Title: Socioecological drivers of complex social structure in an avian cooperative breeder
Description: 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.