Group formation and the evolutionary pathway to complex sociality in birds

Group-living species show a diversity of social organisation, from simple
mated pairs to complex communities of inter-dependent individuals
performing specialized tasks. The advantages of living in cooperative
groups are well understood, but why some species breed in small
aggregations while others evolve large, complex groups with clearly
divided roles is unclear. We address this problem by reconstructing the
evolutionary pathways to cooperative breeding across 4730 bird species. We
show that differences in the way groups form at the origin of cooperative
breeding predicts the level of group complexity that emerges. Groups that
originate through the retention of offspring have a clear reproductive
divide with distinct breeder and helper roles. This was associated with
reproductive specialization, where breeders invest more in fecundity and
less in care. In contrast, groups formed through the aggregation of
unrelated adults are smaller and lack specialization. These results help
explain why some species have not transitioned beyond simple groups, while
others have taken the path to increased group complexity.