In the thorny acacia trees of the Kalahari Desert, avian construction crews are hard at work. White-browed sparrow weavers, a species of social bird, assemble complex roosts and nests from grass–hanging dozens across their small territories of one to a few trees. Yet not all of these woven, tubular structures seem to follow the same blueprint. They vary in shape, ratio, and size.
“The first thing we noticed when we got to see the birds in person is that groups are building differently [from one another],” says Maria Tello-Ramos, a biologist and former research fellow at St. Andrews University in Scotland. Some groups’ roosts and nests were short, almost cylindrical balls of desiccated vegetation. Other groups assembled long and boomerang-esque structures, like horns of plenty made from hay. Others still raised roosts that dangled somewhere in the middle. Structural quirks seemed to stay consistent within a territory.
Tello-Ramos, soon to start a lectureship at the University of Hull in England, had come to Tswalu Kalahari Reserve in South Africa to study sparrow weavers’ unique social building behavior. She wanted to find out how multiple birds coordinate to achieve a shared goal, but now a new question loomed: Why is it that groups living in close proximity to each other (sometimes just a few meters apart) demonstrated such distinct, but consistent architectural styles?
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The answer turned out to be elusive–not clearly revealing itself from the many observations and measurements Tello-Ramos and her colleagues collected. When you’ve crossed every obvious possibility off the list, you have to consider something new. Through process of elimination, in a study published August 29 in the journal Science, the scientists present their best theory of what’s going on with the desert sparrow weavers. The new research suggests that the motley structures are the product of culture, or the “transmission of behavior across generations that is not genetic,” as Tello-Ramos defines it. “I really do think that social learning and social interactions might explain the variation,” she says.
White-browed sparrow weavers live in groups of between two and 14 birds. Each assemblage is composed of a dominant breeding pair and then mostly offspring that stick around year after year to help out their parents. Occasionally an unrelated interloper may join. Sometimes individual birds fly the coop, and go out to make it away from family in a different group.
In these stable but flexible troops, which can last for more than a decade, the birds defend their territory, forage, and build together. Each sparrow weaver spends the night in a separate, woven roost and the breeding pair’s eggs are reared in similarly constructed nests. A group of a dozen birds might have 30 to 40 structures they’ve built within their territory. Each one takes days to complete, multiple weavers (up to eight) pitch in on each project, and new structures are added regularly, especially during the rainy season when grasses are springy and flexible, says Tello-Ramos.
In ornithology, nest variation is often chalked up to a combination of environment and genetics. Species are limited in what they create by their past and their surroundings. For example, shore birds that have never had a ready supply of twigs and trees in their habitats brood their eggs on divots in the sand, not in complicated, arboreal baskets, explains Vanya Rohwer, an 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 constrained by evolutionary history.” Things like temperature are another major factor when it comes to both inter- and intra- species variability, he adds. Birds in colder environments build bigger, thicker, more insulating nests than their warm-weather counterparts.
The new study introduces a third possible variable: avian tradition. Tello-Ramos and her collaborators collected detailed observations on 43 different groups of white-browed sparrow weavers living within an approximately two kilometer square zone. Each group had an average of about 12 members, and altogether the birds built hundreds of structures across their territories. The scientists measured 444 of those structures, documenting the length of the entrance and exit tubes, the diameter of those openings, total length, and other factors.
They found that length and diameter varied significantly more between groups than within groups–even across two years of observation. Some groups’ roosts were as much as 20 centimeters longer than others. And, more importantly, that difference “is repeatable–they continue to do it,” says Tello-Ramos. “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 little ones.’” When a new bird joined a new group, it seemed to quickly adopt that group’s predominant architectural style–conforming to the neighborhood.
To try to determine why that might be, the researchers compared temperature, wind speed, distance from neighbors, bird size, genetic relatedness, and tree height between the groups. Altogether, those variables could only account for less than three percent of the trends they were seeing–leaving the other 97 percent of the mystery unresolved. “I was really impressed with the number of alternative explanations that they probed and examined,” Rohwer tells Popular Science. “I can’t really argue with their data,” he adds.
In lieu of 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 behaviors. Other animals, too, like whales and primates, are known to display traits and behaviors learned from their peer groups. And some studies have indicated birds look to others in constructing nests. In experiments with captive zebra finches, researchers have found that individuals are more likely to select building materials to match their peers’ nests than to stick to their own initial preferences.
“Humans are not the only ones to build and not the only ones to have culture,” Tello-Ramos says.
Combining the new observations and measurements with this prior knowledge, the study authors write “cultural transmission seems to be the most likely explanation for our results. Birds will copy the building behavior displayed by other group members.”
“It’s a novel perspective on what can influence nest-building behavior in birds and it was exciting to see,” says Rohwer. “They’re definitely on to something.” Yet, the study also leaves some loose threads. “These findings are really, really interesting, but they inspire a lot of questions,” he adds.
For instance, Rohwer noted it’s not clear how building style would be decided and passed on within a group. (More research is needed to establish the mechanism of transmission, agrees Tello-Ramos, and she hopes to start on that soon.) Rohwer would also like to know if age of a group has to do with the stylistic changes, as some species of weaver birds adjust their strategy as they mature. He’s also curious how nest architecture varies over larger distances within the sparrow weaver’s range.
Plus, the study does have some limitations. Taking exact measurements of a messy nest is hard, Rohwer points out. The correlation values the researchers found between group and structure variation indicating consistency “aren’t mind-blowing,” he notes. And even if cultural nest building holds true in white-browed sparrow weavers, it might 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 display the type of rigid, architectural group traditions over generations implied by the new research.
Still, “I just have this feeling of being pretty humbled by discoveries like this,” Rohwer says. “Here’s something that has been sitting right in front of us, we’ve always looked at it from one perspective, and maybe there’s more to it than that.”