Peacocks ruffle feathers, make a rumble

Male birds emit low-pitched sounds inaudible to human ears

Web edition : Monday, June 18th, 2012

ALBUQUERQUE ? Unbeknownst to humans, peacocks may be having infrasonic conversations. New recordings reveal that males showing off their feathers make deep rumbling sounds that are too low pitched for humans to hear.

Other peacocks hear it though, Angela Freeman reported June 13 at the annual meeting of the Animal Behavior Society. When she played recordings of the newly discovered sound to peafowl, females looked alert and males were likely to shriek out a (human-audible) call. Peacocks are thus the first birds known to make and perceive noises below human hearing, Freeman said.

?Really exciting,? said Roslyn Dakin of Queen?s University in Kingston, Canada, who studies the visual allure of peacock courtship. If peacocks can rumble, she suspects that other birds may be able to, too. ?I don?t think this is a weird case,? she said.

Such infrasound, or noise below 20 hertz, extends below the limit of human hearing. Biologists watched creatures such as elephants for centuries before recording technology uncovered the infrasound side of those animal conversations. But making infrasound doesn?t always mean communicating with it. Recordings have picked up infrasound from another bird, the capercaillie, but playing back the sounds to those birds has so far revealed no sign that they hear or care about their own infrasound.

Freeman, an animal behaviorist at the University of Manitoba, was inspired to make detailed recordings of male peacocks by her coauthor?s impression that their fanned-out feather display curved slightly forward like a shallow satellite dish. She found no evidence that the extended train gives any dishlike help in perceiving sounds, but her recordings did reveal throbs of sound below 20 Hz. Males were most likely to make them during two common gestures. After spreading open the glory of his train feathers, a male shakes them, which can create a ripple moving down the sides of the array, or else it can send a shudder radiating outward from the base. During both these classic moves, all a person hears is a leaflike rustling, Freeman said. But infrasound thrums can carry for meters to birds out of sight in shrubbery.

Researchers, and anybody else who has heard the piercing shrieks of peacocks, certainly recognize that the birds communicate with sound in the human-audible range. At the same conference, Dakin presented evidence that certain males fake the hoot they give during sex even when they?re alone, possibly a deception that increases the chances that a female will visit. Yet the news that there?s more to peacock communication than biologists had imagined doesn?t particularly surprise Dakin. Familiar as the bird is, she said, ?it?s been talked about to a far greater proportion than it?s been studied.?

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