Life on earth has evolved through a series of transitions, each of which involved independent replicating units forming groups that reproduce cooperatively. For example, organelles evolved through the union of different unicellular organisms, multicellularity evolved by cells joining together, and multicellular organisms joined to form societies. Recent research has highlighted that these transitions in complexity have played a crucial role in overcoming ecological barriers at all levels of life, from bacteria resisting antibiotics by forming biofilms to vertebrates coping with harsh environments by living in cooperative societies. This is a significant advance, as it helps explain the way complex life has evolved and how new ecological niches are created and colonised. However, why some lineages make the transition to higher levels of cooperation while other do not, and why some lineage reverse to a less social life is unclear. To resolve this issue this project will focus on the transition from solitary to cooperative living across birds. We will use comparative genomics to test two central ideas: 1. Cooperative breeding can render individuals vulnerable to pathogens as contact rates between hosts are high and genetic diversity in groups is often low. This phenomenon has been termed the ‘monoculture effect’ due to its importance in agricultural settings, but surprisingly there are no tests of how it influences the formation of animal societies. We will test the prediction that cooperative breeding in birds is more likely to evolve from species pre-adapted to coping with higher pathogen pressure. Comparative genomics will be used to test selection on immune genes in species that represent evolutionary transitions to and from cooperative breeding. 2. Cooperative societies are favoured in hot environments so it is unclear why independent breeding is widespread in these habitats. We will test whether the invasion of hot environments by independent breeding species is explained by pre-adaptations to heat stress that are absent in cooperatively breeding species using phylo-geographical and comparative genomic analyses of birds. We will test the prediction that independently breeding species that have invaded hot environments exhibit changes in heat tolerance genes (heat shock proteins: HSPs and heat shock transcription factors) not present in cooperative breeding species.