Spatial structure and parental aggression in eider broods

The spatial position of young animals within a brood affects their survival, so that marginal individuals are at greater risk of predation. Spatial brood structuring may be caused by differences in offspring size, age, hunger, or active parental manipulation through aggression. Nepotistic manipulation of brood structure would confer fitness benefits for parents accepting nondescendant young. However, insufficient kin recognition has often been considered to preclude such nepotism in birds, particularly in precocial waterfowl. We explored the spatial structure of ducklings within broods of eiders, Somateria mollissima, a seaduck with frequent brood amalgamation. We compared the distribution of ducklings of different origin relative to reference females whose kinship to the ducklings was known. We also observed female aggression towards ducklings, to evaluate the role of parental manipulation of brood structure. We found a nonrandom distribution of ducklings within broods; a female’s own young were on average closer to her than unrelated young were. We also found evidence for parental nepotism: whether the brood contained unrelated young was the strongest predictor of female aggression towards ducklings. The spatial position, hatch weight and relative size of ducklings showed no significant correlations with each other, suggesting that active parental manipulation may be needed to explain the observed spatial structure. Our study conflicts with previous anecdotal evidence suggesting that brood amalgamation in eiders results in the disintegration of parent–offspring bonds, preventing parental exploitation of nondescendant young. It also opens up the possibility that the spatial position of ducklings depends on their mother’s status in the female dominance hierarchy.