From Wildlife Extra:
Why cooperation among sociable weaver birds leads to their amazing nests
A new insight into one of the biggest questions in science – why some animals, including humans, work together to maintain a common good – has been achieved by scientists at the University of Sheffield.
Sociable weavers, a highly gregarious and co-operative breeding bird from the savannahs of southern Africa, build the largest nests of any bird, often weighing tonnes and lasting for decades, and housing colonies of up to several hundred birds.
The massive nests consist of individual nest chambers which are used throughout the year for breeding and roosting and are embedded within a communal thatch.
The thatch covering the nest doesn’t originate from individual chamber building but requires separate investment from colony members to build and maintain it.
As such it provides a public good from which all colony members benefit in terms of buffering extremes of temperature, supporting individual nest chambers and protecting from predators.
The question that researchers from the University’s Department of Animal and Plant Sciences addressed is how sociable weavers work together to successfully build and maintain this public good, while keeping freeloaders at bay.
This is a general problem in such situations because some individuals may cheat the system by benefitting from the public good, without contributing to it.
There are several potential solutions to this problem, one of which is that co-operative behaviour is directed towards relatives.
Dr Rene van Dijk, from the Sheffield research team led by Professor Ben Hatchwell, said: “Our study shows that relatedness between colony members is low, on average, but co-operation over thatch-building is kin-directed, due to the positioning of relatives within nests.
“Sociable weavers do not contribute to thatch building equally, but those that do contribute to it are more closely related to their neighbours within the colony than are non-builders.
“Crucially, related birds are positioned close to one another within nests, so that thatch building investment also benefits their relatives.
“Additionally, relatives visit each other’s nest chambers, suggesting again that the communal benefits are shared among kin.
The study not only demonstrates that the influence of kin selection may stretch beyond that of nuclear and extended family groups thus promoting co-operation in large social groups, but it is also the first study to show that kin selection may promote the communal construction and maintenance of an animal-built physical structure.
Such structures include nests, mounds and burrows.
“This co-operation is similar to how human families may decide to accept a lodger into their home.
“If the lodger isn’t related to the family, he or she may pay rent but otherwise they will not care too much about the upkeep of the house.
“However, if the lodger is a known family member, then you would expect them to maintain the house which he or she may stay in for a longer period and possibly inherit.
“It may seem like a small difference, but it tips the balance towards a more co-operative society.”
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: here.