“You see it dive down, something happens in the bushes, and it comes back up with something to impale,” says Diego Sustaita from California State University at San Marcos. “What happened in the bushes?”
Can a predator really be friends with its prey?
Sustaita and Rubega found out by traveling to California’s San Clemente Island, a Navy-run stretch of land that’s home to an endangered subspecies of loggerhead shrike. Researchers from San Diego Zoo Global have been working hard to save the population, which fell to a low of 14 individuals in 1998. The birds are bred in captivity, and to prepare the youngsters for eventual release, their handlers feed them live prey like mice. That allowed Sustaita and Rubega to film their hunting technique with high-speed cameras.
The team saw that shrikes kill mice by repeatedly biting the weak point at the base of their necks. Often, they would grip that spot with their beaks and rapidly roll their heads from side to side, around 11 times per second, over several short bouts.
These vigorous shakes cause the mouse’s body to accelerate around its neck at six g—six times the strength of gravity. That’s comparable to what people would experience in a low-speed car crash, and more than enough to separate the vertebrae of even a very large rat. At best, a small mouse would experience intense whiplash. At worst, its neck would break. And that’s especially likely since, as Sustaita found, the head and body of the shaken rodent progressively shift out of phase. Which means, in his words, that “the head is moving one way and the body is moving another.”
These calculations are based on footage of just one of the many shrikes that the team filmed. But they clearly explain how a small bird can subdue a relatively large rodent. And they fit with other studies that have shown that shrike victims frequently turn up with severe wounds to their neck vertebrae.
“It would be very interesting to know how much of this killing behavior is learned,” says Michael Habib, who studies bird mechanics at the University of Southern California. “Do shrike populations with unusual diets in the wild use unique killing strokes?”
Shaking clearly matters to shrikes, but Jen Bright from the University of South Florida, who also studies bird mechanics, wonders if scientists have been underestimating its importance in other species. “Most computer models of feeding only look at the force of the jaw muscles,” she says, “so by not including shaking behaviors, we could be underestimating animals’ capabilities. This is particularly important in extinct predatory dinosaurs, where we only have models to tell us what’s going on. Birds are living dinosaurs, so it’s not unreasonable to assume that extinct dinosaurs could have used similar killing strategies.”
But how are shrikes strong enough to pull off their paralyzing head rolls? The bird in Sustaita’s videos was shaking a mouse 30 percent of its body weight, which is equivalent to me shaking two legs of ham, or roughly one Chipotle burrito. In my mouth. With my neck. Eleven times a second. I doubt I could do it. “I hope not,” Sustaita says.