Rather than singing to their mates, Costa's hummingbird males court females with musical, high-speed dives. Their "song" is produced as the wind whistles through their tail feathers. Now, researchers reporting in the journal Current Biology on April 12 have found that the diving males twist half their tails as they whiz through the air, apparently to aim the sound in the direction of their potential mates.
Those acrobatic maneuvers make a phenomenon called the Doppler shift less audible, the new study shows. "Doppler effect" refers to a change in sound frequency as the source and observer move toward or away from each other.
"Doppler shift is the sound of an ambulance passing you by--the sudden down-shift in its tone when it goes from in front of you to behind you," says Christopher Clark at the University of California, Riverside. "Male Costa's dive in a way that minimizes how audible that sound is."
Clark and study co-author Emily Mistick used a device called an "acoustic camera" to record Costa's hummingbird dives. They also conducted experiments in a wind tunnel to examine how the birds' speed and direction influences the sounds they make.
Those videos led to interesting discoveries. First, males dive off to the side of females, in a way that minimizes the Doppler shift effect. High-speed video also showed that males aim their sound by twisting half of their tail vertically by up to 90 degrees.
Clark said he was surprised to realize how difficult it is to calculate velocity from sound. "Once I realized it wasn't trivial for a scientist to measure, I realized it wouldn't be trivial for a female to measure, either," he says.
He also finds it curious that the males spread only half of their tails for reasons that aren't entirely clear. It's possible, he says, that there may be an anatomical limitation such that they can't twist their whole tails around.
While Clark notes that they don't know exactly how these maneuvers sound to female hummingbirds, the findings suggest that males can strategically manipulate the way females perceive their displays, by diving in a way that conceals how fast they are going. Does this mean males are "cheating" to look better (or faster) than they really are? Not necessarily, the researchers say. As far as they know, all males twist their tails in essentially the same way.
"You could argue that this entire species is 'cheating,' although is it really cheating if everyone is doing it?" Clark asks.
Current Biology, Clark and Mistick: "Strategic Acoustic Control of a Hummingbird Courtship Dive" http://www.cell.com/current-biology/fulltext/S0960-9822(18)30322-1
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.