Social structure, individual fitness and the effect of climate in an obligate cooperatively breeding bird, the white-winged chough (Corcorax melanorhamphos)

Abstract

During my PhD I examined the social structure of white-winged choughs (Corcorax melanorhamphos), a highly social species of Australian bird, which are obligate cooperative breeders, meaning they must breed in groups to successfully produce offspring, living in groups from three to up to 20 individuals. Group living in choughs is related to the young individuals' inefficient foraging, with adults providing care to their young for a long time after fledging. Choughs' reproductive success is highly dependent on food availability for the young, and highly susceptible to environmental changes that limit food supply. During droughts these birds enter a period of social instability with increased mortality, dispersal, alliance formation with unrelated individuals and higher aggression from other choughs. This thesis is the fifth study of white-winged choughs over a 60 year period, but the first conducted with genetic methods after a period of major climatic stability. Two older studies were conducted before molecular methods were available and two more recent studies were both conducted after periods of extreme drought with major consequences for population structure and behaviour. Chapter 1 is a general introduction. Chapter 2 explores choughs' population structure using population genetics tools. This is the first genetic study on choughs conducted during a period of climatic stability, with no major droughts registered before or during the investigation; and also the first using genotyping by sequencing method. I calculated within group relatedness, dispersal patterns and genetic differentiation among groups. I found that choughs have female biased dispersal, which contrasts with previous results that showed choughs did not have sex-biased dispersal. I suggest this difference can be due to the different climatic conditions in which studies were conducted. Chapter 3 focuses on the fitness consequences for helpers and breeders under different environmental conditions. Using demographic data available across five different studies done on choughs under different environmental conditions ranging from 1959 to 2017, I investigated how their populations are affected by different rainfall patterns. Genetic data were available from the most recent three studies allowing sophisticated comparisons of population genetic structure and inclusive fitness benefits of group members. I found fitness benefits for helpers and breeders differ according to the prevailing climatic conditions and that advantages to breeding in larger groups tend to be minimal under drought conditions. Chapter 4 focuses on choughs' personality, and demonstrates they have consistent individual differences in boldness. I asked if these behaviours differ within and between family groups, and the habitat where they breed -suburbs versus woodland. I found that flight initiation distance (FID) was an effective method to measure boldness in the wild in choughs. I also found that personality is more variable among groups than within groups; and that individuals breeding in the suburbs are bolder than those breeding in the woodlands. I discuss how group-level similarity in personality may reflect the importance of social learning and cooperation in a social species. Chapter 5 explores patterns of associations among individuals within the group during the breeding season, using social network analysis. I also explore if individual cooperative levels -feeding the fledglings and staying close to them- are affected by genetic relatedness to the group as an indication of kin selection. I found that in most groups, unrelated adult choughs of the opposite sex show strong associations, and discuss how this probably reflects the importance of inbreeding avoidance within chough groups. I did not find that relatedness was associated with higher cooperative behaviour within groups, suggesting the cooperative behaviours observed would be more related to direct benefits of helping.