When you’re filming a bird that specializes at kicking snakes to death, you’d better be careful about your power cables.
Steve Portugal learned that the hard way when studying secretary birds—a long-legged bird of prey with an appetite for snakes and a phenomenal, skull-crushing kick. “Their desire to kick anything that moves and looks a bit like a snake is so innate,” says Portugal. “At the end of a session, I had this long extension lead and I started wheeling it in. Within a fraction of a second, the bird was on it and going mental, trying to kick the extension lead to death. It felt like I was playing with a cat.”
An object doesn’t even need to look like a snake. “You should see what it did to my folder,” says Portugal. “It wasn’t trying to eat it. It was more: What’s this in my aviary? Their default option if they don’t want to mate with it, or raise it, is to kick it.”
“They’re like ninja eagles on stilts,” he adds.
Secretary birds can be found striding through the grasslands of sub-Saharan Africa. They cut a striking and unmistakeable profile, with light grey bodies, black wing tips and shorts, a red mask, and a crest of black quills. The latter, according to one dubious-sounding hypothesis, look like the quill pens that secretaries once tucked behind their ears—hence the bird's name. A more plausible alternative is that “secretary” is a bastardization of the Arabic “saqr-et-tair” for “hunter bird.”
The bird’s hunting technique is simple: Find prey, flush it out, and stamp the living crap out of it. It does so with improbably long legs, which look like a crane's legs glommed onto an eagle’s body. With unerring accuracy, it brings these down onto rodents, lizards, small birds, and snakes—typically on the head.
Portugal, who studies the bodies and movements of birds at the Royal Holloway University, got to see these kicks in action a few years ago. He was studying vulture vision at the Hawk Conservancy Trust in Hampshire, U.K., to see if these large birds could spot wind turbines while looking for food (spoiler: they can’t). And while working with these vultures, he occasionally became distracted by the Trust’s secretary bird—a temperamental male named Madeline. (He was misidentified as a female when he was born.)
Madeline has lived at the Trust for most of his 24 years, and has a fantastic rapport with a trainer named Mike. “Birds of prey are very diva-ish; he likes this one person and scoffs at everyone else,” says Portugal. With Mike's prompting, Madeline regularly takes part in shows where he brutalizes rubber snakes to cheers and applause. Occasionally, he'll even race children. “He doesn't mind children,” says Portugal. “It's only adults he hates.”
As Portugal watched Madeline, he was awed by the speed of the bird’s kicks, and determined to measure the forces they produce.
He put force plates—large metal platforms that can measure the forces applied by walking (or kicking) animals—in the bird’s enclosure and covered them with fake grass. Mike threw a rubber snake onto the plate. And Madeline did Madeline. (First, he attacked the force plate leads; after the team covered those up, then he went for the snake.)
The plates confirmed that Madeline kicks are extremely powerful, landing with forces 5 times his own body weight. For comparison, a barn owl can strike with 14 times its own body weight, but after dropping down on its prey from above. By contrast, a secretary bird applies its forces from a standing start, solely by accelerating its leg.
Diego Sustaita from Brown University says that it's hard to judge the strikes because “little data are available for these kinds of behaviors in other animals.” Still, he says, the magnitude “seems pretty impressive to me, considering that these birds are striking prey far smaller than themselves.”
The kicks are also exceptionally fast, with each foot touching its target for just 15 milliseconds. That’s too quick for the bird to adjust its movements using sensory feedback from its limbs; it just has to aim with its eyes and go for it. That may sound intuitive but it’s actually very different to what other birds do.
A feeding pigeon will close its eyes just before pecking a morsel. Woodpeckers close their eyes just before the moment of impact. Peregrine falcons, the fastest bird on the planet, have their eyes shut just before their talons impale their prey. “They use vision to estimate where the food is and then close their eyes at the last minute,” says Portugal. But a secretary bird keeps its eyes constantly open, and locked onto its prey. “For a bird to rely so much on vision is quite strange. I wasn't expecting that.”
He now wants to see if they behave differently depending on their target—if, for example, they pay more attention when hunting snakes than rodents. Their tough leg scales are largely impervious to snake fangs, but a venomous bite to the body can still be fatal.
Portugal also wonders if similar killer kicks were used by the terror birds, a nightmarish group of extinct predatory birds from South America, some of which were twice the size of a horse. What poor victims might these giant predators have stamped on?