Oh, What a Nest!
Africa's wily sociable weavers are masters of architecture and adaptation
A CAPE COBRA thrusts its yellow head out of a nest in a camel thorn tree, its jaws clenched around two screaming chicks. Leaving another four feet of its body coiled up in the pilfered structure, the snake begins exploring for more victims. Raiding any other bird's nest would count as a mere snack for a mature cobra. But this is the home of the sociable weaver, builder of the largest tree nests in the world.
Here, where hundreds of birds may huddle under one roof, two chicks are but one course in a veritable feast. Before the sun sets on the brick red sand of the Kalahari Desert at the intersection of South Africa, Botswana and Namibia, this reptile will gorge on dozens of eggs and chicks. And when the parents return to an empty roost, they will find yet another reminder that their impressive home is both their sole comfort and their certain curse--a magnet for life in the harsh Kalahari.
For anyone watching a flock of sociable weavers skitter along the sand in search of insects and seeds, it is difficult to imagine that this sparrow-sized, drab brown creature with the black beard could possibly be "one of the ten most interesting birds in the world," as Gordon Maclean, author of Roberts' Birds of Southern Africa, has labeled it. But to lie under one of their sprawling nests and peer up into a honeycomb of a hundred straw chambers held together without mortar of any kind is to behold one of the ornithological wonders of the world. And to study the mousy little bird is to learn some of the natural world's most astonishing lessons of cooperation--a level of overly efficient teamwork that sometimes earns the sociable weaver the wrath of the people who share its Kalahari habitat.
The virtues of the sociable weaver's extraordinary architectural structures are appreciated by other denizens of the Kalahari as well. Lovebirds, barbets, tits and finches warm themselves in the cozy chambers built by the weavers. Vultures, owls, eagles and even geese find the weavers' roof a ready-to-use nesting platform. The pygmy falcon in southern Africa depends entirely on sociable weaver nests for breeding. And to honey badgers, cobras and large-eyed tree snakes called boomslangs, the nest is a dependable larder.
For the weavers, the nest's chief benefit is as an insulator against frigid winter nights. Three American scientists from the University of California at Los Angeles used temperature probes and found that even as the night air plummeted from 63 degrees F to freezing, an occupied chamber in a large nest never fell below 61 degrees. The team, led by physiologist Fred White, calculated that the warm nest cuts the sociable weaver's energy consumption by 40 percent compared to roosting in the open, as most birds do. To burn those extra calories, a colony of 150 weavers with no nest would have to catch and eat 4,500 more insects each day.
Saving energy also helps the birds save water, a resource even more precious than food in the Kalahari, where some areas receive only three inches of rain each year. Sociable weavers need less water, ounce for ounce, than any other bird known to science--not even a teaspoon a day per bird. Most never drink at all, deriving the bulk of their moisture from eating juicy harvester termites, their favorite food.
It is this same dry climate that makes their toasty home possible in the first place. In a wetter environment, the mass of vegetation would decompose. Not here. Many sociable weaver nests have lasted well over a century.
Despite their dropped-haystack appearance, the structures have a sophisticated architecture, with different materials for different purposes. The birds interlock large twigs to form the strong roof and wedge dry grasses into the insulating bulk of the nest. A breeding pair lines the cup inside its nest chamber with soft grass flowers and other fluffies, and uses green grass to mold a firm ridge at the lip of the cup that will keep eggs from rolling out. The tunnel that leads down from each chamber is armed with sharp spikes of straw that make reaching into a nest a painful experience.
Unlike most birds that build temporary nests exclusively for breeding, sociable weavers use and maintain their nests throughout the year. Their nest-building instinct is so strong that when no new chambers are needed, they will fill in a perfectly usable chamber and build a new one below it. The largest nests extend well over 20 feet from side to side and several feet high, containing more than 100 individual chambers. Sometimes, the weight of the giant nests breaks branches and even knocks down entire trees. "Their only problem is they don't know when to stop," notes Nardus Du Plessis, a ranger with the Kgalagadi Transfrontier Park, a vast conservation area jointly managed by South Africa and Botswana.
Even more interesting than the nest itself is the teamwork required to build it. For three breeding seasons, Rita Covas, a Portuguese ornithologist, and her French colleague Claire Doutrelant have regularly visited 18 colonies on a large game farm near Kimberley, South Africa. They probe nests to record the contents of each individually marked chamber and monitor the feeding of chicks from a blind. When Pink-Over-Dark-Green (named for the colored leg bands Covas has affixed) enters Nest 5 in Colony 5, for instance, Covas knows that it is a one-year-old feeding its siblings--a common occurrence.
Cooperation among animals attracts the attention of naturalists because it poses an apparent exception to the idea of natural selection and the belief that each individual's primary goal is to ensure the survival of its own genes. Since the 1970s, evolutionary behaviorists have redoubled the effort to study and explain cooperative behavior in everything from amoebas to hyenas. Somewhere in the stream of data the two ornithologists are gathering may lie the answers to intriguing questions about how and why an individual bird such as Pink-Over-Dark-Green will help another.
The most common form of cooperation among birds is cooperative breeding, when an individual other than mom or dad helps to incubate eggs and feed chicks. About three percent of all bird species are known to use helpers at the nest. The vast majority of these assistants are siblings with a genetic stake in the survival of their brothers and sisters. The simple math of kin selection suggests that ensuring the survival of one brother--who shares half of your DNA--is as good as creating your own chick. Another well-studied reason to feed even unrelated chicks is to prove oneself worthy of a prized nesting site.
In their altruistic behavior, sociable weavers appear to be in a league of their own. Weavers will frequently build a nest chamber to be used by others. Non-siblings will help a breeding pair despite a surplus of nesting space where they could raise their own young. If one pair loses its own brood, the couple may even start providing food for the neighbors' family. "Helping behavior seems to be happening here for some other reason we don't know about," ponders Covas. Are male helpers showing off for females in order to attract a mate in the next breeding season? Are helpers apprenticing in the art of chick rearing in order to improve their chances of successfully rearing offspring in the future?
The ornithologists' search for the answers are complicated by the fact that most of their young subjects end up as snake food--banding rings and all. Doutrelant says that she curses the snakes ("You're eating my data!") whenever she helplessly watches them move through a well-researched colony.
The weavers' primary reptilian enemies are the boomslang and the Cape cobra. Over most of its range in southern Africa, the cobra is considered a ground-dweller, but in the Kalahari, this snake has perfected the technique of keeping its coiled tail end firmly in place in one sociable weaver nest chamber while looping its head into the next. Some boomslangs and Cape cobras appear to live for months on nothing but weaver eggs and chicks, and will even curl up in a nest and make it their home.
Photo: © DON BOROUGHS
SOCIABLE WEAVERS are drab brown, sparrow-sized birds noted for their architectural prowess. Using straw and other natural materials, they build nests that can reach 20 feet across. This one (above, with researchers' colored markers) contains more than 135 chambers and 200 birds.
To illustrate the plight of the weavers, Covas brings up the heartrending story of Blue-Over-Black and Red-Over-Green, a determined pair that waged an eight-month battle of endurance with a boomslang. One year in September, Covas found the pair incubating four eggs in their nest chamber, one of 30 in the colony. Three weeks later, a boomslang robbed the pair of their week-old chicks and cleaned out most of their neighbors as well. Within a few days, the female had laid again, and proceeded to fatten another meal for the boomslang. By April, the pair had cared for eight broods, and not one chick had lived long enough to fly.
No other songbird species is known to lay more than five broods in a year, but the indomitable couple made one last effort for a total of 25 eggs in the season. Unusually late summer rains had allowed them to outlast the boomslang, and on their ninth try, one chick lived long enough to fledge. The next time that young weaver heard the sharp, metallic tip tip tip of an alarmed weaver announcing a snake on the move, it could fly to safety.
Perhaps because of the ordeal of bringing a chick to adulthood, fledglings are spoiled with every possible advantage to ensure their survival. Most live in the same nest chamber with their parents through the first winter, travel to feeding grounds daily with their family's colony and even enjoy free meals from their parents for months after learning to fly. (By contrast, most North American songbirds kick their offspring out of the nest--and the territory--just weeks after fledging.) As a result, Covas has found that sociable weaver juveniles have the same survival rate as adults, despite the youngsters' inexperience. "It's incredible," she marvels, "I know of no other species that does that."
A simpler way to adapt to the threat of snakes is to keep them out of the nest entirely. The key is to find a nesting tree with a long, smooth trunk leading to high branches well out of the reach of any slithering predator. In fact, the ideal nesting tree is not a tree at all; it is a utility pole.
"I'm really gatvol with these birds," grumbles Peet Struwig, using an Afrikaans term that can be politely translated as "fed up." Struwig, the burly head electrician of the Groblershoop District of the South African national electric utility, Eskom, has just spent his morning knocking down weaver nests from power poles. His district's full-time "bird team" drives more than 3,100 miles each month clearing sociable weaver nests that threaten to bridge the gap between conducting wires and the earth wire. And that's the more pleasant part of the job. On stormy nights, when wet nests "flash over" and knock out electricity to farms and towns in the district, he must repair charred poles in the wind and rain.
As a gentle encouragement to the sociable weavers to build away from wires, Eskom and the telephone utility have placed support structures lower on the poles. But the weavers soon outgrow their lower nest and expand their colony back on the crossbars. The birds are even spreading their distribution range south and southwest into treeless plains where they nest exclusively on utility poles.
Struwig proposes killing pole-nesting weavers, but Rudi Kruger, who oversees environmental issues for Eskom's distribution lines, is searching for a less drastic solution. He has a list of 35 possible weaver deterrents--from rubber snakes to ultrasonic waves to wire insulators. Kruger has hired a full-time researcher to experiment with the most promising ideas.
Mortals who try to outwit sociable weavers are up against a formidable adversary. Ornithologist Maclean awards the species an 11 on a scale of 1 to 10 for its ability to adapt to a difficult and constantly changing environment. These wily masters of architecture and adaptation have managed to survive millennia in the hostile Kalahari. They will certainly find a way to survive the vagaries of the twenty-first century, and very possibly, Kruger's 35 deterrents.
Roving editor Don Boroughs has lived in South Africa for the past six years. He is an avid bird-watcher.