Imagine if you could make music with your ponytail or sing using just your beard. It sounds absurd, but some birds perform a feat not all that different. They make songs appear out of feathers and thin air.
The microscopic physics of how exactly feather sound works is a still bit of a mystery, says Richard Prum, an evolutionary ornithologist at Yale University, but scientists know that when air hits certain feathers—at the right speed and angle—it causes them to vibrate. And this rapid oscillation produces sound.
Prum and his colleagues described wing-singing, or aeroelastic flutter, in two species of broadbill, in a paper published last week in the Journal of Experimental Biology. The broadbills are small, mostly unremarkable birds native to East Africa. But the sound the birds make during their mating displays are difficult to ignore. The researchers compare it to the “aroogah!” of a mechanical horn and note that the noise can travel more than 100 yards through dense forest.
The broadbills’ mating flight sound requires what Prum calls a “stylized wing beat” with an “energetic down stroke.” In other words, when the birds flap their wings in everyday flight, it is mostly silent. The feathers only produce the characteristic “brreeeet” when the birds want to be heard.
Prum says broadbills are also unique in that no single feather seems to be crucial to the sound. Instead, different parts of six feathers vibrate in concert, and the birds can still produce sound even if one is taken away. (To figure that out, Prum and his colleagues brought a wing specimen back to the laboratory and manipulated it under various wind tunnel conditions.)
The idea that birds can make music with their wings may seem exotic, but it actually isn’t all that new. Charles Darwin even devoted a section to it in The Descent of Man back in 1871. What’s more, you don’t have to travel to some far-flung forest of Uganda to hear these sounds.
I heard my first wing-song last month in a small park outside of Pittsburgh, Pennsylvania. Dusk was just settling in, and while the rest of the wood was getting ready for bed, a small, long-beaked bird called the American woodcock was apparently feeling frisky.
Every February to April, male woodcocks perform what’s known as a “sky dance.” This involves a barrage of vocal “peents” from the ground before a burst of flight that unleashes whistling from the bird’s wings. For what seems like forever, the birds climb higher and higher into the sky, some 400 feet, before adding another, climactic vocalization and zig-zagging back to earth like a falling leaf—back to the very spot from which the performance began.
Woodcocks employ a combination of sounds made from both their voicebox, called a syrinx in birds, and their feathers. Think of it like the sound created by blowing on a blade of grass held between your thumbs, says Robert Mulvihill, an ornithologist at the National Aviary in Pittsburgh. “These birds are actually playing the instrument that is their wings.”
Mulvihill says aerial flights like the woodcock’s may be linked to reversed sexual dimorphism, a term for when males of a species are smaller than females. Smaller, swifter males may be better equipped to perform aerial acrobatics or create louder, more attractive sounds while doing so—and, thus, be better able to attract a mate.
If you know what you’re listening for, aeroelastic flutter is probably all around you—and this is probably true not just for people who stand in a meadow at dusk waiting for woodcocks. Hummingbirds, some of the most beloved backyard species across the United States, also make surprisingly loud chirps and tweets. And unlike the wing-singing of the woodcock and broadbills, hummingbirds make their music by shaking their tail feathers.
Christopher Clark, a colleague of Prum’s now at the University of California, Riverside, and lead author of the recent broadbill study, has made a career of studying hummingbird feathers and the sounds they generate. Each species emits a different frequency, usually by opening its tail feathers at the bottom of a blink-and-you’ll-miss-it courtship dive. These noises vary from a single, rapid “bleeeep” to fluttering notes that sound like a ray gun’s “pew pew pew.”
While hummingbird courtship can be cryptic, there’s nothing subtle about the common nighthawk. These birds, which are more nightjar than raptor, prey upon insects caught in midair and nest across most of the United States and Canada. While calling out to potential mates, these guys fly in high circles before rocketing out of the sky like a tractor-trailer barreling down a highway. The courtship dive culminates in a “boom!” as air causes the bird’s wing feathers to rapidly vibrate.
Some birds don’t even have to be in flight to play their feathered instruments. The male ruffed grouse just hops up on a log and starts thwapping away in rhythmic bursts that sound like the first few strokes of a gas-powered lawnmower. It’s common to hear this sound while walking in the woods from April to May pretty much anywhere from the Appalachians to Alaska, but actually seeing the bird perform its mating ritual is a rare treat.
While all of these feather sounds are related to courtship, Prum says there is one bird known to make a warning with its wing beats. The crested pigeon of Australia has modified flight feathers that create a distinctive flappety-whistle when it’s alarmed. What’s more, in a study published in 2009, researchers showed that when they played recordings of the pigeon’s wing sound, other pigeons hightailed it out of the area—a pretty clear sign that the sound is pigeon-speak for “run away!”
Credit to Robert Magrath, Australian National University
Whether it’s wings or tails, one modified feather or a bunch of completely normal looking ones, super-quick flights or while sitting still on a log, Mulvihill says birds have come up with just about every way to make noise.
You just have to know what to listen for.