Aggressive females seize central positions and show increased vigilance in brood-rearing coalitions of eiders

Decreased risk of predation is a key benefit of group living, and selfish herd theory predicts competition for the relative safety of central positions. Spatial position also affects behaviour, as individuals trade-off their feeding and vigilance to mitigate the risk of predation. Female eiders, Somateria mollissima, often pool their broods and share brood rearing, and anti-predatory vigilance is a core parental care activity. Females are assumed to trade-off vigilance and feeding, as efficient recovery of condition is crucial for energetically stressed postincubating females. Evidence shows that a female's own ducklings are closer to her than unrelated young in amalgamated broods and that predation of ducklings by gulls is edge biased, so a female's spatial position should correlate with the survival prospects of her brood. We identified determinants of female spatial position (aggression, structural size, body weight, age, clutch size) within multifemale brood-rearing coalitions, and explored whether spatial position affected female activities. Female aggression frequency was the only significant predictor of centrality in brood-rearing coalitions, and female age was positively associated with aggression. Investment in vigilance, but not investment in feeding, increased with spatial centrality; instead, central females devoted less time to other activities (resting, preening and moving). We conclude that central eider females do not need to trade-off vigilance and feeding. Heavy body weight did not guarantee a favourable position in brood-rearing coalitions, which may help explain our previous finding that female eiders in best body condition when their brood hatches tend their brood alone.