Media playback is unsupported on your device Media caption Watch how slow motion reveals the birds' fancy footwork

Scientists have glimpsed songbirds stamping their feet in a rapid-fire "tap-dance" that is invisible to the naked eye.

Both male and female cordon-bleu birds bob up and down while singing to their mates.

Now, using high-speed video cameras, a team from Japan and Germany has spotted a remarkable quick-step that the birds perform mid-hop during this display.

The research appears in the open access journal Scientific Reports.

Because each bird's dance became more vigorous if its mate was on the same perch, the team thinks the vibrations might be adding a tactile element to the courtship ritual.

Alternatively, the rat-a-tat flourish might be a musical accompaniment to the bird's song, or a visual display - or it might be a wooing strategy that targets multiple senses.

"It's a really rare phenomenon that songbirds produce non-vocal sounds," said senior author Masayo Soma, from Hokkaido University in Japan. "Some species produce non-vocal sounds with their wings, but they usually don't use their feet."

Image copyright Nao Ota Image caption Blink and you'll miss it: A single step takes one-fiftieth of a second

Non-singing birds are also known to perform elaborate dances that can include noises from wing or tail feathers - tricks that have, similarly, been unveiled by high-speed video footage.

But the cordon-bleu's striking quick-step is something of a first.

"It is very astonishing," said Manfred Gahr, co-author of the study, from the Max Planck Institute for Ornithology in Germany. "Maybe more birds are doing it, but it just has not been seen."

Choosy partners

It was Dr Soma and her colleagues in Japan, he said, who first suggested turning a high-speed camera on the birds.

"We work mainly on the songs. They thought it would be interesting to also study dancing, and they were right. It's a really fascinating feature."

The team studied 16 blue-capped cordon-bleus - a species of waxbill native to sub-Saharan Africa. Eight males and eight females were paired up randomly for multiple two-hour sessions, totalling more than 200 hours of footage.

"It wasn't very easy to record the behaviours because these birds are very choosy, and they only perform courtship displays to the individuals they like," Dr Soma told the BBC.

So she and her student Nao Ota had to try a few different combinations - but eventually they got the footage they were after: nearly all the males and half the females were filmed, at 300 frames per second, performing their bobbing and singing displays.

Watching the slow-motion footage for the first time was a big moment, Dr Soma said. "We were so excited! It was really interesting. I just kept thinking, this could be a good paper..."

Image copyright Nao Ota Image caption The quick-step was seen in both blue-capped (pictured) and red-cheeked cordon-bleus

When they picked the images apart, the team established that the birds performed bursts of, on average, three or four very rapid steps. A single step lasted as little as six frames of high-speed video - or 20 milliseconds (0.02 seconds).

The dance is all the more remarkable, Prof Gahr explained, because of everything else the birds are doing during the display: they clutch a piece of nesting material in their beaks, tilt their heads upward, bob up and down and sing - all while keeping an eye on their partner.

"It's quite complicated, to do all that without falling from the perch - it's very acrobatic," he said, adding that the team wants to do further experiments to tease apart the cordon-bleus' show-stopping performance.

"It could be that they do the step-dancing at a particular moment during the singing. That's what we hope to see in the future - whether there is some co-ordinated integration of these different motor activities."

The researchers are also keen to observe the animals in the wild.

Image copyright Nao Ota Image caption Cordon-bleus usually dance with a piece of nesting material in their beak

Will Allen, a behavioural ecologist who studies sexual selection at the University of Hull in the UK, said the team had discovered a "very cool behaviour" but was keen to see it explored in more detail.

"We already know that several non-passerine birds perform similar elaborate, multimodal duets, and that many passerines duet in song. What's new here is that there's a passerine species - a songbird - that is duetting in both song and dance.

"This study is an important first step, but we don't know whether the receiver prefers mates that display these dancing movements, or even that the receiver is sensitive to them. There are some suggestions here that they might, but without an experimental-type design, we can't work that out."

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