About five years ago, Clinton Francis and a gaggle of ornithology students were walking toward the ocean at San Simeon Beach State Park in California when they noticed a type of plover called a killdeer about 60 feet away, calling Dee! Dee! Dee! They hadn’t seen the shorebird flush from its ground nest, but Francis, an ecologist at California Polytechnic State University, says that it would have crept slyly away from its eggs after registering the herd of humans as potential predators.
Now that it had an audience, the killdeer (Charadrius vociferus) began to lie through its beak. It contorted its wings in what’s called the broken-wing display, feigning an injury that would make it seem unable to fly. Francis was already familiar with this sort of bird theater, typically performed for earthbound diurnal predators in the hopes that, instead of finding the killdeer’s nest, they’d see a plump, apparently wounded parent as an easy meal. When this deceptive behavior works, the predator charges the killdeer, which then launches skyward to escape.
A plover may fake an injury while running.
While watching the bird, Francis recalls, one of his students asked: “What other species do that display?” Francis knew the behavior was commonly associated with shorebirds, but beyond that, he wasn’t sure. In response, he told the student, Wren Thompson, “You should really look into that for your research question for this class.” Taking the advice and diving in, Thompson found sporadic examples across avian species through an exhaustive literature search, followed by surveys sent to ornithologists, avian ecologists, and experienced birders around the world. In the end, she and her colleagues uncovered evidence that 285 avian species perform the broken-wing display.
Mapping those behaviors onto the avian phylogenetic tree revealed that the trait spans from some of the most basal bird families, including pheasants and ducks, to more recently evolved taxa such as songbirds. “It’s pretty amazing,” Francis says, adding that he was surprised how “particular clades on the avian tree of life really just light up,” including blackbirds, warblers, and sparrows. The frequent and disjointed appearance of the behavior across the tree suggests it evolved independently several times, he adds.
The analysis, published earlier this year, also indicates that predation risk has driven the trait’s evolution. “Birds that experience higher levels of predation, by visual predators in particular, tend to use the display more than those that do not,” Francis says. The team found that the farther the birds’ breeding zones were from the equator, the more likely the animals were to use the broken-wing display. One possible explanation for this relationship, Francis says, is that the portfolio of predators becomes increasingly diurnal—and more visual—towards the Earth’s poles.
“It is certainly surprising to see that broken-wing display is so widespread in phylogenetically distant groups of birds,” Miguel Ángel Gómez-Serrano, a conservation ecologist at the University of Valencia who studies deceptive nest defense behaviors but was not involved in this research, tells The Scientist by email. Plovers have a lot of tactics to distract predators beyond feigning broken wings, he adds. They may begin by calling to catch a predator’s eye. If this doesn’t work, they may escalate to so-called false brooding: lying down to simulate incubating their eggs—something that could trick a predator into looking for the nest away from its true location. Or a plover may begin what’s known as a rodent run, mimicking a flightless mousy snack to entice the predator into chasing an apparently easy meal away from the nest. “The bird runs crouched forward with its chest close to the ground,” says Gómez-Serrano. “Often plovers place the tail folded towards the legs to [better] resemble the shape of a mouse.”
The degree of predation risk seems to dictate the form that a bird’s dishonesty assumes. When the risk is lower, a plover may fake an injury while running, giving the deceiver momentum to fly off and escape. “If nothing seems to work, or the risk of losing their offspring seems obvious—for example a predator that is right next to the nest,” Gómez-Serrano says, “the birds take even more risk by [enacting] the broken-wing display statically near the predator,” imperiling themselves by proximity as well as by losing their running start. Previous work by Gómez-Serrano’s group has found that when Kentish plovers (Charadrius alexandrinus) engaged more in risky stationary displays, their nests survived longer, providing evidence that the potential cost pays off.
Sometimes, though, the price for lying is death. In 2008, on a beach in Spain, Gómez-Serrano saw a Kentish plover enacting an in-motion broken-wing display for a small predator hidden in the surrounding vegetation. While trying to lure that foe away from a nest containing days-old chicks, the plover was itself too distracted to notice a different predator: a kestrel that swooped out of the sky, snatched it up, and flew inland, likely to feed its own chicks.
In addition to broken-wing, false brooding, and the rodent run, other documented dishonest behaviors include playing dead, feigned exhaustion, false feeding, and pseudo-sleeping. Gómez-Serrano says some birds fake eating, pecking at nothing on the ground—perhaps giving predators the impression they’re distracted and easy to sneak up on. Some birds vocalize their lies. Burrowing owls (Athene cunicularia) hiss like rattlesnakes to protect against ground squirrels, and fork-tailed drongos (Dicrurus adsimilis) mimic meerkat alarm calls to scare the mammals into abandoning food. “I think there’s some other really interesting deceptive tactics out there that are worth exploring,” and we may be unaware of many, Francis says.
Filipe Cunha, a behavioral ecologist at Wageningen University & Research in the Netherlands, happened upon a particularly unusual case of avian deception while studying Siberian jays (Perisoreus infaustus). “They’re definitely liars,” he says, explaining how the territorial birds fake an alarm call that’s typically reserved for alerting group members to the presence of predators such as sparrowhawks. Cunha determined that the jays deceive neighboring groups of Siberian jays to scare them into fleeing, after which the liars steal caches of scavenged meat that the tricked birds had hidden to survive the Arctic winter. He says that he hopes studying within-species dishonesty will shed light on how trust evolved in our own species.
Research on avian deception highlights the importance and diversity of these behaviors as survival tools, Francis says. Consider a familiar example of a bird without known deception or indeed any other predation-avoidance behaviors: the extinct dodo, “which [people] were able to just walk up to and club because they had no evolutionary response to approaching humans or any other type of predator,” Francis says. “It’s worth keeping this quiver of tactics because otherwise reproductive success is zero.”