Pre-hatch brood amalgamation in common eiders: why do eiders adopt eggs?
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.