You don’t build the world’s biggest bird nests by putting up with lazybones.
That, anyway, is the finding of a new study into how sociable weavers (Philetairus socius) make and maintain nests that can house up to 500 birds.
Aggressive supervisors identify and punish the slackers while constructing the giant, grass-woven structures in southern Africa. (Take National Geographic's bird quiz.)
Birds that shirk their duties on creating the nest’s main thatch structure, and focus instead on their individual chambers, are chased away from the nest, according to the study, published March 16 in the journal PLOS ONE.
Study co-author Gavin Leighton believes the pushy birds in the weaver colony help to pull it together for the common good.
“The aggression inducing this nest construction, given it’s such a constant behavior that we see, could very well lead to these large nests,” says Leighton, a postdoctoral fellow at the Cornell Laboratory of Ornithology in New York.
Weighing up to a ton or more and up to 20 feet (6 meters) wide, the nests are home to a number of extended families that have their own chambers for roosting and breeding. (Related: "5 Animals That Are Awesome Architects.")
If the families aren’t all closely related, there’s a temptation for individuals to exploit the benefits of the communal nest while leaving the group building to others.
“That’s where we think coercion may be important in protecting this investment and making sure others cooperate as well,” says Leighton, who co-authored the study with University of Miami biologist Laura Vander Meiden.
Sociable weaver researcher René van Dijk of the U.K.'s University of Sheffield isn't so sure.
The idea is “very interesting,” he says, but “it seems to me that this is unlikely to be driving communal nest-building.”
According to van Dijk’s observations, “aggressive interactions seem to be relatively rare,” while “thatch building is only performed by about 50 percent of birds, mostly males. There are thus a lot of freeloaders that would need to be punished for not contributing to the thatch.”
He also questions how the dominant birds can “monitor the behavior of, say, 300 birds in a typical colony.”
But Leighton says has seen nest-builders take frequent time-outs on a nearby branch. “It’s possible that they’re keeping tabs on the other individuals in the colony,” he says. (Read more about Gavin's research in his own words.)
If the study findings are correct, sociable weaver nests may get even bigger than they need to, adds Matthieu Paquet of the University of Edinburgh in the U.K.
“If coercion plays a role, we can imagine birds building more than is actually necessary to avoid aggression,” he says.
A First for Birds
Leighton says his study marks the first known case of a bird using aggression to boost cooperation for the good of the group.
Such behavior is rare in social animals, he adds, with the few reported examples including the naked mole-rat, a species that likewise dishes out punishment to group members that need to pull their socks up. (See "5 of Nature's Wildest Animal Showdowns.")
There’s also us humans, of course. Leighton compares the sociable weaver nest to an apartment complex.
“Individuals who contribute the most to maintaining the shared part of the complex are the most aggressive, and the individuals that work most on their own rooms receive the most aggression,” he says.
But when sociable weavers are angrily shown the door, at least they know there’s a way back.