When I was a kid, teaching myself to bird-watch, I would go out to the arboretum, and I found this one Anna’s hummingbird holding a territory.” Ornithologist Chris Clark’s obsession with hummingbirds like the red-crowned Anna’s began with repeated visits to the University of Washington’s arboretum, in Seattle, at the age of 14. Though he didn’t yet know that the tiny, fast-flying birds would form the basis of his professional life, he quickly became enamored with them. “I would go out every day to check on that bird.”
Fast-forward to 2019. Clark strolls across the campus at UC Riverside, where he has been a professor of biology for the past six years. The springtime sun is shining as a gentle breeze blows from the west. The predominant sound is traffic, but if you close your eyes and ignore the din of nearby I-215, you can just about hear the stits and tyuks produced by the hummingbirds as they flit among the carefully landscaped hedges and shrubs, going about their business of wooing mates and claiming territories.
“As far as hummingbirds and hummingbird biology goes, this is one of the best campuses in the world,” he says just before spotting an Allen’s hummingbird resting atop a flowering Cape honeysuckle rooted in a planter between a parking lot and the psychology building.
Clark explains that while the shrub hails from the southern coast of Africa, Cape honeysuckle evolved alongside a group of small, hovering, long-billed, nectar-feeding avians known as sunbirds and spider hunters. Their flowers therefore offer the perfect source of nutrition for Southern California’s small, hovering, long-billed, nectar-feeding hummingbirds. And Allen’s hummingbirds in particular thrive in the vicinity of heavily manicured lawns and planters like the ones scattered across the university’s campus and in thousands of backyards and gardens in the area.
Male hummingbirds display bright, flashy, iridescent feathers that are meant to reveal their attractiveness to potential mates. On the Allen’s that we stop to observe, there are orange flecks around the eyes and a fiery orange-red patch along the throat. These markings serve the same function as the bright reddish-pink feathers adorning the head and neck of male Anna’s hummingbirds like the one Clark watched as a teenager in Seattle.
Clark’s attire is the visual opposite of a male hummingbird. When you’re trying to get a glimpse of a creature that evolved to notice bright colors, after all, it’s best to dress in dusty earth tones: brown hiking boots, brown shorts, reddish-brown button-up shirt. Like any good bird-watcher, he has a pair of binoculars hanging from his neck.
In 2016, some 57 million Americans spent a total of more than $4 billion on food for their bird feeders, according to the U.S. Fish and Wildlife Service. As a result, most Californians can easily observe the five types of hummingbirds that make a home in the American West—Anna’s, Allen’s, Costa’s, rufous, and black-chinned—in their own backyards. Hummingbirds hover in place, beat their wings up to 80 times per second, slurp nectar from as many as 1,000 flowers in a single day, and dive at velocities of over 60 miles per hour during courtship displays.
What the casual birder might not realize, however, is that hummingbirds are one of only three groups of birds that are vocal learners. (The other two are songbirds and parrots.) They don’t emerge from their eggs already knowing their birdsongs. Instead, newly hatched hummingbirds must learn them by listening to older, more mature members of their species, just as newborn humans pick up language from their parents and caregivers.
And it turns out that hummingbirds don’t just sing with their mouths. They also do it with their feathers.
Eventually, Clark and I find our way to the Department of Evolution, Ecology, and Organismal Biology, where Clark keeps his laboratory and office. Taking a seat behind his desk, he picks up a pair of owl feathers. He rubs the feathers together and asks me to listen, but I don’t hear anything. After all, owls are nighttime ambush hunters, relying on stealth to capture their prey. Covered with a fine layer of velvetlike fibers called pennulae, owl feathers aren’t like those of other birds, even other raptors. Owl feathers evolved to act as an auditory invisibility cloak. An inaudibility cloak.
Clark repeats the demonstration with another pair of feathers lying around on his desk. These sound like fingernails scratching against a window screen. While they’re from a parrot, they could be from any bird that’s not an owl, even a hummingbird. Most feathers are noisy.
Some might be tempted to call these nonvocal sounds, he says, but that isn’t quite right. When people speak in a whisper, they produce sounds without using their voice box. So whispering is technically a nonvocal sound. If you think of vocal sounds more generally as those that originate inside the body, then whispers qualify, Clark says. But then so would flatulence.
In The Descent of Man, Charles Darwin mused about the noises made by birds’ feathers, referring to them as “instrumental” because, like those produced by a musical instrument, they’re produced by the interaction of the feathers’ physical structure with the bird’s movements.
Whatever you call them, feather sounds are simply a by-product of flying, the same way the pitter-patter of footsteps is a by-product of walking. Which is why “I call them locomotion-induced sounds,” says Clark. “Most of the time, we don’t produce them for communication, but sometimes we do. And that’s what birds are doing with their wings and tail feathers.”
It’s not that birds intentionally strum their plumage the way a frustrated child stomps their way into their bedroom, or an impatient parent taps their foot waiting for their son or daughter to clean their room. Feather sounds are a result of the physics of flight. But evolution took those raw sounds and crafted them into something more than just noise: communication. We know this, says Clark, because some hummingbirds have wing and tail feathers with shapes that have been modified to amplify the sounds they make. And because the birds’ behaviors evolved to take advantage of their feathers’ reverberations during courtship.
Here’s how it works: One of the sounds produced by tail feathers is called flutter, which is created when the airflow around a feather causes it to vibrate. Flutter doesn’t typically occur in Anna’s hummingbirds during normal flight, because the birds don’t reach the minimum airspeed necessary to produce it. But male Anna’s tend to exceed that speed during a behavior known as a display dive, which they perform to woo potential mates. If females prefer males that dive faster or from higher up, it turns out, they will also prefer males whose tail feathers sound a certain way.
And that may be the reason male Anna’s hummingbirds developed a modified tail feather. It’s as if evolution wanted to turn up the volume on that flutter.
According to recent research by Clark and his colleagues, among North American hummingbirds—a group known as bee hummingbirds, which includes roughly 10 percent of the planet’s hummingbird species—most males produce sounds with their tail feathers as part of courtship rituals, though the sounds vary from species to species with the size and shape of the tail feathers.
Costa’s hummingbirds, commonly found in Southern California’s more arid landscapes, can even turn their tail feathers into a metaphorical boom box. That’s because their tail sounds have what researchers call directionality, which Clark learned when he put their feathers into the wind tunnel in his laboratory. By orienting their tail feathers directly toward a female during courtship displays, the males ensure that the sound she hears is 11 decibels louder than it would be otherwise.
And there’s more. Nearly a decade ago, when Clark was a graduate student at UC Berkeley, he hypothesized that the similarity between the songs chirped by male Costa’s hummingbirds and the sounds produced by their tail feathers was too great to be a coincidence. Ornithologists were aware that male Costa’s made two similar songs but had assumed that both were produced vocally. But why would the two songs, produced by two very different instruments—the voice and the tail—sound so similar? “Isn’t this fun?” Clark asks with a grin. “Because I have no idea what’s going on. I cannot tell you why they have that match.”
As if all this wasn’t puzzling enough, another of Clark’s recent studies found that nearly half of the bee hummingbirds in existence have lost their songs. Although their evolutionary forebears likely sang songs to entice the opposite sex, somewhere along the way a subset of the species gave it up. Most make sounds with their wing feathers instead.
To understand why these birds use one type of sound and not the other, Clark took note of the contexts in which each species produced one or the other. While tail sounds were used solely in the context of courtship, wing sounds and vocal songs were used more generally, broadcast to whomever might be listening as a sort of territorial claim. It therefore seems reasonable for any bird to pick just one instrument—voice or wing feathers—for that purpose, rather than wasting energy with two.
It’s tempting to wonder whether Clark’s research on hummingbird communication can be used to improve human lives. And indeed, comparing the physics of noisy feathers, like those of hummingbirds, and silent ones, like those of owls, may have applications for stealth flight technologies—everything from planes to drones—and even designing windmills that operate more quietly. And understanding the basis of hummingbird communication could, when combined with songbird research, shed light on human language learning.
Yet Clark is motivated simply by the drive to learn more about this unique group of birds, the ones that fascinated him as a teenager. “Ultimately,” he says, “this is all curiosity-based research.”