Pre-hatch brood amalgamation(intraspecific nest parasitism) was studied for three years (1991-1993) in a colony of common eiders (Somateria mollissima) breeding near Churchill, Manitoba. The amalgamation rate was highest (42.4% of nests) during the year with the highest nest density and good environmental conditions, and was lowest in the year with low nest density and poor conditions (20.2% of nests). Over the nesting season, foreign eggs were laid at the same time as normally laid eggs. Most foreign eggs were laid while the attendant female was laying her first and second eggs. The majority of the foreign eggs were the first or second eggs produced by the non-attendant female, i.e. they were not laying their entire clutch in other birds' nests. One hypothesis explaining the evolution and maintenance of pre-hatch brood amalgamation in birds is egg adoption by nesting females. This hypothesis appears to be the most likely mechanism in explaining the prevalence of foreign eggs in this population. In nests where a foreign egg was laid, before, or on the same day, as the attendant female initiated her clutch, the probability that the attendant females' first egg successfully reached incubation were significantly higher than in nests which did not contain foreign eggs. There is a high rate of partial clutch predation of first and second laid eggs, since nesting females do not attend their clutch until their second or third laid egg. Egg formation is most likely limiting clutch size in eiders, as there are no subsequent costs to incubating or raising extra young. I propose that female eiders perceive nest sites that contain foreign eggs as good quality (low predation risk) nest sites and in the process of choosing these nest sites adopt the foreign eggs at no cost to themselves.