Birds of a Feather

As we learn in our earliest playground adventures, cheaters are out there trying to exploit the labor of others.  Cheaters want to prosper, and sometimes they do.  For every Bernard Madoff languishing in prison, there could be another languishing in a Caribbean bungalow.  Societies are best served by mechanisms that dissuade cheaters, as the cheater’s gain is the honest man’s loss.  That is often easier said than done.

Humans are not the only creatures to face the scourge of cheating.  From microbes to animals, cheaters emerge in the simplest social interactions.  Successful tricksters pass along any genes that helped them cheat to the next generation, further upending social dynamics and forever altering a population’s genetic makeup.  Thus, the exploited are rightly pressured to discourage cheating.  But, what strategies are best to thwart cheaters on a long-term basis?  New observations in birds suggest cooperation is key.

Brood parasites are the ultimate cheaters of the avian playground.  These birds do not build nests to rear their chicks.  Instead, females lay their eggs in the nests of other species, tricking unwitting “adoptive” parents into caring for parasitic young.  Fooled host parents pay a high price in resources shunted away from their own biological offspring and toward their parasitic wards.  But, as a recent dispatch in Science magazine reports, some birds may have an effective strategy to keep the cheaters at bay.

Cooperation is the basis of this anti-cheater scheme.  Dr. Naomi Langmore and her team at Australian National University noticed an intriguing pattern in the global distribution of brood parasite species.  They almost exclusively overlap with birds known as cooperative breeders.  Cooperative breeders form complex social groups that defend nests, rear chicks, and even adopt orphans.  Dr. Langmore’s group was struck by the strong geographic overlap of brood parasites and cooperative breeders.  Within these ranges, brood parasites targeted cooperative breeders to rear their young.  Were the effective group parenting skills of cooperative breeders simply an irresistible target for brood parasites?  If so, how could hosts repel these cheaters?

Female (left) and male superb fairy-wrens. Image kindly provided by Scott Contini.

Ever vigilant? Female (left) and male superb fairy-wrens.
Image kindly provided by Scott Contini.

Dr. Langmore’s team delved into these questions using a six-year field study in Australia of Horsfield’s bronze-cuckoo (a brood parasite) and one of its cooperative breeder hosts, superb fairy-wrens.  Their observations were striking regarding anti-cheating strategies.  Fairy-wren groups deployed warning calls and brutal attacks against cuckoo females that approached their nests.  Cuckoos rarely breached these perimeter defenses.  In addition, the size of the fairy-wren cooperative group mattered.  “It seems that larger groups are better than small groups at keeping cuckoos away,” notes William Feeney, lead author of the study.  Small fairy-wren cooperatives had a diminished per capita output of chicks simply because cuckoo females were more likely to breach their defenses.

In search of a nest? Horsfield's bronze-cuckoo. Image kindly provided by David Kleinert.

In search of a nest? Horsfield’s bronze-cuckoo.
Image kindly provided by David Kleinert.

These findings imply that cooperative behaviors, particularly on a large scale, are key to forming effective defenses against cheaters.  The field observations do not answer whether cooperative breeding specifically evolved to thwart brood parasites.  But, it appears that the cooperative social structure is a firm foundation for developing cheater repellant strategies.  As Mr. Feeney notes, “When a cooperative breeding species becomes a host of a [brood] parasite, the host should stay cooperative.”  Host groups shrink or disband at their peril.

Cooperation is not the only means to defeat cheaters.  Many mammals and birds deploy complex behaviors that punish cheaters after they strike; some non-cooperative birds, for example, foil brood parasites by abandoning parasitized nests.  But, these strategies might cost the host more compared to cooperative approaches.  These findings from Down Under emphasize the potential efficiency and effectiveness of the social group in stopping cheaters in their tracks.  Though I suppose any kid on the playground could have told you that.

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About James Urton

I went to school to become a molecular biologist.  At some point in this long education, I discovered that I love communicating science to the general public: talks, writing, at a pub, on the street corner...  Whatever venue will let me hold your attention for a few moments.  Unfortunately, I can't do this for a living, since no one will pay me.  So, I have a job as a molecular biologist at the University of Washington, where I get to work with great scientists on some really awesome projects, and I'll blog about science here at Muller's Ratchet in my spare time. Why should the general public want to know anything about science? Here's my explanation (which also explains why I chose the name Muller's Ratchet for this site). Briefly as a graduate student (before I had to devote all of my time to graduating), I blogged at Adaptive Radiation.
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