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Drongos 'cry wolf' to score a snack

Fork-tailed drongo perched

One of the aims of the study was to experimentally prove that drongos use deception to their advantage (Source: Tom Flower/University of Cape Town)

Crafty birds African fork-tailed drongos mimic the alarm calls of a range of other local bird species to trick their neighbours into leaving their food unattended over and over again, a new study has found.

The drongos (Dicrurus adsimilis) belong to a group of animals known as kleptoparasites, which make a living by stealing food caught by other species.

In the Kalahari Desert, they perch above groups of ground foraging bird species and meerkats and give the alarm when they spot danger approaching. These target species respond to the alarm calls of other species, as much as signals from their own lookouts.

But according to research published today in the journal Science, led by Tom Flower from the University of Capetown, South Africa, and colleagues, like Aesop's 'The Boy Who Cried Wolf', drongos give their alarm call even when there is no danger.

When their target species — in this case a long-term study group of southern pied babblers (Turdoides bicolour) — take off in a flap, the drongos swoop in and steal their leftovers. After repeated false alarms the wary babblers no longer respond, so the drongos mimic other species' calls to keep the babblers on edge.

"Mostly the drongos' alarm signals are honest, but when a target animal finds a particularly large or juicy snack, the drongos may give a false alarm call, causing the babbler to flee and leave their snack behind," explains study co-author Dr Amanda Ridley from the University of Western Australia.

Deceptive tactics

Ridley says one of the aims of their study was to experimentally prove that animals use deception to their advantage.

"Some animals use camouflage in a certain colour, but this is a fixed deceptive signal," she says. "Drongos are able change their signal depending on whether babblers respond to their alarm call."

During the course of the study, they observed 64 drongos make 688 attempts to steal food from their neighbours. Individual drongo repertoires were found to range from nine to 32 different calls, with a total of 51 different alarm calls recorded during the study.

Mimicked alarm calls were used for almost half of all false alarms, and the researchers found that drongos using multiple alarm calls were not only more likely to deceive their targets; they were also able to trick them repeatedly.

"If the babbler recognises that the drongo is lying and ignores the call, then the drongo changes tactics and mimics a starling, or another locally reliable species," Ridley says.

The ability to mimic the alarm calls of multiple species prevents their neighbours from getting used to the drongos' trickery and ensures their strategy keeps paying dividends over the long term.

Subtle intelligence

Far from their reputation as silly drongos, Ridley says they are very clever and adjust their alarm calls to accommodate the species they are following.

"When they are on their own, they never give the alarm over an approaching ground predator because they are not at risk. But when they are following ground foragers like babblers, they do," she explains.

"They recognise the needs of their target species and adjust their strategy accordingly," she adds. "They have to invest some time in developing the relationship and being seen as a reliable friend, because if they are too dishonest, as is often the case with younger drongos, they are chased away."

"Drongos and babblers are engaged in a dynamic interaction — and in an evolutionary arms race. As the host wises up to the deception, the parasitic drongo needs to stay a step ahead."

Tags: animals, animal-behaviour, birds