The fork-tailed drongo is a master impersonator and a champion of deception. These clever African birds can mimic the warning calls of many other species, tricking them into fleeing for cover. After the other birds and meerkats are scared away, the drongos swoop in and steal their food. This happens over and over again.
In nature, deception pays off, but not for long. When it’s used too often, the victims catch on and start ignoring the false alarms, just like how the villagers got tired of the boy’s lies. But fork-tailed drongos (Dicrurus adsimilis) may have found a way to bypass the “frequency-dependent constraints” that normally limit the pay-offs from deceptive ploys.
To see how these kleptoparasites get away with it, a team led by Tom Flower from the University of Cape Town studied their tactics in the Kalahari Desert, observing 688 attempts of 64 drongos to steal food from their neighbors over the course of 847.5 hours. "They're rather demonic little black birds with red eyes, a hooked beak and a forked tail," Flower describes. They found that drongos have large alarm repertoires, and they know to mix it up so their victims don’t catch on to their racket.
Drongos spend over a quarter of their time following their targets, which include southern pied babblers (Turdoides bicolor) and meerkats (Suricata suricatta), pictured below. The tricky birds produce honest alarm calls when they spot predators, and since all these desert animals eavesdrop on each other, they flee for cover, just as if one of their own had made the call. “It's a bit of an information superhighway where all the animals speak each other's language," Flower tells Reuters. It seems to benefit everyone. However, when a target finds a large food item -- like a fat grub, a scorpion, or a gecko -- the attending drongos produce false alarm calls. "Crime pays,” he adds. Drongos make multiple honest calls and false alarms over several hours each day, and food theft accounts for 23 percent of their daily intake.
In addition to their own alarms, individual drongos can produce between 9 and 32 different calls of other species. A total of 51 call types have been recorded in false alarms: 6 were specific to drongos, 45 were mimics of other species’ calls.
The team played four different, drongo-generated calls at 20-min intervals to individual pied babblers, who were given a food item. The calls were: a nonalarm territory call (the control), a drongo-specific alarm call, a mimicked Cape glossy starling (Lamprotornis nitens) alarm call, and a mimicked pied babbler alarm call. They measured the babbler’s response time, from when it stopped handling its food item and moved toward cover to when it resumed foraging.
Babblers were slower to resume foraging when they heard mimicked alarm calls (of both babblers and starlings) -- compared to drongo-specific alarm calls. Additionally, babblers decreased their fear response when the same alarm call was played three times in succession -- but maintained their response and increased its duration when the third alarm was changed.
The team also observed repeated food-theft attempts by drongos on the same target on 151 occasions. In order to maintain target deception, the birds changed their alarm-call type 74 percent of the time. And they were more likely to switch to a different false-alarm call if their previous food-theft attempt failed. The change usually results in success.
“The drongos are producing their calls tactically. They’re changing their calls in response to the feedback they get from their target,” Flower tells Science. “And that’s how they’re able to overcome the problem of crying wolf too often.” Such a sophisticated con.
The work was published in Science last week.
Images: Tom Flower