Explaining the evolution and maintenance of reproductive helping and cooperation in wild animals have challenged both theoreticians and empiricist biologists. The theory of kin-selection have successfully explained the highly cooperative societies of euso cial insects, but has also provided insights into the cooperative breeding exhibited by several vertebrates. Some species have more transient relationships, only cooperating during the breeding season. The common eider (Somateria mollissima) is such a spe cies were about half of the females abandon their young to the care of other females. Many of the brood tending females care for additional young, alone or together with other mothers forming a créche. The creche may be joined by helping females that assi st in defending the young against predators. We wish to use mtDNA sequencing and microsatellite typing to assess whether the cooperating females in a creche are relativesand if brood tending females ad opt related young. and if brood ten ding females ad This study will be the first to examine possible kinship between cooperating waterbirds, and one of the first to examine kinship in non-territorial species.