By Matthew Stock
Scientists in the UK have found that a bird of prey native to sub-Saharan Africa can deliver precise and powerful kicks with a force five times its own body weight; enough to kill venomous snakes in less than the blink of an eye.
The secretary bird stands over 4 ft. tall on long, crane-like legs. Unlike most birds of prey that swoop down from the air to make a kill, the secretary bird hunts its quarry mostly on foot. Its method for dispatching its prey by stamping on it is distinct from other raptors that typically use their beak to kill after catching their prey with their talons.
"They're unique, actually, there's no other bird of prey that are related to them; they're in their own family group. So taxonomically they're very distinct from any other birds of prey," explained Dr. Campbell Murn from the Hawk Conservancy Trust in Hampshire, adding, "they eat a lot of invertebrates, they eat a lot of small mammals, anything that's basically small enough to be chased down, kicked, disabled, swallowed - it's on the menu."
A team of researchers from Royal Holloway, University of London, the Royal Veterinary College, and the Hawk Conservancy Trust used a male secretary bird called Madeleine to discover just how powerful a kick the creature could deliver.
Madeleine was trained to attack a rubber snake in his enclosure at the Hawk Conservancy Trust. The force of the kicks was measured by a force plate concealed under artificial grass as the rubber snake was pulled across it.
The maximum force they managed to obtain was just over 195 Newton's; about 20 kilogram-force (kgf).
"If you look at it proportionately, or if you scaled it up to somebody my size, it's the equivalent to me kicking with about half-a-tonne of force through my heel straight down on the ground," Campbell told Reuters.
He added: "Importantly, it's happening from a standing start. There are other birds of prey that hit their prey with incredible force but they're usually jumping down from a tree branch or a post, or stooping like a falcon. Madeleine's doing it from a standing start and just going 'whack' with over five times her body weight, from a standing start."
The contact time between the bird's feet and the snake was measured at just 15 milliseconds in average, about a tenth of the time it takes to blink an eye. Since venomous snakes make up part of the secretary bird's diet, the speed and efficiency that they land a fatal blow to the snake's skull is vital to their survival.
The study, published in the journal Current Biology, was instigated by Dr Steve Portugal from the School of Biological Sciences at Royal Holloway, University of London. He said that the secretary birds' rapid strike coupled with their long legs means "the birds can only correct for a missed strike in the next kick - once they've started a kick, they can't adapt it, and they have to wait for the next strike." He added that they are using visual targeting and feed-forward motor control - pre-planned movements - while making a kill.
The research could help understand how prehistoric, particularly large terrestrial birds, located their prey, according to Campbell. He added that there could also be numerous potential technological applications for the research. These include 'biologically inspired' robots and prosthetics.
"There are also applications in terms of potential prosthetics, how we manage artificial limbs, how we design them," he said.
"And the way the bio-mechanics of the secretary bird's working, in terms of its needs for locomotion and its needs for attacking its prey - and how that works from a bio-mechanical perspective can help give us insights into the way we might develop prosthetics."