Have you ever noticed how you slowly stop paying attention to a repeated sound, like a noisy fan or the clockwork clanging of a hammer from a nearby construction site? Getting used to a recurring sensation is known as habituation, and habituation can undoubtedly be a blessing. Who would choose not to ignore the relentless dhik-chik from a stranger’s headphones, for example? But things get complicated if the sound you want to ignore is a sound you can benefit from. This predicament is capitalised on by the fork-tailed drongo.
Closely related and visually very similar to the black drongo that is common in India, the fork-tailed drongo is found across large parts of Africa. These birds often feed alone, doing all the hard work of finding and catching insects by themselves. But sometimes fork-tailed drongos forage more cleverly, using a process that begins with the drongos latching on to groups of socially foraging birds. These social foragers depend upon each other to remain wary of potential predators — individuals who spot a predator deploy a shrill alarm call to alert their foraging group to the presence of an enemy. Foraging birds that listen for the alarm calls of both their own species and other species therefore need not constantly be on the lookout, and can focus most of their attention on feeding. But failing to heed an alarm call could certainly spell death.
The fork-tailed drongo inserts itself neatly into this life-and-death dynamic to serve its own lazy purposes. Fork-tailed drongos have discovered that imitating the alarm call of a foraging babbler or starling can induce these birds to drop whatever they are eating and quickly fly for cover. The drongos then swoop in and gobble up the insect that the social forager had painstakingly collected and was about to eat. Using its brain instead of brute force, the fork-tailed drongo feeds with minimal exertion, and can obtain almost a quarter of its food through such trickery.
So where does habituation enter the picture? The trouble with the drongo’s strategy is that mimicking the foragers’ alarm call too often when a predator isn’t actually present can lead the foragers to start ignoring the call. The foragers become habituated to a sound that earlier signalled danger but now only indicates that their hard-won insect is about to be stolen. (This situation ought to remind you of one of Aesop’s Fables. I’ll let you recall which one.) As a result, the foragers stop fleeing when they hear the alarm-call because the benefit of fleeing from a potential predator is now outweighed by the cost of losing food to an interloper. Needless to say, the drongos are ready with a counterstrategy — they simply switch to mimicking the alarm call of another species. Shifting between over 50 different alarm calls prevents the foragers from getting habituated to a single false alarm call, and ensures the drongos’ continued access to misappropriated meals.
The fork-tailed drongos aren’t all evil, however. They alert the social foragers to real threats as well, thus bearing something of a risk on their behalf. Whether or not you think this exchange of services is fair, I think you’ll agree that fork-tailed drongos are an example of nature at its craftiest.
(Ambika Kamath studies organismic and evolutionary biology at Harvard University)