Vulturine guineafowl (Acryllium vulturinum), a species of large terrestrial bird from East Africa, live in large, multi-male, multi-female groups that associate preferentially with specific other groups, both during the day and at night-time communal roosts, according to a new paper published in the journal Current Biology.
Groups of vulturine guineafowl can become very large, and when multiple groups come into contact the number of birds moving together can reach into the hundreds. However, when these ‘super-groups’ eventually split, they do so back into their original stable group units, meaning that individuals are knowledgeable about who is part of their group and who is not. Image credit: Danai Papageorgiou.
Animal societies can be organized in multiple hierarchical tiers.
Such multilevel societies, where stable groups move together through the landscape, overlapping and associating preferentially with specific other groups, are thought to represent one of the most complex forms of social structure in vertebrates.
For example, hamadryas baboons live in units consisting of one male and one or several females, or of several solitary males, that group into clans. These clans then come together with solitary bachelor males to form larger bands.
Because such social structure means that individuals have to track many different types of relationships at the same time, the assumption has long been that multilevel societies should only exist in species with the intelligence to cope with this complexity.
While many bird species live in groups, these are either open, lacking long-term stability, or highly territorial, lacking associations with other groups.
Vulturine guineafowl, however, present a striking exception: they behave highly cohesively without exhibiting the signature intergroup aggression that is common in other group-living birds. And they can manage this despite having a relatively small brain, even relative to other birds.
“They seemed to have the right elements to form complex social structures, and yet nothing was known about them,” said lead author Danai Papageorgiou, a PhD student at the Max Planck Institute of Animal Behavior.
Papageorgiou and colleagues tracked social relationships in a population of over 400 adult vulturine guineafowl in Kenya.
The researchers individually marked all birds in the population, and by observing them they discovered that the population comprised 18 distinct social groups (with 13 to 65 individuals in each).
What struck the team is that these groups remained stable, despite regularly overlapping with one or more other groups both during the day and at night-time roosts.
To see if these groups preferentially associated with one another, the scientists attached GPS tags to a sample of individuals in each group.
This meant that the position of every single group was recorded continuously each day, which allowed the authors to simultaneously observe how all 18 groups in the population were interacting.
The researchers found that groups associated with each other based on preference, rather than random encounters, and also showed that intergroup associations were more likely to take place during specific seasons and around particular physical features in the landscape.
“To our knowledge, this is the first time a social structure like this has been described for birds,” Papageorgiou said.
“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.”
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Danai Papageorgiou et al. 2019. The multilevel society of a small-brained bird. Current Biology 29 (21): 1120-1121; doi: 10.1016/j.cub.2019.09.072
