Functions and mechanism of recognition systems in social birds

Systems for recognition and discrimination play a critical role in interactions at every level of biological organisation from genes and cells through to individuals and species. For example, immune systems must be able to reliably recognise self from non-self, and females may have to discriminate against related males or males of different species when making decisions about whom to mate with. Similarly, a parent who wishes to invest in their offspring may need to discriminate in favour of their own offspring from among a number of potential recipients. The study of recognition systems is important because the decision-making and discrimination that evolves in these various contexts depends on the effectiveness of recognition. If the costs of making a mistake are very high, we would expect strong selection for accurate recognition and discrimination, but if the costs of such recognition errors are low, then discrimination and recognition would be expected to be much less accurate because of the weak selection imposed on individuals who fail tests of recognition. Therefore, we expect to see variation in the nature and effectiveness of recognition systems according to the context in which that process occurs.

Social organisms provide an obvious case in which the recognition of individuals, kin and other categories of conspecifics is a critical requirement. Indeed, there is abundant evidence for discriminatory behaviour in a wide variety of contexts, often with demonstrable fitness consequences. However, our understanding of recognition systems and hence the ability of social animals to discriminate among different classes of individuals is poor, despite the importance of such mechanisms for defining the constraints on adaptive social behaviour. The aim of this project is to investigate the function and mechanisms of recognition systems at a range of spatial scales and in diverse social contexts using the unique colonial breeding system of the sociable weaver Philetairus socius. The sociable weaver builds an extraordinary communal nest that may house hundreds of birds, that breed either in pairs or in cooperative family groups with helpers assisting their relatives in raising offspring. Dispersal between colonies is limited, and helpers are typically closely related to the breeders that they help. However, the close proximity of kin is also likely to generate a significant risk of inbreeding. At a broader spatial scale, colonies are more or less discrete social groups that exhibit another level of social organisation above that of the family.

These features of the sociable weaver social system make them ideally suited for investigation of recognition and discrimination at a range of social and spatial scales. Specific objectives are to test hypotheses concerning kin discrimination ability in the avoidance of inbreeding and in cooperative investment in brood care by helpers. In each case, we will test whether individuals are able to actively discriminate kin from non-kin, or whether discrimination is based on spatial cues to kinship. We will also use observations and experiments to test discrimination of social groups above the level of family using analysis of social networks. Finally, we will conduct field observations and experiments to determine the development of putative recognition cues (contact calls), and determine the genetic and environmental components of these recognition signals. Through these studies we aim to provide the most comprehensive understanding of the function and mechanism of recognition systems in any social vertebrate.

The primary motivation for the proposed research is to advance our understanding of a fundamental issue in biological systems - recognition systems and their impact on the evolution of adaptive interactions. However, there are three distinct areas where this research is likely to have impact beyond this academic field.

1. The project entails extensive research into the genetic structure of a natural population and the demographic processes generating that structure. In addition to fine-scale processes within a population, the occupation of sociable weaver nests has been described as having metapopulation characteristics, with colonies being founded, abandoned and re-occupied over relatively short periods. Our study will provide important insight into which factors (such as the distance between colonies, colony size, and social cohesion) influence demography, including survival, recruitment and dispersal, of such a metapopulation system. These are fundamental issues in the management of populations, especially those that are fragmented due to population decline and/or habitat destruction. Therefore, the findings of this study will be of broad interest to conservation biologists, who will be invited to the workshops that we will organise in years 2 and 3 of the project (see below, and Pathways to Impact).

2. Sociable weavers are a characteristic species of the arid zone of southern Africa, an ecosystem that is particularly vulnerable to climate change because of the potentially major consequences of minor changes in rainfall and temperature. Indeed, recent evidence suggests that the sociable weaver's distribution is shifting away from the more arid regions of the Kalahari and Namib deserts. To substantiate these shifts and assess the impact of increasing aridification on an arid-zone species, a standardised method of monitoring population sizes is of paramount importance. The sociable weaver's remarkable and conspicuous nests, which may persist over many decades, make this species an excellent candidate for devising such a monitoring system on a large scale. This idea to use sociable weavers to monitor southern Africa's aridification has been discussed with Prof Phil Hockey at the Percy FitzPatrick Institute of African Ornithology and staff of the Department of Tourism, Environment and Conservation, Northern Cape Province, South Africa. Additionally, a weaver monitoring project (PHOtos of Weaver Nests, PHOWN) already exists at the Avian Demography Unit of the University of Cape Town. This will provide an excellent platform into which we will incorporate our detailed monitoring of sociable weavers, facilitating access to a large number of observers. In addition, we will reach this target audience via the workshops to be held at the field site in years 2 and 3 for local researchers, NGOs and policy makers (see Pathways to Impact).

3. The research will contribute to the public understanding of science. Sociable weavers are a charismatic member of southern African fauna and attracts considerable interest from the public. Indeed, we have recently published a popular article about their nests targeted at this audience in the magazine African Birdlife, as well as contributing to broadcast interviews in South Africa. Furthermore, two of the principal objectives of the project concern inbreeding and cooperative behaviour, two topics that have particular traction in the public imagination. The PI and PDRA have a strong track record of engagement with the public in various contexts and would maintain these efforts during the proposed project.