Deception by Flexible Alarm Mimicry in an African Bird

1. What they did

Fork-tailed drongos in the Kalahari spend over a quarter of their time following other animals, such as southern pied babblers and meerkats, often for several hours a day. They act as sentries, giving honest alarm calls when they see a predator approaching, and the other animals flee to cover when they hear them. But when one of those animals finds a large food item, a drongo may give a false alarm, then swoop in to steal the abandoned food. Drongos can produce many alarm calls, including mimicked alarm calls of other species. The authors asked why drongos use so many different false alarms.

They worked with 64 wild drongos that were used to observers and could be recognised by coloured leg rings. The study had four parts:

• Observations. They watched drongos for 847.5 hours and recorded 688 food-theft attempts, noting which alarm call was used on which target.
• Playback experiment 1. They gave 20 babblers (in 10 groups) a food item and played four drongo calls 20 minutes apart: a territory call (not an alarm), a drongo-specific alarm, a drongo mimic of a Cape glossy starling alarm, and a drongo mimic of a pied babbler alarm. They timed how long each babbler took to go back to foraging.
• Playback experiment 2. They played 22 babblers (in 11 groups) three alarm calls in a row, 20 minutes apart. In two treatments all three calls were the same; in the other two, the third call was switched between a drongo-specific alarm and a mimicked starling alarm. Treatments were 3 days apart.
• Repeated thefts in the wild. They recorded 151 occasions when a drongo (42 drongos in total) made repeated food-theft attempts on the same target, and whether the drongo changed its call.

2. What they found

Drongos’ alarm calls

• An individual drongo’s repertoire ranges from 9 to 32 calls (average 17). In all, 51 alarm call types have been recorded in false alarms: 6 are drongo-specific and 45 mimic other species.
• Food theft makes up 23% of what drongos eat, by biomass.
• Drongos that could mimic a target’s alarm call did so more often when stealing from that species than when stealing from other species (147 food-theft attempts by 10 drongos).

Experiment 1: does mimicry work better?

• Babblers took longer to go back to foraging after mimicked alarms (of babblers or starlings) than after drongo-specific alarms, and longer after any alarm than after the territory call.
• Babblers did not respond differently to mimics of their own alarm than to mimics of the starling alarm.

Experiment 2: does changing the call work better?

• When the same alarm call was played three times in a row, the babblers responded less each time.
• When the third call was changed, the babblers kept responding as strongly, and they responded longer to a changed third call than to a repeated one. This held whether the drongo-specific or the mimicked starling call came first.

In the wild

• On repeated attempts on the same target, drongos changed their alarm call type on 74% of occasions.
• They were more likely to change their call after a failed attempt.
• After a failed attempt, changing the call made the next theft more likely to succeed than repeating the same call.

3. What they concluded

• By varying their false alarms, drongos gain twice: they use the calls most likely to fool a particular target, and they avoid the target getting used to one repeated false alarm.
• This lets drongos get around the usual limit on deception, where a lie stops working once it is used too often compared with the honest signal. The authors compare this to disease organisms that keep changing their surface proteins to stay ahead of the immune system.
• Flexible variation may not work forever: targets might eventually ignore all the calls in a drongo’s repertoire, or recognise individual drongos and judge their reputation.
• The study shows a deceptive use for vocal mimicry, a behaviour for which few benefits have been shown, and suggests that paying attention to the target’s response can be adaptive.
• Behaviour like this, honest and dishonest alarms used tactically, has been taken as evidence of advanced mental abilities similar to theory of mind. The authors note that similar behaviour in different species need not have the same underlying mechanism.

6. Other technologies, skills or innovations

• Playback experiments with recorded drongo calls, including drongo mimics of other species’ alarm calls, shown as spectrograms.
• Long-term habituated study population: drongos, babblers and meerkats used to observers, with individuals recognised by colour rings.