Are elaborate bird nests built using simple rules?

Determining how animals achieve seemingly complex behaviours is central to our understanding of the evolution of cognition. Complexity in behaviour is itself not sufficient for confirming a role for learning and memory. For example, although the nests some birds build appear structurally complex, they may, like the structures built by ants and termites, be achieved through relatively simple building processes. We attempted to use observations of nest building by male Southern Masked Weavers to determine whether one aspect of nest building, the completion of a nest, could be described usefully by either of two rule-based explanations that explain the construction of complex physical structures by invertebrates. The first possibility, stigmergy, is that birds use feedback from local nest cues to determine the building sequence and nest completion. The second possibility is that birds follow a stereotypical series of behaviours. We found that male Southern Masked Weavers returned to work on apparently complete nests even when they had begun to build a new nest. Neither stigmergy nor stereotypy explains our observations of nest building behaviour in Southern Masked Weavers. If weaverbirds do complete their nests using a set of rules, those rules are not the simple ones that have been used to describe building by invertebrates. This leaves open the possibility that learning and memory play a greater role in nest building than is currently thought to be the case.