Many birds sing to woo females, but some hummingbirds go to great lengths to do so. They climb to between 5 and 40 metres before plummeting past perched females in death-defying dives. They pull up at the last minute, spread their tail feathers and produce a loud chirpy song. The song comes not from the birds' mouths, but from their tails. The splayed tail feathers vibrate as air rushes past them, causing them to flutter.
Flutter sounds colloquial and innocuous, but it can be deadly. It's what happens when air, moving at just the right speed, zooms past objects with just the right stiffness, setting up large and potentially disastrous vibrations. Flutter brought down the passenger plane Braniff Airways Flight 542, killing everyone on board. Flutter wrecked the Tahoma Narrows Bridge, causing it to warp and twist like a piece of rope. But flutter also ensures that male hummingbirds get some action.
Christopher Clark from the Peabody Museum of Natural History has found that males of different hummingbird species have their own unique tail songs. Their feathers that flutter at different frequencies, and the sounds produced by different feathers blend into one another. This allows each male bird to create an entire symphony with its bottom, independently of its calls or the famous humming noise produced by its wings. These songs tell eavesdropping females about the male's species, as well as his quality.
Clark clipped tail feathers from 14 different hummingbird species and placed them in a wind tunnel. As the air swept past, Clark measured the feathers' vibrations using a laser Doppler vibrometer – a device that engineers use to inspect everything from speakers to bridges. He also recorded the sounds they made using a microphone.
At both low and high wind speeds, the feathers were silent. The tail feathers only fluttered in the right way at intermediate speeds, matching those that the birds dive at. The feathers are very finely tuned musical instruments. Tiny changes in their weight, stiffness, size and shape can greatly change the pitch and volume of the sounds they create.
Clark also found that neighbouring feathers interact with one another. In Anna's hummingbird, the second tail feathers from the outside amplify the sounds created by their outermost pair, making them four times louder. In Allen's hummingbird, the third and fourth pairs of tail feathers flutter at different frequencies, creating a symphomy of two tones – the bird also adds a third by singing in the more traditional way. In the Calliope hummingbird, the tail feathers strike one another during the dive, creating a sputtering noise on top of the normal song.
Clark thinks that the female hummingbirds are behind the great diversity of the males' tail feathers. By developing preferences for songs of subtly different characters, they drove the males to evolve different types of feathers that produced different noises. Just as birds of paradise evolved elaborate visual displays, and bower birds evolved unique collector instincts, so hummingbird tails evolved into a diverse range of musical instruments.
It's possible that the females originally judged the males by the quality of their dives, and used the fluttering sounds simply as a criterion to review them by. These flights are certainly no simple feat. Anna's hummingbird is just 7 centimetres long and at the fastest point of its dive, it covers 385 times that in every second. For its size, it is the fastest flight of any bird, faster even than the legendary peregrine falcon. It also copes with g-forces that would cause even jet fighter pilots to black out.
In an earlier study, Clark found that the bird's tail feathers create a louder noise the faster it dives. By listening out, a female can get a pretty good idea of how fit a male is, simply by the sound of wind vibrating its tail. It's a brutally honest signal – there is no way that for a weaker male to cheat and produce a loud song. Loud means strong, and probably sex too.
Clark also thinks that such fluttering tones are more common among birds than we realise. You can hear them when ravens, vultures, ducks, loons, woodcocks and flycatchers take to the skies, and the list goes on. Perhaps they too use these sounds to communicate with one another.
Reference: Clark, Elias & Prum. 2011. Aeroelastic Flutter Produces Hummingbird Feather Songs. Science
This post originally appeared on Not Exactly Rocket Science. Images by Anand Varma (top) and Ed Yong (middle).