Cooperation and population dynamics in the Sociable Weaver | FitzPatrick Institute of African Ornithology

Our long-term project at Benfontein Reserve uses Sociable Weavers Philetairus socius and their fascinating social behaviour to study questions such as whether individuals that are more cooperative are preferred as partners, or whether sociality contributes as a buffer against the extremes of the Kalahari climate. Extreme weather has been leading to natural disasters, and fires have become more frequent in our study areas. A new line of research is investigating the social and demographic effects of natural catastrophes.

The Sociable Weavers at Benfontein have not been going through their happiest times. While the population was still recovering from a very destructive fire in 2021, another fire entered the study area in September 2023. One more colony was destroyed, adding to 9 others that had burned down in 2021 and bringing the population, which is part of a long-term study led by Rita Covas and Claire Doutrelant, to ca. 50% of what it was previously. In addition, the fire caused great damage to a set of sophisticated automatic feeders that had been built with great dedication by PhD student Babette Fourie. This led her to postpone an ambitious experiment where selected PIT-tagged birds open feeders to a group of birds or only to themselves, artificially creating ‘pro-social’ or ‘selfish’ individuals. The experiment is part of an European Research Commission (ERC) funded study that aims to assess whether individuals that are more cooperative are preferred either as sexual or social partners.

Despite its consequences, the fire created an interesting new line of investigation into how animals cope with natural catastrophes that cause sudden changes in the environment. MSc student, Mariana Sousa, used data collected pre- and post- fire to describe how the destruction of colonies caused by the 2021 fire led to a ca. three-fold increases in colony size at the surviving colonies, with an associated increase in levels of aggression. The massive immigration led also to a marked change in genetic structure at the colonies, which in turn influenced social structure. Mariana is now further exploring these results. The effects of the major stress and increased levels of aggression caused by the fire are also being studied by new post-doc Sophie Dupont, who will investigate how the fire influenced telomere attrition and survival.

PhD student, Marta Marmelo joined the project to study whether individuals differ in their propensity to cooperate and whether they help with different tasks or tend to specialise. This will help us to understand whether cooperation is a reliable trait that others can use when making decisions about who to associate with. As part of her PhD, Marta started a study of mobbing in Sociable Weavers by presenting models of snakes and Pygmy Falcons Polihierax semitorquatus. A different cooperative behaviour, nest building, was studied by PhD student Nicolas Silva, who found that male sociable weavers, and especially older males, prefer to build with larger straws, but females show no preference. These results are part of his first PhD paper, and part also of larger projects studying nests as extended phenotypes (French Research Agency funding) and the role of partner choice on the evolution and stability of cooperation (ERC grant).

Activities in 2023

PhD student Babette Fourie built a sophisticated system of automatic feeders based on RFID technology to manipulate feeding behaviour and associations.
PhD student Marta Marmelo, is investigating reliability of helping behaviour and initiated the study of mobbing behaviour in Sociable Weavers.
Post-doc Sophie Dupont initiated a study of telomere dynamics in Sociable Weavers and will investigate how telomere attrition was influenced by the 2021 fire.
Detailed data collection on nest building was conducted throughout the year and will be used to understand how the communal nest built by Sociable Weavers, one of the largest structures built by any animal, is achieved. These detailed data will allow us to better quantify benefits and costs of nest building.
Data collection on social associations (feeding and roosting) was conducted during the non-breeding season for the third consecutive year. Collecting data outside the breeding period gave us crucial new insights into social structure and benefits of group membership in this species.
Advances in video analysis led by Liliana Silva are now allowing the automatic extraction of data from videos of nest provisioning. .

Highlights

A BBC crew filmed the sociable weavers at Benfontein as part of a new series on parental behaviour.
Rita Fortuna published one more paper from her 2022 PhD in Oecologica on how egg components and offspring survival vary with group size and laying order.
The existing long-term data on sociable weavers was included in a global study, published in PNAS, investigating the effect of climate change on avian offspring production.
Liliana Silva has established a pipeline for automatic video analyses that allow to automatically identify individuals and the behaviours performed in ca. 20% of the time taken with manual analyses. .

Impact of the project
The long-term nature of this project allows unique insights into the evolution of cooperation and the mechanisms that allow it to persist. The demographic data allow for examination of the factors affecting population dynamics, provide a baseline against which to study environmental change and natural disasters, which are increasing in frequency under climate change, and provide insight into whether and how sociality mediates the response to a changing environment.

Key co-supporters
European Research Council (ERC); French Research Agency (ANR); Portuguese Foundation for Science and Technology (FCT)

Research team 2023
Dr Rita Covas (FIAO, UCT and CIBIO, U. Porto)
Dr Claire Doutrelant (FIAO, UCT and CNRS, France)
Dr Fanny Rybak (U. Paris-Sud, France)
Dr Pietro D’Amelio (CNRS, France)
Liliana Silva (CIBIO, U. Porto)
Dr André Ferreira (U. Zurich, Switzerland)
Dr Sophie Dupont (CNRS, France)

Students: Babette Fourie (PhD, Porto and Montpellier); Marta Marmelo (PhD, Porto); Nicolas Silva (PhD, Montpellier); Mariana Sousa (MSc, Porto).

Research Assistants: Franck Théron, Lesedi Moagi Carlotta Bonaldi, Paola Stefanini, Shiara Covenden, Leo Jhaveri, Antoine Grissot, Jana Rau, Anne Cillard, Rozenn LeFur, Justin Jacobs.

Sociable Weavers build and maintain their nests communally throughout the year, but who builds, where and when, and - more interestingly - why, remains an elusive question. We are now recording nest building behaviour throughout the year to address these questions (Photo: Maxime Loubon).

Two Sociable Weavers feed at one of the project's feeding stations. These stations are equipped with RFID antennas that record the PIT-tags carried by the birds on a leg ring. The data obtained allows us to study social associations in this species and answer questions such as whether birds that cooperate more have a better social position in the colony (Photo: Cecile van Steenberg).

A devastating fire at Benfontein reserve in September 2021 burned down most of the Sociable Weaver colonies in our long-term study area. Franck Théron, who has worked on the project since 2013, stands next to a scorched Vachellia erioloba that used to house 45 pairs of Sociable Weavers (Photo: Tanguy Deville).