The only bird known to sing with its wings contains some secrets of its performance in its bones, researchers have found.
The club-winged manakin, which lives in the cloud forests of Colombia and Ecuador, performs a mate-attracting song by rubbing its wings together.
A Cornell University team from the US scanned its bones.
They found unlike most birds, it has dense, solid wing bones that help it to emit a violin-like sound.
Lead researcher Kim Bostwick, curator of birds and mammals at the Cornell University Museum of Vertebrates, and her colleagues carried out CT scans of manakin wings.
These revealed that, while most birds have hollow wing bones, the club-winged manakin's are bulky and solid.
Birds tend not to want to carry around a lot of extra weight, said Dr Bostwick.
[So the fact] that the club-winged manakin is carrying around such enlarged, solid and densely mineralized bones, must mean they have some great contribution to sound production.
During a courtship display, male club-winged manakins (Machaeropterus deliciosus) knock their wings above their backs to create sound.
Dr Bostwick thinks that having ridged, vibrating feathers attached to a solid, stiff mass is the best way to make sure the vibrations are emitted from the feather as sound, rather than being absorbed into the bone.
Dr Bostwick was the first to decode the mechanism behind the manakin's unique sound - revealing a new kind of birdsong.
She began travelling to South America to study and film the birds in 1997, as a graduate student of Yale University ornithologist Richard Prum.
But the bird's wing movements were so fast that the footage she obtained then yielded no clues.
It was only when Dr Bostwick returned to Ecuador with a portable high-speed camera, which recorded images 30 times faster than previous attempts, that the exact movements of the wings became clear.
Her footage, along with analysis of the diminutive bird's anatomy, showed that manakins knock their wings together more than 100 times per second in order to sing.
When the wings meet, a specialised feather, with a stiff tip bent at a 45 degree angle, rubs against another feather that has seven separate ridges. This mechanism produces a mating call.
Similar performances are well known in the insect world, but nothing the same has been recorded in vertebrates before.
Charles Darwin was fascinated by club winged manakins and wrote in 1871 about the remarkable diversity of the sounds made by these birds and their importance for sexual purposes.
The evolution of their violin wings is an example of sexual selection, a term used to describe how the mating preferences of females can impact on male characteristics.
Dr Bostwick said the fact that it existed revealed that female manakins have a preference for the males that make this fascinating sound.
Exactly why females were initially attracted to it is less clear. Dr Bostwick's next step is to find out more about the evolutionary story of how the strange features we see in the club-winged manakin came to be.
She has already discovered some clues about how the club-winged manakin derived from the ancestors it shares with normal-winged birds.
Some of [its] closest relatives 'share', in an evolutionary sense, behaviours, sounds, and even some anatomical precursors of the crazy traits we see in the club-wings, she explained.
But a century and a half after Darwin was dazzled by it, the club-winged manakin apparently still has a lot to teach us about evolution.
