Drongo Mimics Alarm Calls To Steal Food
Despite the prevalence of vocal mimicry in animals, few functions for this behaviour have been shown. I propose a novel hypothesis that false mimicked alarm calls could be used deceptively to scare other species and steal their food. Studies have previously suggested that animals use their own species-specific alarm calls to steal food. However none have shown conclusively that these false alarms are deceptive, or that mimicked alarm calls are used in this manner. Here, I show that wild fork-tailed drongos (Dicrurus adsimilis) make both drongo-specific and mimicked false alarm calls when watching target species handling food, in response to which targets flee to cover abandoning their food. The drongo-specific and mimicked calls made in false alarms were structurally indistinguishable from calls made during true alarms at predators by drongos and other species. Furthermore, I demonstrate by playback experiments that two of these species, meerkats (Suricata suricatta) and pied babblers (Turdoides bicolor), are deceived by both drongo-specific and mimicked false alarm calls. These results provide the first conclusive evidence that false alarm calls are deceptive and demonstrate a novel function for vocal mimicry. This work also provides valuable insight into the benefits of deploying variable mimetic signals in deceptive communication.
Drongo mimics alarm calls to steal food
The drongo, an African bird, deceives other species, including meerkats, by mimicking their alarm calls in order to scare them away and steal their abandoned food, according to a new study published in the 2 May 2014 edition of the journal Science.
To conclusively demonstrate that false alarm calls (both species-specific and mimicked) are deceptive, it must be shown that: (i) false alarms are specific to deceptive contexts and are rarely if ever made in direct physical attacks on targets; (ii) the type of calls made in true alarms at predators are the same as the types of calls made in false alarms to scare target species and steal their food; and (iii) that the target species react with a similar alarm response to playbacks of alarm calls originally recorded in a true or false context.
(a,b,c) Sonograms from recordings; of drongo-specific chink alarm calls made in true and false contexts by three drongos; of true alarm calls made by model species in response to predators, and mimics of these calls made by drongos in false alarms: (d) glossy starling, (e) crowned plover, and (f) pied babbler. Photo credits: T. Flower, M. Boerner.
(a) Meerkats (n = 22) responded for longer and (b) were more likely to abandon food in response to playback of true and false chink alarm calls than to non-alarm calls, but did not differ in their response to the true or false chink alarm calls (LMM response time: χ22=48.29, p
(a) Meerkats (n = 20) responded for longer and (b) were more likely to abandon food, in response to playback of false (drongo-mimicked) and true glossy starling alarm calls than to non-alarm calls of these species, but did not differ in their response to the false (drongo-mimicked) and true glossy starling alarm calls (LMM response time: χ23 = 50.44, p
Animals commonly eavesdrop on the alarm calls of other species in their environment [6,24,25] and drongos appear to exploit this behaviour by using deceptive alarm calls to steal food from target species [16]. These deceptive alarm calls are likely to work because the cost to target species of ignoring drongo true alarms is greater than the cost of responding to false alarms [9,26]. However, deceptive signals typically become ineffective when made too frequently relative to their honest counterpart [8,27]. This could explain why pied babblers did not abandon their food in response to playbacks of drongo-specific chink alarms, one of the most frequently made false alarm calls. Conversely, they did abandon food in response to playback of mimicked glossy starling alarms. Vocal mimicry could therefore have large adaptive benefits in this system as drongos could change their alarm call type when their own drongo-specific calls become ineffective.
A fascinating study by Sheppard [28] revealed that frequencies of visually mimetic butterfly species in the natural environment correlated with those of distasteful model species. By analogy, we may expect drongos to reach an equilibrium point where the frequency with which they vocally mimic the alarm calls of other species reflects the natural occurrence of the model alarm call. However, alarms may vary in their reliability, drongos are likely to differ in their call repertoires and individuals may strategically adjust their alarms to match target species, or even make some call types infrequently to maintain their efficacy. Further investigation of drongo call use could therefore provide valuable insights into the strategies employed by signallers in deceptive communication.
To find out how the drongos keep their victims from habituating, a team of international researchers traveled to the Kalahari Desert in southern Africa. They spent more than 800 hours in the field, meticulously watching 64 of the birds and recording their calls and behaviors. Through this, the researchers witnessed around 700 attempted food robberies.
"But when the drongos saw a meerkat with a large food item such as a gecko, larvae or even a scorpion, it would make a false alarm call that sounded the same as the calls they made at predators, even though there were no predators around," he told BBC News.
Although most of the species they impersonated were other birds, drongos even managed a meerkat alarm call. Mr Flower thinks the birds may have learned by trial and error that meerkats are likely to find their own alarm call "particularly convincing".
Except when it's lying. Because sometimes drongos, which are about the size of a scrub jay, make false alarm calls, causing their listeners to drop whatever juicy morsels they were dining on and flee the scene. Meanwhile the deceptive birds have swooped in and made off with their victim's meal. (Related: "The Bird That Cries Wolf Changes Its Lies")
To study the drongos' alarm calls, Tom Flower, an evolutionary biologist at South Africa's University of Cape Town, has habituated and banded about 200 of the birds in the Kuruman River Reserve in the Kalahari Desert. (It's the same area occupied by the meerkats in the television series Meerkat Manor.)
Flower has trained individual drongos to come to him when he calls. In return, he gives the bird a tasty mealworm; then the drongo leaves to go about its usual, natural tasks: flying up from its perch to catch insects, and following other bird species or meerkats. Flower goes with them.
If a drongo is following a meerkat, for instance, and the small mammal turns up juicy larvae or a gecko, the drongo is likely to switch from honest sentry to deceptive thief. Indeed drongos get as much as 23 percent of their daily food by making false alarms and stealing their target's dinner.
Sometimes drongos can deceive their victims by using their drongo alarm call. But after a while, if the drongo has called repeatedly, the targets stop responding. That's when the bird tries another tactic: vocal mimicry.
To find out why drongos need such an extensive repertoire of calls, Flower and his colleagues carried out several playback experiments, using the alarm calls of drongos (both their own and those they mimic).
For instance, the scientists measured the length of time a pied babbler stayed away from a tasty tidbit it had been handling after it heard a recording of a drongo making a warning cry, or a drongo imitating a babbler's alarm call, or that of a starling. Tellingly, the babblers stayed away longest when the mimicked alarm was their own or a starling's.
The drongo, an African bird, deceives other species, including meerkats, by mimicking their alarm calls in order to scare them away and steal their abandoned food, according to a new study published in the 2 May 2014 edition of the journal Science. However, just as in Aesop's fable about the boy who cried wolf, the drongo can make too many false alarms and cause members of the exploited species to wise up. But when one false alarm call stops working, drongos mimic a different alarm call, keeping up the deception racket and their access to stolen food.
Other species in the area are also habituated to people watching them at distances of less than five metres, including the drongos and another important species from which they steal food, a bird called the pied babbler. Dr Flower said: "That means that I and other researchers can get right into the thick of the action. We can unravel the interactions between all these animals because different individuals are identifiable by coloured leg bands (in the case of the birds), or L'Oreal hair dye marks on the fur of the meerkats (don't worry, it's been tested on humans)."