A white-browed sparrow weaver inspects a roost under construction, after just receiving some grass brought by another member of its group.
Maria Cristina Tello-Ramos
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Maria Cristina Tello-Ramos
Neighboring groups of birds within the same species can create very different-looking nests — showing that their nest-building choices aren’t solely controlled by instinct and the environment.
Instead, these birds seem to learn rules for nest-making that get passed down within a family group from generation to generation.
That’s the conclusion of researchers who studied nearly 450 grassy structures built over two years by white-browed sparrow weavers living in the Kalahari Desert in South Africa.
These small, brown-and-white birds live communally, and it turns out that groups of birds that lived quite near to each other nonetheless built distinctive architectural forms, according to a new report in Science.
The finding suggests that when people look up and see a nest in a tree, it might not be the product of innate behavior alone.
“It might be that we’re seeing a tradition,” says Maria Cristina Tello-Ramos, a researcher who did this study while working at the University of St. Andrews.
Learning by example
Scientists already knew that groups of animals, including birds, can form their own cultures, in the sense that important information gets transmitted through social learning rather than just genetics.
Birdsongs, for example, can have regional “accents,” and birds also look to their elders to learn about foraging and migration, says Tello-Ramos.
Lab studies have suggested that the same can be true for nest-building.
Experiments show that inexperienced male zebra finches will watch familiar males choosing nest-building materials and then use that information to modify their own choices, says Tello-Ramos.
“If before they preferred a pink ribbon, rather than an orange one, then they will prefer the orange one if they see others using the orange,” she says.
She wanted to see how social learning might affect the nest building of white-browed sparrow weavers. These highly social birds live in extended families of two to 14 individuals.
“They do everything together. They forage together. They defend the territories together,” says Tello-Ramos.
Intriguingly, she and her colleagues noticed that trees that were home to one extended family often had nests that looked quite different from the nests that belonged to their neighbors in another tree.
The long and short of it
This bird species builds grass structures that serve two purposes: They can be turned into a safe spot to incubate eggs, but they’re also shelters where a single bird can safely roost inside at night. One tree will be dotted with many such structures.
Building each one is a communal activity. The birds weave grass into a tube, which ends up being shaped basically like an upside-down letter “U.”
When it’s used to incubate eggs, one end of the tube gets sealed up to form a cup. But when it’s used as a nighttime roost, the tube has an entrance at one end and an exit at the other.
“And what we saw is that in some of the families, the birds build roosts with very long tubes,” says Tello-Ramos.
Other families, meanwhile, build short tubes.
“They do it consistently,” she says. “Families that are maybe ten meters apart are building different things.”
To learn more, she and her coworkers measured every single structure built by 43 family groups over two seasons. They analyzed everything from the length of the tube to the tube’s width to the angle between the U-shaped tube’s “arms.”
Then they checked to see what might account for the observed differences, carefully considering factors like temperatures, wind speed, and the number of birds in each family.
They couldn’t find any explanation for why the birds were building such different structures.
And since it’s known that these birds are highly social, says Tello-Ramos, it seems probable that family members are just copying each other, creating a culture of nest-building that gets passed down again and again and again.
Family is everything
Sometimes a family will take an outsider under their wing and let it join the group. These outsiders seem to adopt the ways of their new home.
Researchers know this because bird families that incorporated a lot of outsiders nonetheless maintained consistent nest-building traditions.
“What I found most fascinating was the fact that when a new sparrow weaver joins the group, it doesn’t bring the cultural transmissions of its old group,” says Catherine Sheard, an evolutionary biologist at the University of Aberdeen in Scotland, who studies diversity in bird nests but wasn’t part of the research team. ”That wouldn’t have been what I predicted.”
Humans transmit a lot of information to each other through social learning, notes Sheard, “but there’s sort of this old-fashioned belief that animals don’t do that at all and it’s purely genetic, or you build things out of whatever’s laying around.”
As this study shows, that’s just not true, she says. And in addition to learning from other birds, birds can learn from their own experiences.
Lab studies show that if they build a nest and fail to successfully raise a chick, says Sheard, they’ll try a different nest-building material the next time. Birds that enjoy reproductive success, however, will stick with the nest-making materials that worked.
All of this may be part of why, even within a single species, individual birds can construct nests that look quite different — a kind of architectural diversity that often goes unrecognized, says Sheard.
“I feel like when we draw illustrations and show nests to kids, we show the most beautiful, sort of prototypical nest,” Sheard says. “But actually birds will do a lot of very strange things.”