New study published : The importance of sociable weaver colonies to the surrounding animal community

14 Jun 2021 - 12:00
Sociable weaver colony in a camelthorn tree (Photo: Anthony Lowney)

A new study by Anthony Lowney and Robert Thomson demonstrates the importance of sociable weaver colonies to the surrounding animal community was recently published in the Journal of Animal Ecology.

Sociable weavers build some of the largest bird-built structures in the world. These colonies consist of multiple nesting chambers, and some of the larger colonies contain over one hundred chambers up to 500 sociable weavers. However, many other species have also been recorded using these structures, these include: Kalahari tree skinks that are found in greater numbers on trees containing a colony than on trees without, cheetahs that climb to the top of a colony to use the roof as a vantage point, and African pygmy falcons who throughout their southern Africa rely solely on sociable weaver nest chambers for breeding.

The different species may gain different benefits from weaver colonies. Nest chambers are insulated against the extreme ambient temperatures providing a thermal refuge against the high summer and cold winter temperatures. Soil directly below a colony is highly enriched with nutrients from the weaver droppings and this could have knock-on effects for the surrounding vegetation and any of the animals that feed on this vegetation. The colony structures themselves create a platform that tree climbing mammals and large birds can use as a safe refuge. These structures are also constantly maintained, meaning that they can remain in the landscape for many years and larger, older colonies may act as landmarks for the local animal community.

Species that alter the availability of resources in such a way are often called "ecosystem engineers" and although many ecosystem engineers have been identified, birds are often overlooked. This is surprising as birds build nests that come in many shapes and forms, from nests underground that alter the local vegetation structure and complexity to large communal nests that likely provide resources for the species that gravitate towards them. However, identifying ecosystem engineers is only the beginning, and the real challenge is to how determine how big an impact these species have on the wider community and if this changes through time and/or space.

Anthony Lowney and Robert Thomson set out to test the impacts of sociable weavers on the surrounding animal community and discovered that they were used by many more animals than they originally thought. This study revealed that weaver colonies create localised biodiversity hotspots with more species using trees with colonies than trees without and that the importance of these structures to other animals did not change over time. Colonies were equally as important during the harsh times of the year as they were during times when there was plenty of resources. Colonies were used by large mammals for shade, to forage on vegetation nearby and territorial behaviours that include scent marking, horn rubbing and fighting). These mammals included cheetahs, kudu and jackals. Tree climbing mammals including wild cats, mongooses, and genets, each used the top of the colony as a platform as a place to rest, forage and scent mark.

Eighty-nine percent of colonies that we surveyed also contained birds of a different species. These included acacia pied barbets, ashy tits, and scaly feathered finches, who all used weaver nest chambers for roosting, and pygmy falcons that used nest chambers for breeding and roosting. Furthermore, outside this particular study we also found red-headed finches breeding in weaver chambers, spotted eagle-owls breeding on top of colonies, and barn owls nesting inside colony cavities.

These findings show that sociable weaver colonies play an important role in structuring the surrounding animal community and that this results in year-round biodiversity hot-spots. As climate change advances, the importance of these structures may increase, especially to those species that use them as a refuge as a refuge against harsh ambient temperatures.