Genetic evidence for kin-based female social structure in common eiders (Somateria mollissima)

Kin-based social groups are commonly studied among cooperatively breeding species but have been less studied in nontraditional group breeding systems. We investigated the presence of kin-based sociality among females in the common eider (Somateria mollisima), a colonial nesting sea duck that exhibits high levels of natal philopatry in females. Previous studies of female sociality in common eiders have been restricted to observations during brood rearing. However, aggregations of female common ciders are also observed during other periods of the life cycle such as colony arrival and nesting. Here we apply a novel, empirical framework using molecular markers and field sampling to genetically characterize female social groups at several stages of the common eider life cycle. When compared with mean estimates of interindividual relatedness for the entire colony, significantly higher levels of relatedness were found between females within groups arriving to the colony in flight, between females and nearest neighbors at the time of nest site selection, and between groups of females departing the colony with ducklings. Both full-sibling and half-sibling equivalent relationships were also found within these groups. Therefore, throughout each of several stages including in-flight colony arrival, nesting, and brood rearing, we provide the first genetically confirmed evidence of female kin-based social groups in common ciders and anseriformes in general.