Variable social organization and breeding system of a social parrot revealed by genetic analysis

Social organization and contributions to reproduction vary widely within and between species that breed in groups. Such variation often arises from the process of group formation, which drives patterns of relatedness and hence the degree of social conflict and co-operation between group members. Using field observations and molecular genetics, we investigated breeding behaviour in an urban population of the highly social Monk Parakeet Myiopsitta monachus to address two objectives. First, we investigated breeding-group frequency, composition and formation, finding that 19% of breeding units were co-operative groups, ranging in size from three to five birds, the remainder being pairs (81%). Group composition was variable with multi-male, multi-female and multi-male–female groups. Relatedness in breeding groups also varied with many containing kin, but some groups containing only non-kin. This variation reflected alternative routes to group formation, including offspring retention by pairs, sibling coalitions and aggregation of unrelated individuals. Secondly, we investigated productivity, reproductive investment and patterns of parentage in relation to the size of breeding units. Productivity did not differ significantly between pairs and groups. We detected extra-pair paternity in 27% of broods raised by pairs, and parentage shared among more than two members of most breeding groups, with joint-nesting by females detected in multi-female groups. In conclusion, the breeding system of the Monk Parakeet defies simple definition, instead showing variable reproductive roles, with potential for both indirect and direct fitness benefits.