Science & Environment

Lanky bird's killer kick quantified

secretary bird kicking rubber snake Image copyright Hawk Conservancy
Image caption Five times the bird's weight lands on the snake in one-tenth of the blink of an eye

When the lanky leg of a secretary bird kicks a snake in the head, the killer blow can transfer five times the bird's own weight in a hundredth of a second.

So say UK researchers who have been studying the kicks of a male bird called Madeleine, kept at the Hawk Conservancy Trust in Hampshire.

Madeleine is trained to strike rubber snakes as part of public displays.

The scientists made their measurements by dragging his vulcanised victims over concealed force plates.

Writing in the journal Current Biology, they say that studying such extreme examples of animal movement could help design fast-moving robot limbs or prosthetics.

Media captionRight, right, left: Madeleine in action

"A comparable task might be playing baseball with a prosthetic arm, which requires very fast, forceful and accurate arm movements for pitching and batting," said Dr Monica Daley, a senior lecturer at the Royal Veterinary College.

Secretary birds live in sub-Saharan Africa, stand about 1.4m tall and eat snakes, lizards and small mammals.

Natural 'ninja'

Their fearsome kick-hunting makes quite a sight, according to animal physiologist Dr Steve Portugal from Royal Holloway, University of London.

"They look amazingly dinosaur-like; they strut through open plains... looking down the whole time. They wait for a snake to be flushed out ahead of them - and then they suddenly run over and start to deliver the kick to the head."

With their accuracy, speed and ferocity, but strutting on spindly legs instead of soaring through the sky, Dr Portugal said he sometimes describes the birds as "ninja eagles on stilts".

So the team knew they were studying a unique and agile predator. But when they put Madeleine's lightning kicks to the test, measuring the power of the blows with a force pad and observing the strike in high-speed video, they were still taken aback by the numbers.

Image copyright Janet Marshall
Image caption Madeleine the secretary bird is 24 years old, weighs 4kg...
Image copyright Linda Martin
Image caption ...and has an "innate desire to kick the [stuffing] out of anything that vaguely resembles a snake"

A single kick delivered some 195 Newtons of force - and Madeleine's foot touched the rubber snake, on average, for just 15 milliseconds (0.015 seconds). Blinking your eyes takes 150 milliseconds.

"So they deliver five to six times their own bodyweight in a tenth of the time it takes to blink an eye, which is really quite surprising," Dr Portugal told BBC News.

Other birds of prey reach similar levels of violence - a barn owl can pounce with 14.5 body weights, for example - but few of them do it from a standing start.

Preying on predators

"Most of the time when you measure forces, the whole animal is moving towards the target," Dr Portugal explained.

"Whereas with these guys, they're essentially standing still; that force is coming from one leg. It's a huge amount of force to deliver with a static kick."

The birds may have evolved its fancy footwork out of necessity, he added. When your prey is itself a venomous snake, mistakes are costly.

"They're preying on another predator. You've only got a fraction of a second before the snake will turn round, so you've got to stun it within the first two or three kicks at most."

In fact, that well-honed instinct to dispatch snakes on sight caused a few problems for the researchers.

Image copyright Janet Marshall
Image caption Secretary birds are found across sub-Saharan Africa

During one of the first experiments, Madeleine spotted the cables powering the force plate and set about destroying those instead of the rubber snake.

"We had a few teething issues where Madeleine got distracted by extension leads, because obviously they were very long - so they looked like a humungous lunch," Dr Portugal said.

The findings have implications for studying how other species move, the team suggests, including extinct ones like the prehistoric South American "terror bird", which may also have been a kick-hunter.

Dr Daley said: "Despite their very unusual appearance with exceptionally long legs, the secretary bird's striding gait is remarkably similar to that of ground birds such as pheasants, turkeys and ostriches.

"This suggest that specialisation for their remarkable kick-hunting technique has not unduly compromised their locomotor abilities."

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