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“Gangster” bird found to charge for protection

Nov. 19, 2010
Courtesy of BBSRC
and World Science staff

In what some sci­en­tists are liken­ing to a mob pro­tec­tion rack­et, a spe­cies of birds in Africa has been found to guard oth­er birds from preda­tors in ex­change for food.

On one level, the security ser­vice of­fered by the dron­go birds of the Ka­la­ha­ri Des­ert would seem to be le­git­i­mate. They do prov­ide some true pro­tect­ion—un­like quite a few ma­fios­os, who cre­ate the very threat that for a fee they will sup­posed­ly ward off.

A drongo perched in a tree above pied bab­blers. (© Tom Flo­w­er, U. of Cam­bridge)


Yet there’s a hint of the crim­i­nal in the dron­go opera­t­ion too, sci­en­tists say. The avian guards help them­selves to “pay­ment” by oc­ca­sion­ally scream­ing out false warn­ings of pred­a­tors, then us­ing the re­sult­ing con­fu­sion to snatch food from their feath­ered clients. Nor do the lat­ter seem to spe­cif­ic­ally re­quest prot­ect­ion; rath­er, the dron­gos are just—there.

The draw­backs not­with­stand­ing, the “client” birds, pied bab­blers, gain some­thing from the ar­range­ment, ac­cord­ing to sci­en­tists. That’s be­cause the self-ap­point­ed sen­t­in­els, through their pres­ence, al­low the bab­blers to fo­cus on for­ag­ing for in­sects rath­er than watch­ing their backs.

The be­hav­ior may be a rare ex­am­ple of two spe­cies evolv­ing from a par­a­sit­ic to a “mu­tu­al­ist­ic” rela­t­ion­ship, say the in­ves­ti­ga­tors, re­port­ing the find­ings in the re­search jour­nal Ev­o­lu­tion.

“Dron­gos are par­a­sit­ic birds who swoop in to steal food from oth­er spe­cies,” ex­plained An­drew Rad­ford the Uni­vers­i­ty of Bris­tol, U.K., one of the re­search­ers. Giv­en this un­sa­vory way of life, he went on, it was some­what sur­pris­ing to find that dron­gos perched above for­ag­ing bab­blers ad­ver­tise their pres­ence rath­er than keep a low pro­file.

They an­nounce them­selves “by is­su­ing a call called a ‘twank’ eve­ry four or five sec­onds,” Rad­ford said.

The ex­plana­t­ion, he added, seems to be that the “twank” re­as­sures bab­blers some­one is keep­ing a look­out against pred­a­to­ry birds. This lets the bab­blers for­age for in­sects more ef­fec­tive­ly. That, in turn, leads to bet­ter op­por­tun­i­ties for the dron­gos to filch some of the catch. “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,” Rad­ford said.

But when the dron­gos cry wolf about the pres­ence of preda­tors, they scare oth­er an­i­mals in­to drop­ping their catch, which the dron­gos then pounce on, said Rad­ford and col­leagues. So pied bab­blers have basic­ally 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 on duty.

Like a “good gang­ster,” Rad­ford said, the dron­gos—not par­tic­u­larly large bird­s—pro­vide real pro­tec­tion some­times, both by mak­ing true alarm calls and by “mob­bing” preda­tors as a group.

But “de­spite all of the use­ful ser­vic­es dron­gos pro­vide, the for­ag­ing birds are still more re­spon­sive to [a­larm] 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 could pro­vide in­sight in­to oth­er im­por­tant mu­tu­al­is­tic and par­a­sit­ic rela­t­ion­ships in na­ture, said Doug­las Kell, chief ex­ec­u­tive of the U.K. Bi­o­tech­nol­ogy and Bi­o­log­i­cal Sci­ences Re­search Coun­cil, which funded the stu­dy. “Ev­o­lu­tion­ary arms rac­es, in­clud­ing those be­tween par­a­sites and their hosts, and plants and an­i­mals and the dis­eases that they suf­fer, un­der­lie a whole range of so­cially and eco­nom­ic­ally im­por­tant ar­e­as of bi­ol­o­gy,” he noted.


* * *

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Homepage image: A drongo in the Kalahari (© Andy Radford, U. of Cam­bridge)







 

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In what some scientists are likening to a “protection” racket, a species of birds in Africa’s Kalahari Desert has been found to guard other birds from predators in exchange for food. The security services offered by the drongo birds would seem to be more legitimate than many a mafia racket: at least the drongos—unlike numerous gangsters—aren’t the same party creating the threat to begin with. Yet there is something of a criminal flavor about the drongos’ operation, scientists say. They help themselves to “payment” by occasionally screaming out false alarms, and using the resulting confusion to snatch food from their feathered clients. Nor do the latter appear to specifically request the drongos’ help; rather, the drongos are just—there. Despite all this, the “client” birds, pied babblers, gain something from the arrangement, according to scientists. That’s because by keeping watch, the drongos let the babblers focus on foraging for insects rather than watching their backs. The behavior may be a rare example of two species evolving from a parasitic to a “mutualistic” relationship, say the investigators, reporting the discoveries in the research journal Evolution. “Drongos are parasitic birds who swoop in to steal food from other species,” explained Andrew Radford the Universities of Bristol, U.K., one of the researchers. Given this unsavory way of life, he went on, it was somewhat surprising to find that drongos perched above foraging babblers actually advertise their presence rather than keep a low profile. They announce themselves “by issuing a call called a ‘twank’ every four or five seconds,” Radford said. The explanation, he added, seems to be that the “twank” reassures babblers someone is keeping a lookout against predatory birds. This lets the babblers forage for insects more effectively. This, in turn, leads to better opportunities for the drongos to filch some of the catch. “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,” Radford said. But when the drongos cry wolf about the presence of predators, they scare other animals into dropping their catch, which the drongos then pounce on, said Radford and colleagues. Pied babblers have in effect 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 on guard. Like a “good gangster,” Radford said, the drongos—not particularly large birds—provide real protection sometimes, both by making true alarm calls and by “mobbing” predators as a group. But “despite all of the useful services drongos provide, the foraging birds are still more responsive to [alarm] 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 could provide insight into other important mutualistic and parasitic relationships in nature, said Douglas Kell, chief executive of the U.K. Biotechnology and Biological Sciences Research Council, which funded the study. “Evolutionary arms races, including those between parasites and their hosts, and plants and animals and the diseases that they suffer, underlie a whole range of socially and economically important areas of biology,” he explained. “From drug and pesticide resistance to the biodiversity of ecosystems, deepening our understanding how a range of organisms have evolved into complex relationships will help us address important social issues in a smarter, more holistic way.”