Like gangsters running a protection racket, drongos in the Kalahari Desert act as lookouts for other birds in order to steal a cut of their food catch. The behaviour, revealed in research led by Andy Radford and Linda Hollén from the University of Bristol, may represent a rare example of two species evolving from a parasitic to a mutualistic relationship.

The research showed that victimised pied babblers gained a mitigating benefit from the presence of thieving drongos because, by keeping watch for predators, the drongos allowed the babblers to focus on foraging and so catch more insects.

Many apparently harmful parasites seem to be tolerated by their hosts and the provision of compensatory benefits to victims could underpin a range of host-parasite relationships. The insight the research provides into complex co-evolution between species gives scientists a valuable way of understanding other important relationships, such as those between drugs and bacteria and between pathogens and hosts.

Dr Andy Radford said: “Because drongos are parasitic birds who swoop in to steal food from other species, you’d expect them to keep a low profile while waiting. Rather surprisingly, however, drongos perched above foraging babblers advertise their presence by issuing a call called a ‘twank’ every 4 or 5 seconds.

“When we played back these ‘twank’ calls to a babbler group, we found that they spread out over a larger area and lifted their heads less often, indicating that they were less fearful of predators when they thought a drongo was keeping watch. We think that drongos have evolved to alert babblers to their presence because helping the group forage more effectively leads to more frequent opportunities for theft.”

Drongos have an ingenious way of catching food: by crying wolf about the presence of predators they scare other animals into dropping their catch, which the birds then pounce on. This research indicates that pied babblers have evolved to tolerate the drongos giving false warnings and stealing some of their hard-earned gains in exchange for the chance to forage in relative safety when a drongo is keeping watch.

Dr Radford continued: “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. 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.”

The research was carried out with the support of BBSRC, the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, and Magdalene College, Cambridge.

The research is published in Evolution and reported in Nature’s Research Highlights today.

Paper

Singing for your supper: sentinel calling by kleptoparasites can mitigate the cost to victims by Andrew N. Radford, Matthew B. V. Bell, Linda I. Hollén and Amanda R. Ridley in Evolution