The Bird That Builds Nests So Huge They Pull Down Trees

At some point, a tree becomes more nest than tree. That sounds like the kind of proverb that old guy at my local bar would tell me.

My father worked for over 30 years in construction, falling off of ladders and getting slivers of metal in his eye and generally bleeding profusely. He toiled like a maniac so our family could eat, all while furthering one of humanity’s most indispensable inventions: large-scale construction of shelter. From the most modest roof that my dad once nearly tumbled off of, to Dubai’s 2,716-foot Burj Khalifa, the tallest building in the world, nothing builds like a human.

For its size (and lack of opposable thumbs) though, Africa’s incredible social weaver surely comes close. These birds, about the size of the sparrows here in the States, come together in colonies of as many as 500 individuals to build by far the most enormous nests on Earth, at more than 2,000 pounds and 20 feet long by 13 feet wide by 7 feet thick. The structures are so big they can collapse the trees they’re built in, and so well-constructed they can last for a century, according to Gavin Leighton, a biologist at the University of Miami. Occupying as many as 100 chambers, these are quite possibly the biggest vertebrate societies centered around a single structure—outside of human beings and their skyscrapers, of course.

The social weaver with some building material. Or is that a tiny cigar. I can’t tell.

Calling the semi-arid plains of Namibia and South Africa its home, social weavers make use of several different materials, building the nest by weaving in twig after twig. Then they line the insides of the chambers with luxurious grass and feathers and, occasionally, cotton balls that Leighton accidentally drops in the field (perhaps it’s their keen sense of symbolic justice—he uses the cotton after drawing blood from the birds for genetic sampling).

The weavers will pack into the nest’s chambers three or four at a time, and when they do, the benefits of the enormous structure become clear. Winter nights here regularly dip below freezing, even down under 20 degrees Fahrenheit in the coldest places. At one point in his field work Leighton dropped thermal recorders into chambers, which weavers later that day took up residence in. “I think the nighttime temperature was 30 or 35, and the temperature inside the chamber with three or four birds in it was 70 or 75 degrees Fahrenheit,” he said. “So there’s this really huge thermal benefit to staying in these giant nests.” In the baking summer, too, the chambers provide the birds with a fairly tolerable climate both day and night. Far from the top of the nests, which bake in the sun, the chambers enjoy the shade. And the nest as a whole lags behind the ambient air temperature a bit like a swimming pool, whose waters retain the cool of the evening into the morning and the heat of the afternoon back into the night.

Social weavers build entrances to their nests at the bottom, which makes them more inaccessible to predators other than the dreaded tape-measure-handed human being.

The perks don’t stop there. According to Leighton, by situating the chamber entrances at the bottom of the nest, the birds also likely protect themselves from predation by hawks flying above. And this positioning also helps keep water out. (Interestingly, chambers are never connected—each has a single entrance. Leighton reckons social weavers may have long ago started off building typical weaver nests, which are essentially a tube with an entrance at the bottom, then started bundling them into larger structures.) And from an evolutionary perspective, living in such big groups boosts an individual’s chances of not getting picked off by a predator. You don’t have to run faster than the bear to get away, the maxim goes, you just have to run faster than the guy next to you.

The social weaver’s setup is so sweet that other species are more than happy to squat in their digs. African vultures are fond of hanging out on the roof, and red-headed finches will raise their families in the chambers—as do pygmy falcons, birds of prey more than capable of killing their landlords. Indeed, while it’s not common, “it has been documented that pygmy falcons will sometimes eat social weavers,” said Leighton. “Which is kind of a depressing thought, because social weavers are building and maintaining this giant apartment complex, and then a predator moves in and starts eating them.”

A pygmy falcon just chillin’ and looking cool as hell.

“But given that it is very rare for pygmy falcons to take sociable weavers,” he added, “it may be worth the risk to have them around if they intimidate not only snakes but potentially other bird species looking to squat in one of the nest chambers.” Thus do the falcons act as the apartment complex’s doormen of sorts—doormen that may lose their cool and eat a resident every once in a while.

That’s not to say social weavers don’t stand up for themselves. Should a snake wander up the tree or a pygmy falcon land on their roof, they’ll crowd around the menace and fire out high-pitched alarm calls—incessantly. While this may scare away smaller birds, it does little good on falcons and serpents. Curiously, the weavers won’t aggressively take to the air and flock to scare away their enemies, as crows or mockingbirds do. Why that is, Leighton can’t say for sure.

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