Sound and High-Speed Video Reveal Secret of the Sage-Grouse Swish
By Leeann Louis
July 15, 2009
A student uses sound and high-speed video to discover how sage-grouse make a “swish”
Most birds use their voices to communicate, but the Greater Sage-Grouse has evolved a more complicated means of communication. Male grouse strut to court females. During the strut, a sage-grouse inflates and deflates two large air sacs on his chest, creating two sharp pops and two swishing sounds, as well as other sounds.
As an undergraduate student working with Kimberly Bostwick at the Cornell University Museum of Vertebrates, I’m fascinated by how sage-grouse produce these non-vocal sounds. Marc Dantzker, an ornithologist at the Cornell Lab of Ornithology, introduced me to this exciting species. He was working out how the two pops of the display are produced, but he was also uncertain how the swishsounds were made. So he gave me high-speed video recordings of grouse performing their display and asked me to figure out how they made the swish. Having such an open-ended question made my research both fun and challenging.
In slowing down and studying the videos, I learned that the Greater Sage-Grouse rubs his wings over the stiffened feathers of his inflated air sacs to produce the swishing sounds. But the sounds change frequency as they are produced. How is this change created? Kim and I predicted that either the properties of the feathers on the air sacs vary across the sacs, or the grouse varies the speed at which he moves his wings over the sacs during the display. To understand this, think of the sound you make when rubbing your finger over a comb. If the comb has two kinds of teeth, the frequency will change as your finger rubs the two different types. You can also change the frequency by changing the speed as you run your finger along the comb.
We’re using three approaches to figure out how the bird changes the frequency of the swish. First, we’re studying recordings from the Macaulay Library to determine the frequencies of swishes. We’re also scrutinizing a Greater Sage-Grouse specimen, measuring the lengths and widths of the feathers on its air sacs to determine whether the feathers change across the sacs. Finally, we’re studying high-speed videos of birds performing the swish to figure out if the speed at which the wings move along the air sacs changes during the swish.
Preliminary results show that the Greater Sage-Grouse’s air sac feathers may vary in size, and also that the birds may also be changing their wing speed. This suggests that birds can evolve both anatomical and behavioral adaptations to produce nonvocal sounds.
Greater Sage-Grouse have inspired a great many people, including Native Americans, naturalists, behavioral ecologists, photographers, hunters, and me. I hope their declining population recovers for their own sake and so that this fascinating bird can provide us with intriguing puzzles long into the future.
Leeann Louis is a student in Biological Engineering at Cornell. In addition to this grouse project, she has studied the morphology and mechanics of sound production in manakins with Kim Bostwick.
Originally published in the July 2009 issue of BirdScope.