Sociality represents one of the major evolutionary transitions in life: a transition from individuality to societies as complex as our own. Animals exhibit a wide variety of societies, from temporary aggregations, such as schools of fish, to complex organizations where individuals maintain stable relationships, such as cooperative groups in birds, mammals, and eusocial ants. However, the diversity of social systems and the mechanisms driving the formation of complex societies remain unclear. In this dissertation, I aim to understand a less known complex social system, a colonial cooperatively breeding bird on the African savannah, grey-capped social weavers (Pseudonigrita arnaudi). I monitored daytime and nighttime relationships among more than 600 birds in over 100 colonies across three subpopulations for five breeding seasons using an auto-tracking system. In Chapter 1, I examine the social organization of this species and test the role of kinship in the organization of the society. In Chapter 2, I focus on the inter-annual variation in the fission-fusion process of colonies and test how social factors (i.e., group membership and colony membership) and ecological conditions (i.e., rainfall) may underlie individual and group decisions about settlement of colonies. In Chapter 3, I investigate the conflict in the form of infanticide behavior (i.e., egg tossing) in this social system and test nest predation as an alternative hypothesis against a presumed hypothesis that group conflict is the cause of the tossing behavior. In sum, my dissertation provides the first detailed study of a colonial and cooperatively breeding bird with high resolution of movement at the individual level. Understanding a new social system will not only expand our knowledge of the variety of animal societies, but will also give insight into how social complexity has evolved, including our own.

Cooperatively breeding societies, in which one or more non-parental individuals (“alloparents”) care for young alongside the parents, show considerable variation in social structure. Traditionally, such societies have been thought to comprise small, kin-based family groups where offspring from previous broods delay dispersal and help raise closely-related offspring to gain indirect fitness benefits when independent breeding opportunities are unavailable or yield lower fitness outcomes. However, genetic evidence is increasingly revealing cooperatively breeding species whose social groups comprise unrelated individuals as co-breeders or alloparents or both (for e.g., 45% of all avian cooperative breeders). Such social groups exhibit complexity in social structure such as large group size, multiple breeders, and low and varied group kin structure. To understand why such complex societies form and how are they maintained when the opportunity to gain indirect benefits via kin selection is low and variable, I investigated the direct and indirect benefits driving a key demographic process, dispersal, and the resulting variation in group social structure on the individual, group, and population levels in an obligate, avian cooperative breeder, the superb starling (Lamprotornis superbus). I used a combination of long-term, individual-level data spanning 15 years from nine groups monitored at the Mpala Research Centre in Kenya and fine-scale genetic and environmental data sampled across 22 social groups that included the long-term study population. In Chapter 1, I show that (i) dispersal decisions in superb starling males are driven by temporal environmental variation experienced by their parents pre-laying, (ii) both dispersal and philopatry result in equivalent lifetime inclusive fitness outcomes, and (iii) oscillating selection due to high temporal variability in the environment likely maintains the two alternative dispersal tactics, resulting in the formation of mixed-kin groups. In Chapter 2, I show that (i) immigrants are vital to the stability of superb starling social groups in light of low and variable offspring recruitment in a harsh, unpredictable environment, (ii) plural breeding likely arises as a result of reproductive concessions provided by group members as joining incentives to recruit immigrants, and (iii) despite smaller groups providing more reproductive concessions, immigrants gain higher fitness in larger social groups and thus prefer to immigrate into larger groups which are found in higher-quality territories. Finally, in Chapter 3, I find genetic signatures of directional dispersal from social groups in low- to high-quality territories across an environmental gradient which likely generates considerable within-population variation in group social structure. Overall, my dissertation underscores the importance of direct benefits derived from group augmentation in the formation and maintenance of cooperative social groups with a complex social structure in a harsh and unpredictable environment.