16 November 2012

Bird Talk

Welcome back. Following last week’s blog post on sounds you might hear from your home, I thought you might be ready for sounds you can’t hear. Since sounds from houses, apartments and the like can get pretty boring, you’ll understand if I switch to peacocks.

Why peacocks? Well, last June, a grad student from the University of Manitoba, in Winnipeg, Canada, presented a paper on peacocks at the annual meeting of the Animal Behavior Society. She reported that, along with the screeching we’ve come to know and love from peacocks, the birds are also communicating in infrasound, beyond human hearing. That’s a first.

A white peacock--
could be male or female.
Infrasound

I’ll back up. You may know or care less that sound is a mechanical wave that can be characterized by its frequency. Frequency can be measured in Hertz, which is how many times the wave goes up and down per second--1 Hertz is 1 cycle up and down each second. I’ll abbreviate Hertz as Hz.

Humans can typically hear sounds that have frequencies of about 20 Hz to 20,000 Hz. Whether or not you have a dog or a dog whistle or even like dogs, you probably learned that dogs hear better than humans. That’s only sort of true. Dogs can normally hear sounds of much higher frequencies than humans can hear (greater than 20,000 Hz), but they don’t do as well on the lower frequencies.

(Note: Before you go on about your super hearing dog to a cat owner, be aware that cats hear much better than dogs do at both high and low frequencies. Cats can certainly hear dog whistles. Would they come? Oh, sure.)

Getting back to infrasound, consider sound frequencies below 20 Hz. We may feel those waves, like the throbbing bass of my son’s car sound system, but we’re not generally going to hear them. Frequencies from 20 Hz down to 1 Hz are nominally in the infrasonic range.

Peacock Communication

Infrasound isn’t new, of course, but the idea that animals communicate at those frequencies made the headlines in the mid-1980s with elephants. It’s now known that many animals not only produce sounds in the infrasound but actually communicate infrasonically (e.g., whales, alligators, giraffes). And now add the first bird, peacocks.

The University of Manitoba researcher, an animal behaviorist, was motivated to record male peacock sounds by her coauthor, who viewed the peacock’s open fan-like tail (aka, “train”), with feathers curved slightly forward, as somewhat comparable to an antenna dish.

Although she found no evidence that the open tail helped the bird to perceive sounds, the recordings captured infrasonic pulses. The sounds were most commonly associated with males. After spreading his tail feathers, the male shakes them, which can produce a ripple down the sides of the array or a shudder radiating from the base.

When the researcher played the sound recordings for other peafowl, peahens perked up and peacocks were likely to screech.

Wrap Up

If you’re wondering whether your pet communicates infrasonically and what it might be saying, you’ll need special equipment to collect in the infrasound. You can analyze the recordings by playing them back at higher speeds. Didn’t we do something like that to hear the hidden messages in the Beatles records? Anyway, thanks for stopping by.

P.S.

Article on the peacock infrasound study:
http://www.sciencenews.org/view/generic/id/341606/title/Peacocks_ruffle_feathers%2C_make_a_rumble
 
Animal Behavior Society conference, held at the University of New Mexico: http://abs-hbes.unm.edu/index.html
I found the paper on the conference schedule--“Acoustic properties and signals of peacock tail displays: How low can they go?” A.R. Freeman, A.R. and J. Hare--but I could not find the abstract.

Discussion of infrasound and elephant communication:
http://www.birds.cornell.edu/brp/elephant/sections/dictionary/infrasound.html

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