Parrot politics: social decision-making in wild parrots relies on both individual recognition and intrinsic markers.

Dominance hierarchies are generally thought to form over time via memory of repeated interactions. Yet dominance hierarchies are also occasionally reported in species with fission-fusion social dynamics, where individuals may encounter large numbers of individuals, leading to incomplete social information. In these cases, three potential mechanisms have been proposed. First, the complex decision-making required could increase selection for social cognition. Second, so-called 'badge-of-status' could evolve as shortcuts. Third, mixed strategies could evolve that rely on memory for interactions with familiars and status signals for strangers. Here, we test these hypotheses in wild sulphur-crested cockatoos (Cacatua galerita), recording social associations and aggressive interactions of 411 individuals across three neighbouring roosts. We find cockatoos use a twofold social strategy when initiating or reacting to aggression. For familiar individuals, aggressions were initiated or responded to based on differences in dominance rank. However, when facing less familiar individuals, decisions to interact-or respond-were based on body weight, with interactions directed towards, and more likely to respond to, individuals of similar weight. Our results suggest that social knowledge remains an important determinant of aggressive interactions in fission-fusion systems, but that individuals can dynamically incorporate other cues of competitive ability when knowledge is lacking.