In Aesop’s fable The Boy Who Cried Wolf, the titular boy repeatedly lies to nearby villagers by shouting that a wolf is attacking his flock. When a wolf actually attacks, the villagers ignore the boy’s now-genuine cries. The moral, as parents tell their children, is: Don’t lie. But it could equally be: If you’re going to lie, mix it up a little. Maybe, cry bear now and then.
In southern Africa, there’s a bird that epitomises this lesson: the fork-tailed drongo. “They’re demonic little birds—black with forked tails, red eyes and a hooked beak,” says Tom Flower from the University of Cape Town. They’re also accomplished impressionists. They make at least 51 different alarm calls, and only six of these are their own. The rest are cover versions of the alarm calls of other species.
While working with meerkats in the Kalahari Desert, Flower noticed that the drongos would often scare them away from food by mimicking the alarms of other species—including, possibly, the meerkats themselves—even though no predators were around. He started following them, tracking a wild group of 64 drongos through the baking days and freezing nights of the Kalahari. So far, he has clocked more than 850 hours of observations.
He eventually found that the birds spend a quarter of their time following other animals like meerkats and pied babblers. They act as sentries, warning their neighbours about approaching predators with genuine alarm calls. But they’re also thieves. As Flower saw, the drongos sometimes sound a false alarm when one of its companions finds food. The meerkats and babblers flee from the non-existent predator, and the drongos swoop in to snatch the morsels. These thefts account for a quarter of their daily calories.
The drongos are very specific. They tend to mimic the calls of the species that they’re targeting, and for good reason. When Flower played different alarms to pied babblers, he found that they react more strongly to their own alarms than to those of the drongo itself.
This strategy clearly works, but why does it keep on working? Why don’t the meerkats and babblers get wise to the false alarms, in the manner of Aesop’s villagers? Well, actually, they do. Through his playback experiments, Flower showed that babblers react less strongly if they hear a second false alarm 20 minutes after the first one, and even less strongly if they hear a third. But if he swapped the third alarm to a different one, the babblers reset their reactions and become watchful and attentive again.
That’s exactly what the drongos do. They often try to steal food from the same individual and on three-quarters of these repeated attempts, they swap their alarms. They’re also more likely to swap if their first attempt fails; when they do, they’re more likely to succeed on the next go.
This might explain why the drongos can mimic so many calls—their varied repertoire helps them to keep on deceiving their targets. They’re actually quite similar to infections like HIV, influenza and malarial parasites, which can all change the proteins on their surfaces to fool the immune systems of their hosts.
“It’s not clear yet how strategic the drongos are in terms of what sounds they produce and when,” says Laura Kelley from the University of Cambridge, who studies vocal mimicry in birds. Flower showed that they change alarms after an unsuccessful theft attempt, but what do they switch to? Do they pick another species that shares the same predators as the pied babbler? Do they mimic birds that are especially reliable as alarm callers? Or do they choose another impression at random? “We might expect that every target species does not pay attention to all alarm calls equally – some will be more ecologically relevant than others,” says Kelley.
And what does this say about the drongo’s intelligence? Some scientists have suggested that tactical mimicry implies that the mimic understands something about the mental states of its targets—an ability known as theory of mind. But Nathan Emery from Queen Mary University of London, who studies animal intelligence, says that simple rules can produce the same behaviour. If a babbler with food is present, produce a drongo alarm call; if babbler leaves, take food; if babbler stays, produce babbler alarm call; and so on. It’s a “win-stay, lose-shift” strategy.
Flower himself says “I don’t think drongos intentionally manipulate the minds of other animals, which is how a human might accomplish the same behaviour.” There’s probably a simpler explanation. “Basically, they just keep doing what has previously got them food and if nothing work, then perhaps experiment a bit or generalise from previous experience with other species?”
That’s still impressive, though. “What the drongos are doing is not trivial,” says Emery. They’re flexibly using alarm calls, a behaviour that evolved in one context, to influence behaviour in a different context—food-stealing.
Flower wants to understand exactly how the drongos pull off their flexible deception. “This could show that apparently complex behaviour can arise from simple mechanisms,” he says. “It would also illustrate that when animals do something that we only thought humans could do, it doesn’t mean that they have similar cognitive ability to humans.”
PS: Do the drongos use false alarms to steal food from each other? “Yes, they do,” says Flower. “Whether they are less gullible than other species is unclear, and whether some individuals are more or less likely to be fooled is also unknown.”
Reference: Flower, Gribble & Ridley. 2014. Deception by Flexible Alarm Mimicry in an African Bird. Science http://dx.doi.org/10.1126/science.1249723