Patterns of intraspecific nest parasitism in the High Arctic common eider (Somateria mollissima borealis)

Intraspecific nest parasitism was studied during two breeding seasons at two common eider (Somateria mollissima borealis) colonies in Kongsfjord, Svalbard (79°55′N, 12°10′E). The density of breeding eiders differed considerably from year to year and also among colonies. Parasitic eggs were identified through deviations from a normal laying pattern. Parasitic eggs (8%) were laid in 16% of the clutches at the dense colony. The corresponding values on the island with low breeding density were 1.6 and 2% in 1989 and 1990, respectively. Parasitic eggs were laid in clutches of all sizes according to their availability. The parasitizing females laid 70% of their eggs during the host's laying period. Parasitic laying occurred despite the constant availability of suitable nest sites. The hosts did not reduce their clutch size in response to egg parasitism. The rate of egg predation was significantly higher in parasitized than in nonparasitized nests. Parasitic eggs were laid late in the breeding period, suggesting that birds which were immature, in poor body condition, or had previously failed are salvaging some reproductive effort.