Common snipe 'flies the flag' in search for love
High-speed video has revealed for the first time how male common snipes - a species of wading bird - generate their distinctive drumming mating calls.
A team of UK researchers found the birds' tail feathers "flapped like a flag" in the wind, something that had not been seen before.
When attempting to attract a potential mate, the males perform a dive to create the drumming noise.
The findings appear in the Journal of Experimental Biology.
"The exciting thing was that the video revealed that the tail feather actually flaps backwards and forwards," said co-author Roland Ennos, from the University of Manchester's Faculty of Life Sciences.
"The tail feather has special adaptations, which means its acts just like a flag blowing in the wind, which has not been seen before," he told BBC News.
Dr Ennos said that people previously thought the feathers were particularly strong to generate the sound during courtship displays.
"Most tail feathers are rigid, so would be stiff in the wind in order to provide aerodynamic lift."
But, he explained, the snipes' (Capella gallinago gallinago) "special line of weakness" in its feathers resulted in drag, and actually slowed the birds down.
"Therefore, by going fast and making a lot of noise, the bird is showing prospective mates how fit it is."
The males have a "special tail feather that it can stick out", Dr Ennnos said.
"The feathers have a weakened hinge region in their rear vane. The birds dive to increase their speed and make a more attractive higher-pitched sound."
During its courtship display, the male climbed to an altitude of about 50m (165 feet) before diving at about 40 degrees with its two outer tail feathers extended.
The researchers found that when the male birds reached a speed of 50km/h (31mph), the outer feathers produced an audible sound.
The feathers continued to produce the sound until the bird reached speeds of more than 86km/h (53mph).
All of a flutter
The team reported that the drumming was created by an "aeroelastic flutter", a self-feeding, potentially destructive vibration.
Dr Ennos said it was an aeroelastic flutter that caused the Tacoma Narrows suspension bridge in the US to famously wobble and collapse in November 1940, just a few months after it opened.
The team decided to investigate how the snipes created the drumming sound after reading research by US researchers, who also used high-speed video to show that a species of hummingbird produced its chirping call with feathers, not vocally.
"In order to measure the frequency of sound that the feather made, we simply stuck a feather in a wind tunnel," said Dr Ennos. "But to capture the footage, that was more tricky.
"The wind tunnel was very dark and we could not get enough light. So we put the feather in front of a hair dryer and filmed it when it was fluttering."
He said the findings could be used to shed light on how different species of snipe produce different pitched calls.
"Some species do have narrower tail feathers, and these have higher pitched calls. Our findings could help to explain this."