The evolution of cooperative behaviour is a fundamental and persistent problem in evolutionary biology. Natural selection is expected to select for selfish behaviour because individuals should seek to maximise their own reproductive success to leave more offspring in the next generation. Therefore, when individuals cooperate in joint tasks, there is inevitably conflict over how hard each should work: every individual would like others to work harder and to do less themselves. Hence, in social groups, cooperative behaviour will be vulnerable to exploitation by selfish individuals, or freeloaders. Public goods games capture the essence of the paradox: groups composed of cooperating individuals should out-compete groups of selfish individuals, but selfish individuals do better than cooperative ones within groups. This social dilemma is known as the tragedy of the commons and it is found in virtually all biological systems, including human societies, where over-exploitation of common resources such as fish stocks or failure to curb carbon emissions are obvious examples. An extensive theoretical literature shows that selfishness replaces cooperation under most conditions, but that cooperative behaviour may be maintained under certain circumstances. These include: 1. When non-cooperating individuals are punished or coerced into cooperation through a system of enforcement. 2. When individuals must achieve a good reputation through cooperation in order to maximise their own fitness. 3. When cooperation benefits relatives so that cooperators gain fitness benefits through kin selection. The aim of this study is to use the unique communal breeding system of the sociable weaver of southern Africa to test these alternative hypotheses for resolution of a tragedy of the commons. The sociable weaver builds massive communal nests in which a single nest mass provides roosting and breeding sites for up to several hundred birds. The communal nest has two distinct but integrated components: a thatched dome that has a supportive and thermoregulatory function; and individual nest chambers embedded beneath the thatch in which pairs or small groups of individuals roost and breed. A key feature is that the dome is not a self-organising structure because it is not an emergent property of investment in individual nest chambers. Thus, colony members must invest time and effort in building and maintaining the communal dome as well as their own nest chamber. How do sociable weavers resolve the conflict among individuals over cooperative investment and avoid a tragedy of the commons that would result in social collapse? This study will first determine the individual contributions made by colony members to public goods, and determine whether this effort is costly. The function of the public goods will also be quantified by relating the thermoregulatory properties of the thatch to its size and the position of nest chambers. Finally, and most importantly, this study will use field observations and experiments to test whether a tragedy of the commons is averted by cooperating with kin, through enforcement of cooperative work by other colony members, or because cooperative work is a signal to other colony members of an individual's quality with associated benefits to an individual's own reproductive success. Previous studies of sociable weavers show that the programme of research is tractable and the project's objectives achievable. The study will be conducted on an established study population of sociable weavers at Kimberley, South Africa, in collaboration with the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town. Because of the ubiquity of the tragedy of the commons in biological systems, the findings of this study will have broad relevance across a wide range of disciplines.