Sociable weavers (Philetairus socius) build the largest communal bird nests in the world. Native to Southern Africa, they team up to build grass structures with hundreds of individual cavities for raising young. Nestlings are raised by both their parents and helper birds, which are usually but not always related.
These avian societies are far from utopian though – new research published in the journal bioRxiv shows that sociable weavers regularly commit infanticide, the intentional killing of a hatchling or immature animal, in this case of often unrelated nestlings.
“They usually peck the chicks on the head to kill or weaken them, and then throw them out of the nest. Very small chicks they just throw out,” says lead author Rita Covas, a behavioural ecologist and evolutionary biologist at the University of Porto in Portugal.
She and her colleagues analysed video taken between 2014 and 2023 at a set of eleven sociable weaver colonies in Benfontein Nature Reserve, Northern Cape Province, South Africa. They found that 40 different birds made 50 attempts to kill nestling birds. They were successful in 33 cases.
"The intruders usually only manage to attack chicks when no adults from their group are around. If one of the parents or helpers from the group sees an intruder in the nest, it will chase it,” Covas says.
Most of the aggressors were born outside of the breeding group (82.5%) and most were female (59%). Male sociable weavers typically remain in their natal colonies, while females usually disperse to other colonies. "Males have lots of relatives in the colonies where they breed, and hence should have less incentive to commit infanticide, as they would have a higher chance of killing relatives, with whom they share genes," Covas says.
A majority of the killers (62.5%) were not born in the colony they attacked. They did not reproduce in the chambers they attacked or pair with one of the parents – except in one case, over a year later. This suggests that the birds gain no direct reproductive benefit from killing nestlings. That is: they did not clear the nesting chambers so they could set up their own families there.
The researchers hypothesise that the benefits of these infanticides are more indirect. Killing young birds may reduce resource competition – either in the present or in the future, benefiting potential offspring of the killers.
Top image: sociable weaver nest. Credit: Getty
More wildlife stories from around the UK
- In 2020, humans suddenly vanished from a popular bay in Hawai‘i. What happened next was remarkable
- Panama’s ocean upwelling fails for the first time in 40 years. Now scientists are worried
- India's deadliest snakes to become even more dangerous, new study suggests
- 1.1-million-year-old mammoth discovered in Siberian wilderness. What scientists find in its DNA has got them excited