sonations', this is the first time the mechanism has been proven.

Similar to a reed in a saxophone, the tail-feathers of male Anna's hummingbirds (Calypte anna) can be made to vibrate to produce a loud chirping sound when courting females. (Right click and download a wav file of the sound here.)

"This may explain a wide diversity of non-vocal sounds produced by birds." write Christopher Clark and Teresa Feo, zoologists at the University of California in Berkeley and authors of a study reported today in the U.K. journal the Proceedings of the Royal Society B.

Swoop and sing

During breeding season, male hummingbirds ascend 30 m into the air before diving headfirst and swooping over a female at speeds of up to 23 m/s. During this dive a series of sounds are emitted (see below for video).

In order to discover how the sound was produced, Clark and Feo elicited display dives from male hummingbirds by placing a stuffed or caged female in a conspicuous location within the male's territory.

By recording high-speed videos and audio recordings of 25 dives, they found evidence that the fluttering of the outermost tail feathers produces the sound.

The pair then experimentally trimmed the tail feathers of ten males and re-recorded the birds' display dives. They found that although they performed normal dives, the removal of certain tail feathers prevented the loud chirps.

Further tests on the feathers in a wind tunnel showed that the position of the bird's tail during a dive causes rapid airflow over the feathers, which produces the whistling sound once the male reaches a speed of over 65 km/h.

Curiously, the researchers revealed that the hummingbird can sing at the same frequency as the mechanical chirp it creates, raising questions as to why it requires it at all. However, they note, the tiny species vocal cords are too weak to produce loud sounds and the feathers can produce a much more impressive mating call.