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 elder (Somateria mollissima borealis) colonies in Kongsf jord, Svalbard (79 degrees 55'N, 12 degrees 10'E). The density of bree ding elders differed considerably from year to year and also among col onies. 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 b reeding density were 1.6 and 2% in 1989 and 1990, respectively. Parasi tic eggs were laid in clutches of all sizes according to their availab ility. The parasitizing females laid 70% of their eggs during the host 's laying period. Parasitic laying occurred despite the constant avail ability of suitable nest sites. The hosts did not reduce their clutch size in response to egg parasitism. The rate of egg predation was sign ificantly higher in parasitized than in nonparasitized nests. Parasiti c eggs were laid late in the breeding period, suggesting that birds wh ich were immature, in poor body condition, or had previously failed ar e salvaging some reproductive effort.