Saturday 20 November 2010

"Gangster" bird found to charge for protection:another way we are not unique = an avian protection racket

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.

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 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 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.

http://www.world-science.net/othernews/101119_racket.htm

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