This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Dillon Marsh's photographs of sociable weaver nests, taken in the Kalahari Desert of Southern Africa, beautifully illustrate traditional nature--the realm of wild animals--overlapping with human civilization. The apparent bales of hay draped over the tops and sides of telephone poles are home to hundreds of songbirds, which construct and maintain their monstrous nests communally.
While the merging of human and wild landscapes is powerful, perhaps the bigger oddity here is the nests themselves. Why would birds choose to live in a nest complex at all?
These nests are far more than apartment complexes for sociable weavers (Philetairus socius): they house multiple generations that work together to raise chicks. While most songbird species breed before they even turn a year old, sociable weavers rarely breed before the age of two. Instead, these younger birds help raise other nestlings--their siblings as well as unrelated chicks--by gathering food and maintaining the nest's fluffy interior chambers and external sticks and grass.
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Why would these young birds decide to help out at home instead of raising families of their own? Part of the answer lies in their life history. The desert of Southern Africa is hot and dry, sure, but it's a very stable environment. The birds don't have to migrate or brave severe seasonal changes. The most dangerous time of their lives is when they are a chick in the nest: some 70% of baby sociable weavers are killed and eaten by snakes.
Once they grow up, they live for a long time (up to 10 years!) and mostly survive year-to-year. Long lives make for lots of time to breed, and once they start breeding, it's all they do. Most pairs of sociable weavers lay 2-6 eggs at least 4 times each year (and up to 9 times), and spend a lot of time raising these chicks.
However, while conditions are pretty constant in their habitat, sociable weavers still have trouble finding food. They mostly eat insects and seeds, which vary in abundance depending on the amount of rainfall in the desert--so not very much. A group of researchers from the University of Cape Town wanted to test if food availability affected the the 2-year delay in social weaver first-time breeding. When they scattered bird seed daily to boost food availability and reduce the risks of foraging, more first-time breeders bred early at just a year, and fewer birds stuck around to help out around the nest.
Evolutionarily speaking, this makes sense: a long-lived species would produce more offspring by waiting until they had preferential access to food for their chicks, instead of risking their own lives by trying to raise chicks when there's not enough food around. And, while they wait for their turn to access food, they may as well help out around the nest and ensure that it sticks around, for the present and future.
All photos courtesy of © Dillon Marsh. See the rest of the Assimilation series and his other work at his website.
Covas R., Doutrelant C. & du Plessis M.A. (2004). Experimental evidence of a link between breeding conditions and the decision to breed or to help in a colonial cooperative bird, Proceedings of the Royal Society B: Biological Sciences, 271 (1541) 827-832. DOI: 10.1098/rspb.2003.2652