You might think being able to hover in place and dip your long, stealthy beak into the sweetest nectar would be enough as a male hummingbird to attract a mate, but it turns out female Costa’s hummingbirds are a little more demanding. Scientists researching the birds have observed unusual flying patterns, in which the male birds produce an unexpected type of song: from their tails.
Unlike regular birdsong, the male Costa’s have developed a new way to sing to potential mates. Researchers at UC Riverside used an acoustic camera to record how the birds flew around, observing that whereas most male hummingbirds would dive in front of the females, the Costa’s opted to fly to the side instead.
Turns out, it’s sound not speed that they’re trying to demonstrate. By twisting their tails up vertically, by as much as 90-degrees, the birds can cause the outer tail feathers to flutter and create a “song” of their own. However there’s also a sly reason for it, that goes beyond merely music.
The careful twisting and diving also allows the birds to minimize the Doppler effect as they whoosh past their preferred mate. That’s the same effect that changes how a sound is perceived depending on whether it’s moving toward you or away from you. You’ve probably noticed, for example, how the sound of the siren from an ambulance or fire truck alters once it goes past you.
So, too, the female Costa’s hummingbirds can use that Doppler effect change to work out how fast the males are diving. With speed a key factor in deciding which male to produce offspring with, the ability to mask just how fast you’re going could certainly be useful, especially if you’re not the fastest bird in the flock.
“Recent studies in birds and other animals suggest that females prefer higher speeds during male athletic displays,” Christopher Clark, study lead and an assistant professor of biology in UCR’s College of Natural and Agricultural Sciences said. “By concealing their speed, males are not necessarily cheating, but instead have evolved this placement of trajectory out of female choice.”
Actually figuring out the speed, in fact, provided tough for the researchers; even with a wind tunnel to put the birds in for controlled observation. The fact that even their equipment struggled, Clark points out, led him to conclude that female Costa’s could similarly struggle. “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.”
There are still some questions remaining, mind. For a start there’s why Costa’s hummingbirds evolved to behave in this way in the first place, given it’s at odds with how other birds in the genus do. More specifically, the scientists still aren’t sure why only half of the tails twist during the maneuvers, though they speculate that it could be down to limitations of their anatomy when in flight.