Most birds have to open their beaks to sing, but when it comes to the dainty broadbill, all it has to do is wiggle its wings. In African and Rufous-sided Broadbills, circular flight displays (see the video below) are accompanied by a pulsing song that can be heard from more than 300 feet away. The brreeeeet sound, which resembles a klaxon horn, doesn’t come from vocalizations, but rather from the fluttering of wing feathers, as explained in a study published today in the Journal of Experimental Biology.
Some types of birds, such as hummingbirds and doves, can vocalize with both their syrinx—the avian equivalent of a voicebox—and their wings and tails, says study author Christopher Clark, a researcher at the University of California, Riverside. Clark knew that broadbills make some pretty cool noises, too, but wasn't sure whether they came from the bill or the feathers.
To find out, Clark and his colleagues traveled to Uganda to take video and audio recordings of African and Rufous-sided Broadbills during their circular flight—a behavior the scientists speculate might be for romancing and defending territory. Back in the lab, they analyzed the clips and correlated the sound pulses with the down strokes of the wings. The scientists also noticed gaps between a handful of the 10 primary feathers—located at the outermost part of the wing and connected to the bird’s wrist bone.
To determine if the feathers were the source of the sounds, Clark put a dried African Broadbill wing in a wind tunnel and made it “fly.” The detached wing was able to reproduce the song, but it was still unclear which specific feathers were responsible.
Clark’s colleagues then returned to Africa to continue the study with wild birds. They removed less than an inch of the tips of target feathers from wild broadbills, and found that clipping certain ones changed the horn-like sound to something that more resembled a wind-up toy. This test helped them hone in on the “key players” among the wing feathers.
Broadbills do have the ability to vocalize, Clark says, but only faintly; their flight sounds are their primary mode of singing. He thinks that somewhere along the line, “wing song replaced vocal song” in the birds, giving them a unique way of showboating their talents.