In the thorny acacias of the Kalahari Desert, bird-building teams are hard at work. White-browed weavers, a gregarious species of bird, build complex roosts and nests out of grass – dozens hanging from one or a few trees in their small territories. But 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 person was that the groups built differently [from one another]”, says Maria Tello-Ramos, a biologist and former research fellow at St. Andrews University in Scotland. The roosts and nests of some groups were short, almost cylindrical balls made of dried vegetation. Other groups built long and boomerang-like structures, like cornucopias made of hay. Others even built roosts that dangled somewhere in the middle. Structural peculiarities seemed to remain the same within a territory.
Tello-Ramos, who will 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 pygmy weavers. She wanted to find out how several birds coordinate to achieve a common goal, but now a new question arose: Why do groups living in close proximity to one another (sometimes just a few meters apart) exhibit such different but consistent building styles?
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The answer proved elusive—it did not emerge clearly from the many observations and measurements that Tello-Ramos and her colleagues made. Once you have eliminated all the obvious possibilities from the list, you have to consider something new. By process of elimination, a study published August 29 in the journal Sciencethe scientists present their best theory about what's going on with the desert sparrow weavers. The new research suggests that the colorful patterns are the product of culture, or the “transmission of behavior across generations that is not genetic,” as Tello-Ramos defines it. “I really think social learning and social interactions could explain the variation,” she says.
White-browed weavers live in groups of two to 14 birds. Each aggregation consists of a dominant breeding pair and then mostly offspring that stay year after year to help their parents. Occasionally an unrelated intruder joins. Sometimes individual birds leave the cage and move to another group to get away from their family.
In these stable but flexible groups, which can last for more than a decade, the birds defend their territory, forage for food and build together. Each weaver woodpecker spends the night in its own woven roost, and the breeding pair's eggs are raised in similarly built nests. A group of a dozen birds may have built 30 to 40 structures in their territory. Each one takes several days to complete, several weavers (up to eight) help with each project, and new structures are added regularly, especially during the rainy season when the grass is springy and flexible, Tello-Ramos says.
In ornithology, nest variation is often attributed to a combination of environment and genetics. Species are limited in their diversity by their past and their environment. For example, wading birds that never had an adequate supply of branches and trees in their habitat incubate their eggs on depressions in the sand rather than in complicated tree baskets, explains Vanya Rohwer, an ornithologist and curator of the bird and mammal collection at Cornell University's Vertebrate Museum, who was not involved in the research on the sparrow weaver. “A lot of this is limited by evolutionary history.” Things like temperature are another important factor when it comes to variability within and between species, he adds. Birds in colder environments build larger, thicker and more insulating nests than their counterparts in warm climates.
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 weavers living in an area about two kilometers in size. Each group had an average of about 12 members, and in total the birds built hundreds of structures in their territory. The scientists measured 444 of these structures and documented the length of the entrance and exit tubes, the diameter of these openings, the total 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' roosts were up to 20 centimeters longer than others. And, more importantly, this difference “is repeatable – they do it over and over again,” says Tello-Ramos. “This was not an isolated case. It was as if they said, '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 the group's predominant building style – thus assimilating into the neighborhood.
To find out why this might be the case, the researchers compared temperature, wind speed, distance to neighbors, bird size, genetic relatedness, and tree height between groups. Overall, these variables could only explain less than three percent of the observed trends—the other 97 percent of the puzzle remained unsolved. “I was really impressed by the number of alternative explanations they examined,” says Rohwer. Popular science. “I can’t really disagree with their data,” he adds.
With no clear answer, researchers turned to the scientific literature on social species. Previous research has documented regional accents in bird song and socially learned foraging behavior. Other animals, such as whales and primates, are also known to display traits and behaviors learned from their peers. And some studies have shown that birds take cues from others when building nests. In experiments with captive zebra finches, researchers have found that individuals are more likely to select building materials that match the nests of their peers than to stick to their own initial preferences.
“Man is not the only one who builds culture, nor is he the only one who has culture,” says Tello-Ramos.
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 copy the building behavior of other group members.”
“This is a new way of looking at the influences on nest-building behavior in birds and it was exciting to see,” says Rohwer. “They're definitely onto something.” However, the study also leaves some questions unanswered. “These results are really, really interesting, but they also raise a lot of questions,” he adds.
For example, Rohwer noted that it is not clear how building style is established and transmitted within a group. (More research is needed to determine the transmission mechanism, Tello-Ramos agrees, and she hopes to start doing that soon.) Rohwer would also like to know if the age of a group has anything to do with the stylistic changes, since some weaverbird species adapt their strategy as they age. He is also curious to see how nest architecture changes over longer distances within the pied weaver's range.
The study also has some limitations. Measuring a messy nest precisely is difficult, Rohwer points out. The correlation values the researchers found between group and structural variation that suggest consistency “are not mind-blowing,” he notes. And even if cultural nest-building practices hold true for white-browed 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, group architectural traditions across generations that the new research suggests.
Still, “discoveries like this fill me with humility,” says Rohwer. “Here we have something that was right in front of us, we've always looked at it from a certain perspective, and maybe there's more to it.”
