December 15, 2010
Announcing your presence would seem to be a bad strategy for a bird that survives through kleptoparasitism—stealing food from others. But that’s just what the fork-tailed drongo (Dicrurus adsimilis) of the Kalahari Desert does. And the drongos seem to have taken some lessons from the Italian mafia, say scientists reporting in Evolution.
Let me explain.
The drongos steal food from many different birds, but the ones in this scenario are pied babblers (Turdoides bicolor), medium-size black-and-white birds that forage for invertebrates along the ground. Drongos hang out near these babblers and act as sentinels, scanning for predators such as snakes, raptors and mongooses and giving warnings when they approach.
Drongos also produce a “twank” sound every four to five seconds that lets the babblers know they are there. When babblers hear this sound, they know they’ve got someone looking out for them and they become more efficient foragers, according to the study: the babblers can spend more time looking for food and less looking for threats. They can spread out more on the ground, instead of gathering together for safety. And they can spend more time foraging out in the open, potentially giving them access to better or more food.
There is a cost for the drongos’ protection, though. Not all of their alarm calls are real; the drongos sometimes make false calls so they can fly down and snatch a meal. “Like any good gangster, as well as lying and stealing, the drongos also provide protection by mobbing aerial predators and giving true alarm calls on some occasions,” says the study’s lead author, Andrew Radford of the University of Bristol. “But, despite all of the useful services drongos provide, the foraging birds are still more responsive to calls from other babblers. It seems likely that the babblers simply don’t trust the drongo mafia as much as their own flesh and blood.”
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