Long-term stability of dominance hierarchies in a wild parrot with fission-fusion dynamics

Dominance hierarchies allow animals living in stable groups to limit the costs of fighting over access to resources. However, little is known about whether hierarchies are maintained in more open, fission-fusion societies, where individuals interact with large numbers of individuals. We recorded social associations and aggressive interactions in a highly social, communally roosting parrot, the sulphur-crested cockatoo (Cacatua galerita). By following 515 individuals across three neighbouring roosts and recording social interactions during foraging, we show that sulphur-crested cockatoos form clear linear hierarchies. Hierarchies are primarily roost based, but also incorporate birds from outside the roost. We find that an individual's dominance rank is affected by sex and age, with males ranking higher than females and adults ranking higher than juveniles. Finally, we find despite highly dynamic social associations (fission-fusion dynamics) among individuals, hierarchies are stable, with dominance ranks being highly repeatable across years. This study demonstrates that stable and/or closed group membership are not pre-requisites for dominance hierarchies to emerge. ### Competing Interest Statement The authors have declared no competing interest.