Birds can be picky building their nests. They experiment with materials, waffle over which twig to use, take them apart and start again. It’s a complex, fiddly process that can seem to reflect careful thought.
“It’s so fascinating,” Maria Tello-Ramos, a behavioral ecologist at the University of St. Andrews in Scotland, said. “But it hasn’t been studied much at all.”
New research led by Dr. Tello-Ramos, published on Thursday in the journal Science, provides the first evidence that groups of birds that build their homes together learn to follow consistent architectural styles, distinct from groups just a few dozen feet away.
The finding upends longstanding assumptions that nest building is an innate behavior based on the birds’ environment and adds to a growing list of behaviors that make up bird culture.
As important for survival as nest building is, scientists know relatively little about it. Most of what is known about bird nests has come from studying their role in reproductive success, focusing on their usefulness in protecting birds and eggs from cold, wind and predators.
“The focus has been on the structure, not the behavior that built it,” Dr. Tello-Ramos said. She said she found that surprising because nest building is one of the rare behaviors that has a tangible product, something that can be measured and provide insight into why birds behave the way they do.
Part of the reason nest-building behaviors haven’t been researched much, Dr. Tello-Ramos said, boils down to one cliché: bird brain.
Nest building is such a complex behavior that, for decades, scientists thought “the little brains of birds couldn’t possibly deal with such a large amount of information, so it must be innate,” she said. Recent work has shown birds repeating others’ nest building, but those studies were often limited to individuals or small groups in labs.
White-browed sparrow weavers, sociable brown-and-white birds with a “grass-centric” lifestyle, provided Dr. Tello-Ramos and her colleagues the perfect opportunity to study nest building. The birds spend almost all their time together and previous findings hinted that they built nests cooperatively. They are also prolific builders, communally crafting up to a dozen temporary roosts for sleeping each year in addition to several nests for eggs. That gave researchers an opportunity to track how individuals and groups built over time.
Dr. Tello-Ramos and her colleagues spent months at a time observing 43 groups of sparrow weavers in the South African part of the Kalahari Desert, tracking their building progress. The groups were relatively close to each other, separated by as much as a mile and as little as 30 feet.
Over the study period, the birds built more than 400 roosts and nests. They went into a building frenzy when it rained. Everyone pitched in gathering material, ferrying it to the builders, tucking the grass into place and pushing against the structure to solidify it.
At a glance, the roosts look like haphazard piles of straw, but they are, in fact, carefully woven grass domes. There’s a cozy central space and separate tube-shaped passages for entering and exiting. (In nests for eggs, the exit passage is replaced by an egg nook.)
The researchers measured the dimensions of the nests and roosts. They also analyzed factors classically thought to determine nest shapes: climate, the size of the birds and tree height. They even compared the genetics of the groups to see if closely related birds built similar nests.
The results were clear. “Birds that live together build together, and they have distinctive architectural styles,” Dr. Tello-Ramos said. Groups consistently built in the same style across generations. If new birds arrived, they conformed to the group’s architectural style.
The biggest differences were in the length of entrance and exit passages, which varied by as much as four inches. That may sound negligible, but it’s significant for the small birds. And, more important than details of the architectural differences are two facts: The nests were built cooperatively and families stuck to their styles.
None of the classical explanations, like weather, explained the variability across groups and the consistency within groups.
“We propose that this is due to social learning and culture,” Dr. Tello-Ramos said. “Once they started, everybody followed, and now they have a certain tradition.”
The main question now is how the birds do it. “How do they operate?” Dr. Tello-Ramos said. “How do they transmit the information?” The findings also invite questions about intelligence. “What do these birds understand about the nests they’re building?”
Iliana Medina, a behavioral ecologist at the University of Melbourne in Australia who was not involved in the study, agreed, but added: “It’s tempting to assume that there’s some kind of elevated cognitive ability behind this, but there might not be. They might have no idea what they’re actually doing.”
Nonetheless, Dr. Medina said it was “really exciting” to see how a group of birds could come together to build something and then transmit that information culturally.
“That there can be cultural transmission for a behavior that was thought to be completely innate means there’s much more flexibility and a whole set of skills involved,” she said.