Birds
Sociable Weaver
Philetairus socius · also called social weaver
Drive through the Kalahari and you will see trees that seem to be wearing haystacks. Those are the nests of the sociable weaver, a sparrow-sized bird that builds, together, the largest nest of any bird in the world, and keeps it going for a century.
Overview
Philetairus socius lives in the dry interior of southern Africa, in arid savanna with scattered large trees. It eats mostly termites, ants, and beetles and gets nearly all its water from food.
The largest bird nest on Earth
A colony builds a single communal thatch structure up to 6 m across and 3 m deep, weighing a tonne or more, with over a hundred nest chambers. Generations add to and repair it, and a nest can stay in use for 50 to 100 years.
An apartment block
Each chamber has a downward entrance lined with sharp grass to stop snakes, and the deep chambers stay warm on cold desert nights. The nest also houses pygmy falcons, which almost always nest in weaver colonies, plus finches, lovebirds, and small reptiles and mammals. Pairs breed cooperatively, helped by grown offspring.
Status
Least Concern and common, but colonies are lost when a nest tree collapses or catches fire, and heat waves have caused mass die-offs at some colonies.
Sociable Weaver: common questions
How big is a sociable weaver nest, really?
Enormous by bird standards. A well-established colony nest can be six metres wide and three metres deep, roughly the size and shape of a small haystack, and weigh a tonne or more when it is full of grass, sticks, and accumulated droppings. It hangs over the crown of a large tree, sometimes bending or breaking the branches, and it is easily the largest nest built by any bird. From a distance the nests look like giant untidy bundles of thatch caught in the trees, and a single big camel thorn can carry one that dwarfs the tree itself.
Why do the birds all nest together in one structure?
Because in the Kalahari it pays to share walls. The desert swings between scorching days and near-freezing nights, and a small bird alone loses or gains heat quickly. Inside the thick, insulating mass of the communal nest, the deep chambers stay much warmer than the outside air at night, saving the birds a great deal of energy, and the outer chambers stay cooler by day. Building the structure is also a shared job spread across many birds and many years, so no single pair pays the full cost. The trade-off is that a big, obvious nest attracts predators, which is why the entrance tunnels are lined with sharp spikes of grass.
What else lives in the nest?
A whole community. The pygmy falcon, Africa's smallest raptor, is almost entirely dependent on sociable weaver colonies for nesting and roosting, using spare chambers and generally tolerating, and being tolerated by, its hosts. Rosy-faced lovebirds, red-headed finches, familiar chats, and other birds also move into empty chambers, and the structure shelters snakes, geckos, and small mammals. The nest is effectively an apartment block that provides scarce shelter in a landscape with very little of it, so it draws tenants far beyond the weavers that built it.
Are sociable weavers affected by climate change?
Yes, and in a way that reveals a weakness in the strategy. The communal nest is superb insulation against cold nights, but it does much less to help on extremely hot days, and sociable weavers cannot pant or sweat efficiently enough to shed heat when air temperatures climb past the mid-40s Celsius. Researchers studying colonies in South Africa have recorded mass mortality events during heat waves, with many birds dying in or around the nest over a few days. As heat waves become hotter and more frequent, this is emerging as a real threat to a species otherwise doing well.