This project examines the importance of Sociable Weaver Philetairus socius nests to Kalahari animal and plant communities. The objectives are to investigate the diversity of animals associated with the nests, the interactions between these species, and to gain insights into the life histories of associated species. We also aim to understand how the ‘ecological engineer’ potential of these nests may have community-wide impacts on structure and function, and how this impact may change across environmental gradients.

Evidence of the importance of facilitation in communities has accumulated, challenging the traditional emphasis on negative species interactions in ecology. In particular, facilitative interactions are predicted to increase in importance in stressful environments and may become a crucial component of the adaptive responses of communities under stress due to global change. Ecological engineers – species that modify habitats and ameliorate abiotic stress for other species – are a key research focus. Identifying and understanding the impact of ecological engineers is vital, especially in arid environments that are expected to become harsher due to global climate change.

Pygmy Falcons Polihierax semitorquatus are the most controversial user of Sociable Weaver colonies. They never construct their own nests and depend entirely on weaver colonies, which is a unique obligate nesting association. Pygmy Falcons also, albeit rarely, prey on weaver nestlings and even adults, suggesting a semi-parasitic relationship between the species. We study the natural history and ecology of Pygmy Falcons and assess whether the falcons provide benefits to the weavers.

As colonial breeders, Sociable Weavers bring material back to their nest trees in the form of faeces, feathers, and carcasses. This nutrient input results in weaver nests being islands of fertility in the landscape. We study how this alters the soil chemistry, as well as soil nematode and plant communities. We further investigate the effect of this fertile island on host tree seedlings, host tree productivity and the potential costs of supporting such a huge nest.

Activities in 2023

  • The 13th season of detailed monitoring of Sociable Weaver colonies and the individually marked Pygmy Falcon population at Tswalu Kalahari was completed. The 2023 breeding season featured the latest nest initiation dates of Pygmy Falcons since the monitoring started in 2011. This was also the poorest breeding season on record with only eight pairs breeding (out of ~40), and only six nestlings were ringed.
  • Data collection towards the Kalahari Endangered Ecosystem Project (KEEP), a collaboration with WITS, UP, UNISA and UWC researchers, continued. These long-term data will contribute to identifying the impacts of global change on the Kalahari ecosystem.
  • Robert Thomson and PhD student Olufemi Olubodun hosted a film crew from NHK (Japanese national television) on Tswalu Kalahari in December. The crew is shooting a full-length program for their Darwin’s Amazing Animals series that explores the ecosystem engineering role of the Sociable Weaver and activity of other animals at their nest colonies.


  • Timothy Aikins Khan submitted his thesis for examination in February 2023 and graduated with his PhD in December 2023. He has since returned to Ghana where he works as a Lecturer at the University for Development Studies in Tamale.
  • Olufemi Olubodun published his first PhD chapter in the Journal of Ornithology. This study examined the variation in some breeding parameters of the pygmy falcons over a ten-year period between 2011 and 2020.
  • Anthony Lowney and Robert Thomson published a paper in Ibis that investigated the cost and benefits to Sociable Weavers of hosting Pygmy Falcons.
  • Timothy Aikins Khan published two papers from his PhD thesis. The first paper, published in Journal of Arid Environments explored the “islands of fertility” created by savanna trees and how these are amplified by Sociable Weaver colonies. The second paper was published in Plant Ecology. Timothy established  that all savanna islands of fertility are not equal but colonial birds influence soil nutrient stoichiometries with consequences for tree seedling growth. Both papers were co-authored by Prof. Michael Cramer and A/Prof. Robert Thomson.

Impact of the project

This project provides unique insights into the community ecology and between-species interactions in the Kalahari. It highlights fascinating natural history stories and brings attention to this unique system. We quantify the ecological engineering role of the Sociable Weaver and determine the potential role of Sociable Weaver nests in a warming and increasingly arid Kalahari. The outputs of this project also contribute to eco-tourism information to enhance the experience of visitors to landscapes within the distribution of the Sociable Weaver.

Key co-supporters

DSI-NRF CoE grant; Tswalu Foundation; University of Cape Town launching grant; Suzuki South Africa.

Research team 2023
A/Prof. Robert Thomson (FIAO, UCT)
Dr Anthony Lowney (U Hartpury, FIAO Research Associate)
Prof. Michael Cramer (Biological Sciences, UCT)
KEEP team (led by Prof. Andrea Fuller and Prof Graham Alexander, both WITS)

Students: Timothy Aikins Khan (PhD, UCT); Olufemi Olubodun (PhD, UCT).