The Berkeley Science Review: Articles

A Musical Tail
Hummingbirds seduce at high speeds (view PDF)
by Greg Alushin

With his wings frantically flapping fifty-six times per second, the male Anna's hummingbird ascends one hundred feet into the air before plummeting downwards, zooming along at over sixty miles per hour at the base of his dive. What could be the reason for this garish display of bravado? Why, to impress a lady of course, although he will also dive for other males and even people, perhaps simply for the sheer joy of it. But this bold acrobat does not seek to entice with dazzling visual effect alone. "Swooping over whatever it is that they are diving to, which is often a female hummingbird, they'll make this 'BEEP!' at the bottom of the dive," explains graduate student Christopher Clark of the integrative biology department at UC Berkeley.

The mechanism by which the bird produces this sound has been a matter of controversy. Biologists had posited that the diving chirp of the hummingbird is mechanically produced by its feathers during the dive, although more recently, others had rejected this notion, citing the striking similarity between the pitch of the sound and the bird's song. Clark and undergraduate Teresa Feo definitively settled the matter: the Anna's hummingbird uses his tail feathers to chirp. Their result is one of the first documented cases of non-vocal sonation—the act of communicating by sound.

Clark's interest in hummingbird physiology provided the motivation for this discovery. His research focuses on how the shape of hummingbird tails are optimized for the mechanics of flight and other functions like attracting mates. He decided to study the Anna's hummingbird because of its peculiar tail feathers. "The males in particular have outermost tail feathers that are somewhat narrow. And they are sexually dimorphic; the two sexes are different," he says. This clue provided an indication that the shape of the tail feathers might be linked to a function specific to one sex, which Clark hypothesized was the diving chirp of the male.

Clark also embraced the opportunity to do fieldwork without the hassle it usually entails—the Anna's hummingbird is a Berkeley native. Indeed, prime specimens can be found zipping up to a feeder mounted in a special chamber built into the window of Clark's lab. (Its presence is not entirely aesthetic or altruistic, as the birds are sometimes captured and harmlessly experimented upon.) All of Clark's field experiments were performed locally in Albany Bulb park, which he says provides its own set of challenges in contrast to those of more exotic locales. "In the Albany Bulb, the problem was the people. My field notebook has lots of pages with big muddy dog prints from dogs running off leash. A guy also tried to steal my camcorder once."

That camcorder provided some of the initial exciting evidence for the tail chirp. High-speed video recordings showed that the hummingbird spreads its tail feathers for a few hundredths of a second at the bottom of its dive, when the chirp is produced. Clark then captured a few birds and plucked their tail feathers (which grow back in a few weeks), thereby eliminating their ability to produce the dive sound. At this point the evidence that the sound was produced nonvocally was compelling, but the actual physical mechanism of its generation remained mysterious.

Clark and Feo decided to get to the bottom of this question with a series of lab experiments. By placing a tail feather into a jet of air, they were able to reproduce the tone of the dive-sound, proving that the sound was produced by that feather. They then put the feather into a wind tunnel, where they could precisely vary the speed of the passing air. The pitch of the sound did not vary with wind velocity, suggesting that the frequency of the sound wave is based on the intrinsic mechanical properties of the feather. This result was confirmed by high-speed video of the feather in the air jet, which showed it vibrating at the same frequency as the pitch of the sound produced.

There is no hard evidence as to why natural selection has looked favorably on the tail chirp, although there is plenty of room for speculation. The dive sound of the Anna's hummingbird is significantly louder than its vocal song; therefore, the ability to dive and produce the sound could signify the fitness of a male. "But this is just a hypothesis," says Clark. "Maybe the females prefer loud sounds at a particular frequency. It is possible that this is because the architecture of their ear is tuned to a particular sound." A number of birds are believed to make tonal sounds with their feathers, although thus far there has been little investigation into whether these sounds are used for communication. Clark believes that many other hummingbirds, in which funny-shaped tail feathers and elaborate courtship displays abound, may also produce similar non-vocal sonations. In the future, he plans to study how those sounds relate to the evolutionary relationships between different hummingbird species. Soon that little 'BEEP!' may turn out to be just one note in a humming, cavorting symphony.

Greg Alushin is a graduate student in biophysics.

Want to know more? Check out:
http://sciencereview.berkeley.edu/media/15/hummingbird.mov

Comments on this article? Drop us a line at with 'letter to the editor' in the subject!