Social bonds and rank acquisition in raven nonbreeder aggregations.

Complex social life has been characterized as cognitively challenging and recently, social relationships such as long-term social bonds and alliances have been identified as key elements for brain evolution. Whereas good evidence is available to support the link between social relations and cognition in mammals, it remains unsatisfying for birds. Here we investigated the role of avian social bonds in a nonbreeder aggregation of ravens, Corvus corax, in the Austrian Alps. We individually marked 138 wild ravens, representing approximately half of a population that uses the area of a local zoo for foraging. For 2 years, we observed the dynamics of group composition and the birds' agonistic and affiliative interactions. We identified two levels of organization: the formation of an unrelated local group and the individuals' engagement in social bonds of different length and reciprocity pattern. Whereas belonging to the local group had no significant effect on conflicts won during foraging, the individual bonding type did. Birds that engaged in affiliative relationships were more successful when competing for food than those without such bonds. Bonded birds did suffer from aggression by other bonded birds and, probably as a consequence, most of the ravens' social relations were not stable over time. These results support the idea that social bonding and selective cooperation and competition are prominent features in nonbreeding ravens. Proximately, bonding may qualify as a social manoeuvre that facilitates access to resources; ultimately it might function to assess the quality of a partner in these long-term monogamous birds.