There are benefits and costs for Sociable Weavers and Pygmy Falcons when the two species nest side by side, according to new research.
Sociable Weaver, which is endemic to Southern Africa, builds huge colonial nests. These structures, which can contain hundreds of nesting chambers, make them ecosystem engineers by influencing the behaviour, distribution and movements of many other species.
As well as providing a refuge for birds such as Acacia Pied Barbet and Rosy-faced Lovebird in the harsh savannah conditions, the platforms offered by Sociable Weaver structures are used as nest sites for large birds of prey, and benefit tree-climbing mammals including Leopard. Beneath them, their shade makes life easier for antelopes, porcupines and other animals.
Pygmy Falcon is entirely reliant on Sociable Weaver nest structures in Southern Africa (Alistair Rae via Wikimedia Commons).
Close relationship
Pygmy Falcon has a strong association with weaver structures. In Southern Africa, the species is entirely dependent on them for nest and roost sites, and the bird of prey is not present in areas lacking weaver colonies.
It has been put forward that the falcons may assist the weavers by driving away snakes from the colony, a potentially significant benefit as up to 70% of weaver breeding attempts end in predation by snakes. However, there has been no evidence for this improving productivity, so Anthony Lowney, from Hartpury University, and Robert Thompson set out to understand the implications of having falcon 'squatters'.
The researchers compared snake encounter rates at colonies with falcons to those that had none. They discovered that snake encounters more than halved at colonies with falcons. More snakes were seen at larger colonies, and snakes were almost seven times more likely to be found at colonies while the weavers were breeding.
To test whether the falcons actively kept snakes away, they placed snake stimuli, and control stimuli, at colonies with breeding Pygmy Falcons, finding that the falcons attacked the 'snake' more than the control (especially if they had larger broods). However, when not breeding, the falcons stayed clear.
Similar breeding success
That said, Lowney and Thompson found no difference in weaver breeding success between colonies with nesting falcons and those without. The researchers suggested that this may be explained in part by the fact that falcon and weaver breeding does not always overlap, with the falcons only providing a defensive benefit when they are breeding.
However, the researchers still found no difference in weaver breeding success even when they were nesting at the same time as the falcons, suggesting any snake-guarding benefits the latter offer may be offset by direct predation of weaver nestlings. Pygmy Falcons have been seen feeding on weaver chicks, although this is considered to occur only infrequently.
A reduction in the number of abandoned falcon breeding attempts due to lack of resources hinted that the weavers provide a failsafe food supply for breeding falcons. However, the birds of prey experienced higher nest predation, probably from snakes drawn to the weavers, when their hosts were nesting.
Sharing a colony seems to have costs and benefits for both Sociable Weaver and Pygmy Falcon. The former probably has less trouble with snakes with falcons around, but this is balanced out by the birds of prey taking some young. The falcon has a ready supply of weaver chicks if other prey becomes scarce, and doesn't have to build its own nest, but its young may become the target of snakes drawn to the weaver colony.
The researchers say further investigation is needed to fully unpick the advantages and disadvantages for both species in the ever-changing Kalahari environment.
Reference
Lowney, A M, and Thomson, R L. 2023. Cost and benefits in extreme nesting associations: do sociable weavers benefit from hosting African pygmy falcons. 2023. IBIS. DOI: https://doi.org/10.1111/ibi.13283
