Birds can form complex, multilevel societies, a new study finds, a feat previously known only in humans and certain other big-brained mammals, including some of our fellow primates as well as elephants, dolphins and giraffes.
This challenges the idea that large brains are required for such a complex social life, the researchers say, and may offer clues about how multilevel societies evolve.
The subjects of this study are vulturine guineafowl, a heavy-bodied, ground-feeding species native to scrublands and grasslands in northeast Africa. These birds are an impressive sight, with a vivid blue breast and long, glossy neck feathers leading up to a bare, "vulturine" head with intense red eyes. And now, as researchers report in the journal Current Biology, we know they live in impressive societies, too.
Vulturine guineafowl are highly social, living in flocks of a few dozen birds. Of course, there are lots of social birds and other animals around the world, many of which live in much larger groups. A murmuration of starlings, for example, may number several million. A multilevel society is defined less by size, however, than by "different structural orders of grouping," according to Current Biology Magazine, forcing members to use more mental energy tracking multiple kinds of relationships.
"Humans are the classic multilevel society," study co-author Damien Farine, an ornithologist at the Max Planck Institute of Animal Behavior, tells The New York Times. In fact, he adds, people "have long hypothesized that living in complex society is one of the reasons why we’ve evolved such large brains."
A multilevel society may also exhibit "fission-fusion" behavior — in which the size and composition of social groups change over time — but not all fission-fusion societies are multilevel. Fission-fusion "refers to fluid grouping patterns," researchers explain in Current Biology Magazine, but "is not tied to a particular social organization."
Living in a multilevel society can offer big benefits, with different levels of the society serving specific adaptive purposes that evolved in response to different cost-benefit trade-offs. This includes reproduction and social support at the lowest tier, for instance, as well as perks like cooperative hunting and defense at higher tiers.
Due to the mental demands of managing relationships in a multilevel society, scientists have long believed this social structure only evolves in animals with the brainpower to deal with its complexity. And until now, multilevel societies have only been known in mammals with relatively big brains, the researchers note. While lots of birds live in large communities, these tend to be either open groups (lacking long-term stability) or highly territorial (not friendly with other groups).
Birds of a feather
In the new study, however, researchers reveal vulturine guineafowl to be a "striking exception," according to a statement from the Max Planck Institute of Animal Behavior. The birds organize themselves into highly cohesive social groups, the study's authors report, but without the "signature intergroup aggression" common among other birds that live in groups. And they achieve this with a relatively small brain, which is reportedly small even by avian standards.
"They seemed to have the right elements to form complex social structures, and yet nothing was known about them," says lead author Danai Papageorgiou, a Ph.D. student at the Max Planck Institute of Animal Behavior. Faced with a dearth of research on this species, Papageorgiou and her colleagues began investigating a population of more than 400 adult vulturine guineafowl in Kenya, tracking their social relationships across multiple seasons.
By marking and then observing each bird in the population, the researchers were able to identify 18 distinct social groups, each of which contained 13 to 65 individuals, including multiple breeding pairs plus various solo birds. These groups remained intact throughout the study, even though they regularly overlapped with one or more other groups, both during the day and at their night roosts.
The researchers also wanted to learn if any of the groups were preferentially associating with each other, a hallmark of a multilevel society. To do that, they attached GPS tags to a sample of birds in each group, giving them a continuous record of every group's location throughout the day. This generated data that could reveal how all 18 groups in the population with interacting.
The results showed groups of vulturine guineafowl were associating with each other based on preference, the researchers say, as opposed to random encounters. The study also found that intergroup associations were more likely during specific seasons and around specific locations in the landscape.
"To our knowledge, this is the first time a social structure like this has been described for birds," says Papageorgiou. "It is remarkable to observe hundreds of birds coming out of a roost and splitting up perfectly into completely stable groups every single day. How do they do that? It's obviously not just about being smart."
The discovery of a multilevel society in vulturine guineafowl suggests this form of social organization might be more common than we thought, researchers say. (Photo: Sumeet Moghe [CC BY-SA 4.0]/Wikimedia Commons)
We already know birds aren't as simple as their brain sizes might suggest. Not only do many birds perform impressive cognitive feats — like using or even making tools — that seem too advanced for them, but research suggests many birds have significantly more neurons packed into their brains than do mammalian or even primate brains of the same mass.
And now, according to the authors of the new study, these small-brained birds are challenging what we thought we knew about the evolution of multilevel societies. Not only have vulturine guineafowl achieved a format of social organization once thought to be uniquely human, but their long-overlooked society suggests this kind of phenomenon may be more common in nature than we realized.
"This discovery raises a lot of questions about the mechanisms underlying complex societies, and has opened up exciting possibilities of exploring what is it about this bird that has made them evolve a social system that is in many ways more comparable to a primate than to other birds," Farine says in a statement. "Many examples of multilevel societies — primates, elephants and giraffes — might have evolved under similar ecological conditions as vulturine guineafowl."