Female sociality in the common eider duck during brood rearing

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Abstract
This study investigates the potential adaptive significance of the behavior of female common eider ducks (Somateria mollissima) which have no young of their own ("aunts") but accompany other females and young. "Aunts" exhibited ambivalent aggression and protection toward ducklings. There was no evidence that the presence of "aunts" enhanced the survival of the ducklings they accompanied. Both females which had their clutch removed and females which had not laid eggs behaved as "aunts." The level of circulating prolactin hormone in "aunts" was lower than in incubating females and similar to the basal level of males. We therefore rejected the hypothesis that maternal care was misdirected during hormonal adjustment from breeding to nonbreeding. We suggest that "aunts" which have lost a clutch or brood, spend some time feeding away from the colony, and then return to the nesting grounds after replenishing their nutrient reserves. Upon returning, "aunts" may select sites for nesting in future years. During this time, they are temporarily attracted to broods. We propose that "aunts" flock and fend off predators during an attack in an attempt to seek safety in a flock rather than to protect ducklings of other females.