Birds have ‘culture’. Just look at these nests.

In the thorny acacia trees of the Kalahari Desert, bird construction crews are hard at work. Black-browed sparrow-weavers, a social bird species, build complex roosts and nests from grass; they hang dozens scattered across their small territories from one to a few trees. Yet not all of these woven, tubular structures appear to follow the same blueprint. They vary in shape, proportion and size.

“The first thing we noticed when we saw the birds in real life is that groups build differently [from one another]”, say Maria Tello-Ramosa biologist and former researcher at St. Andrews University in Scotland. The roosts and nests of some groups were short, almost cylindrical spheres of desiccated vegetation. Other groups collected tall and boomerang-like structures, such as horns of plenty made of hay. Others still set up roosts dangling somewhere in the middle. Structural idiosyncrasies seemed to remain consistent within a territory.

Tello-Ramos, who would soon take up a lectureship at the University of Hull in England, had come to the Tswalu Kalahari Reserve in South Africa to study the unique social building behavior of sparrow weavers. She wanted to discover how multiple birds work together to achieve a common goal, but now a new question emerged: how come groups living close together (sometimes just a few meters apart) have such different, but consistent architectural styles showed? ?

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The answer proved elusive and did not emerge clearly from the many observations and measurements that Tello-Ramos and her colleagues collected. Once you’ve crossed all the obvious possibilities off the list, you need to consider something new. By a process of elimination, in a study published on August 29 in the magazine Sciencethe scientists present their best theory about what’s going on with the desert sparrow weavers. The new research suggests that the variegated structures are the product of culture, or the “intergenerational transmission of behavior that is not genetic,” as Tello-Ramos defines it. “I really think social learning and social interactions can explain the variation,” she says.

Black-browed Sparrow Weavers live in groups of two to fourteen birds. Each group consists of a dominant breeding pair and then mainly offspring that stick around year after year to help their parents. Occasionally an unrelated intruder may join. Sometimes individual birds fly through the coop and move to get away from family in another group.

In these stable but flexible flocks, which can last more than ten years, the birds defend their territory, forage and build together. Each sparrow weaver spends the night in a separate, woven roost and the eggs of the breeding pair are raised in similarly constructed nests. A group of a dozen birds may have 30 to 40 structures that they have built within their territory. They all take days to complete, multiple weavers (up to eight) collaborate on each project, and new structures are added regularly, especially during the rainy season when the grasses are resilient and flexible, Tello-Ramos says.

In ornithology, nest variation is often attributed to a combination of environment and genetics. Species are limited in what they create by their past and their environment. For example, coastal birds that have never had enough twigs and trees in their habitat hatch their eggs on grass turf, rather than in elaborate tree baskets, explains. Vanya Rohweran ornithologist and curator of the bird and mammal collection at the Cornell University Museum of Vertebrates, who was not involved in the sparrow weaver research. “A lot of that is limited by evolutionary history.” Things like temperature are another important factor when it comes to both inter- and intra-species variability, he adds. Birds in colder environments build larger, thicker, and better insulating nests than their warm-weather counterparts.

The new study introduces a third possible variable: bird tradition. Tello-Ramos and her collaborators collected detailed observations of 43 different groups of white-browed sparrows living within an area of ​​about two kilometers. Each group averaged about twelve members, and in total the birds built hundreds of structures throughout their territory. The scientists measured 444 of those structures, documenting the length of the entrance and exit tubes, the diameter of those openings, the overall length and other factors.

They found that length and diameter varied significantly more between groups than within groups, even over two years of observation. Some groups’ bunks were as much as 8 inches longer than others. And most importantly, that difference “is repeatable – they keep doing it,” Tello-Ramos says. “It wasn’t a one-off. It was like, ‘No, this is our thing. This is what we do. We build long tubes and they build small ones.’” When a new bird joined a new group, it seemed to quickly adopt that group’s predominant architectural style to match the neighborhood.

To try to determine why that is the case, the researchers compared temperature, wind speed, distance to neighbors, bird size, genetic relatedness and tree height between the groups. All told, these variables could only explain less than three percent of the trends they saw, leaving the remaining 97 percent of the mystery unsolved. “I was very impressed by the number of alternative explanations they explored,” says Rohwer Popular science. “I can’t really argue with their data,” he adds.

Instead of providing a clear answer, the researchers turned to the scientific literature on social species. Previous research has documented regional accents in birdsong And socially learned foraging behavior. Other animals, such as whales and primates, are also known to exhibit traits and behavior that they have learned from their peers. And some studies have shown that birds look to others when building nests. In experiments with captive zebra finches, researchers have found that individuals are more likely to select building materials match the nests of their peers than sticking to their own initial preferences.

“People are not the only ones who build and they are not the only ones who have culture,” says Tello-Ramos.

Combining the new observations and measurements with this prior knowledge, the study authors write: “Cultural transmission seems the most likely explanation for our results. Birds copy the building behavior of other group members.”

“It’s a new perspective on what can influence nest-building behavior in birds and it was exciting to see,” says Rohwer. “They are definitely planning something.” Yet the research also leaves some loose threads. “These findings are really interesting, but they raise a lot of questions,” he adds.

For example, Rohwer noted that it is not clear how architectural style is determined and passed on within a group. (More research is needed to determine the mechanism of transmission, Tello-Ramos agrees, and she hopes to begin that soon.) Rohwer would also like to know whether the age of a group has anything to do with the stylistic changes, as some Weaver bird species adapt their strategies as they grow older. He is also curious to see how nest architecture varies over greater distances within the sparrow weaver’s range.

Furthermore, the study has some limitations. Precisely measuring a messy nest is difficult, Rohwer emphasizes. The correlation values ​​the researchers found between group and structure variation, indicating consistency, are “not surprising,” he notes. And even if cultural nest building applies to black-browed sparrow-weavers, it may not be an applicable framework for understanding other bird species. “The vast majority of bird nests are built by a single individual,” he says. So many species may not exhibit the kind of rigid, architectural group traditions across generations that the new research suggests.

Still, “I just feel quite humbled by these kinds of discoveries,” Rohwer says. “Here’s something that’s right in front of us, we’ve always looked at it from one perspective, and maybe there’s more to it than that.”