There are places in the southern United States where the roadsides are fenced with barbed wire, and the wire is adorned with corpses. The carcasses belong to lizards, rodents, small birds, and even snakes, all impaled on the sharp prongs. These grisly dioramas are the work of the unlikeliest of butchers: a small bird called the loggerhead shrike.
The shrike is a hawk trapped in the body of a finch. From a distance, it looks like any other songbird, perched on a high vantage point. But from those perches, the two-ounce bird frequently swoops onto its prey, and subdues it with the murderous, hooked tip of its beak. “It’s like seeing a rabbit running around with long canines and a mane and acting like a lion,” says Margaret Rubega from the University of Connecticut. “I personally think they’re very badass.”
Without the powerful talons of true raptors, shrikes only tear into their meals after first impaling them onto sharp points, like thorns or barbed wire. This behavior is the source of the bird’s infamy, and its nickname: the butcherbird. But it’s fundamentally just an act of storage; the bird only impales prey that it has already killed. And although most of its victims are insects, shrikes have been known to dispatch mice and lizards that are up to three times their body weight.
“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?”
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.
Rubega notes that the bird doesn’t fully pick its prey up, so it doesn’t have to support its victim’s full weight in its mouth. Sustaita points out the shrike’s unusual bill: Like that of a falcon, it has sharp ridges called tomial teeth on its side. Scientists have historically assumed that the shrike uses these to paralyze its prey by slipping them between the vertebrae of its neck. But perhaps they also help the bird to maintain its grip, “so it can hold on and shake without needing to bite so hard,” Sustaita says.
It’s also possible that shrikes are unusually strong for their size. “If you watch the impaling, it does take a fair bit of work to put a sharp point through the body of something that’s half as big as you,” says Rubega. “Their technique will be very interesting to look at.”
“I’ve got a freezer full of shrikes and I’m looking forward to digging into their musculature,” Sustaita adds.