That heap of hay in a tree is not a typical animal commune. Huge group nests of sociable weaver birds across southern Africa are about as close as nature gets to building condos.
Ant nests, beaver lodges and many other marvels of animal architecture enclose shared space. But small, sparrowlike Philetairus socius push together beakful after beakful of grass to create a haystack of apartments. The nests can grow to weigh a ton and last about a century. Tunnels opening from the shaggy underside lead to each family’s unit. For better and worse, a weaver bird nest “in practice is like a block of flats,” says evolutionary biologist Rita Covas of CIBIO Research Center at the University of Portugal.
The condos have great insulation, an important perk for birds that don’t migrate from the hot-then-cold Kalahari. In summer, Covas can feel shady relief when she reaches up into a nest. In winter, condos are heated by snuggle power. The thatch keeps a chamber with a lone bird at about 12° Celsius. An apartment crowded with five birds reaches a toasty 33° C, Covas and colleagues reported in June in the Journal of Avian Biology.
An apartment can fill up as a few young birds linger for a gap year before venturing away. The stay-at-homes pitch in to hunt food for the newest nestlings. The help lets parents ease off a bit in foraging, though oddly enough, mom’s life lengthens when her offspring stay close while dad’s tends to shorten. Why is still a puzzle.
Even with help, raising chicks is chancy when a nest offers a feast for marauders. “Those poor birds,” Covas says. “Snakes climb the trees to get into the colony and then they inspect every single chamber — they’re very thorough.” Adult weavers will mob loudly and frantically, but a cape cobra or boomslang ignores them, bingeing on eggs and chicks by the dozen.
The birds can cause trouble for each other, too. “There are lots of chases,” Covas says. Outright murder is rare, but during food shortages, a neighbor on occasion pushes into the chamber next door and kills the chicks for causes still unknown. “Reducing
competition?” Covas speculates. “Spite?”