When it comes to wooing females, male hummingbirds have something in common with World War I fighter pilot the Red Baron. During the mating season, these bright-throated males climb high into the air and then nose-dive, belting out sharp squeaks or trills to impress watching females. A new study shows how the tiny birds emit their high-pitched calls. As they fall, stiff breezes vibrate their tail feathers, giving each species a unique whistle.
Researchers had observed other birds squeaking while flying. When some doves jet away to avoid a predator, for instance, their bodies start to whistle, says Kimberly Bostwick, an ornithologist at Cornell University, who was not involved in this study. "We hear it, but we've never really understood what's going on."
Hummingbirds may be some of the squeakiest fliers. Male Anna's hummingbirds (Calypte anna), which look as if they're wearing bright-pink scarves, swoop at speeds over 20 meters per second, emitting a shriek like a startled rodent. In 2008, Christopher Clark, a physiologist now at Yale University, and colleagues first identified the source of the noise: the birds' tail plumage. When his team plucked the hummingbirds' thin, outermost tail feathers, the boisterous animals became as silent as stealth bombers.
But it still wasn't clear how the hummingbirds' plumage sang. So in the new study, Clark and colleagues put tail feathers from 14 species of "bee" hummingbirds—a rowdy group that includes the Anna's hummingbird—into a wind tunnel. At gentle breezes, the feathers just ruffled, but when the winds sped up to around the birds' normal dive velocities, about 7 to 20 meters per second, something strange happened: The feathers started to ripple rhythmically, much like the Tacoma Narrows Bridge, which famously began undulating and then collapsed in 1940 when winds hit it at just the right speed. But unlike that infamous Washington State roadway, the feathers emitted sometimes-piercing noises when they vibrated, Clark and his colleagues report today in Science.
Many feathers whistled in harmony, too, Clark says. When placed side by side, for instance, some of the Anna's hummingbird's middle tail feathers started to mimic the vibration of those on the edge of the tail, producing a much louder but also uniform noise. The orange-throated Allen's hummingbird, which sounds a bit like a chirpy machine gun, has two sets of tail feathers that each whistle separate notes. This nimble flier also makes a trilling noise with its wings before it dives, Clark says. "You can think of a bird as being a one-man band," he says.
Clark and his colleagues' follow-up work also suggests that each bird's feathers have a signature flutter. When his team popped feathers from a wide range of non-hummingbird species into the wind tunnel, each one started to vibrate at a certain speed and with a signature flutter. That suggests that flutter-based communication could be more common than thought, he says. "All the feathers I've tested ... every single feather has made a sound," Clark says. "It's just that in some cases, the sounds are pretty quiet."
Bostwick is impressed. "It's just a really neat study," she says. Breezy bird communication may be so common, she adds, because the same properties that make feathers good for flying—they're stiff but elastic—also prime them for making noise. The squeakiest fliers, including hummingbirds and some of New Guinea's birds of paradise, also tend to be those under strong pressure to compete for mates. In other words, they're the birds already most apt to make ridiculous displays, from wild dances to dive-bomb runs.