Sociable Weaver

A Sparrow-Sized Bird, a Six-Meter Nest

At 14 centimeters tall and just 24 to 32 grams, the sociable weaver lives in the semi-arid acacia savannas of southern Africa’s Kalahari. Colonies of these little brown birds build the biggest nest structure made by any bird on Earth: one shared, haystack-like mass with a roof of twigs and chambers of dry grass that can stretch more than 6 meters wide and close to 3 meters tall, packed with over 100 separate chambers and home to as many as 400 birds. The whole thing can weigh several tons, heavy enough to bring down the tree holding it up. Some of these mega-nests have stayed occupied, and constantly added to, for more than a hundred years.

Built-In Air Conditioning

The nest isn’t built for size alone. Researchers White, Bartholomew and Howell found that easing the impact of low outside temperatures on the birds is one clear job it does. In a follow-up study during a Kalahari summer, they measured air outside a nest swinging from 16°C to 33.5°C, while the temperature inside an occupied chamber moved by only seven or eight degrees, staying inside the birds’ comfort zone the whole time.

Headcount is the thermostat. The main way a colony keeps its chambers in that comfortable band is by changing how many birds sleep in each one at night.

The engineering has limits. Straw spikes guard the tunnel entrances, but even the largest nests don’t stop a Cape cobra in the warm season, and one snake may eat the eggs and chicks in every chamber of a large nest.

A Cooperative With Hundreds of Roommates

A single nest can shelter a colony of five to more than 300 birds. Males who help build it tend to be more closely related to the colony members around them than males who don’t build at all, and chicks raised with extra helpers survive better than chicks raised by their parents alone. Helpers cut both ways for the breeders: females breeding with more helpers survive better themselves, but males with more helpers actually survive worse. About four-fifths of a weaver’s diet is insects, especially harvester termites, with grass seeds making up the rest, and most never take a drink, getting all the moisture they need from their food. Weavers are relatively long-lived: the oldest one on record made it to 16.

Landlords of the Kalahari

Other animals move in too. The pygmy falcon, which also preys on the weavers, roosts and nests nowhere else. Kalahari tree skinks are more abundant in trees with a weaver nest than without one, take refuge in the nest when threatened, and even eavesdrop on the weavers’ own alarm calls to know when trouble is near.