The common eider differs from many other ducks in being a colonial 'capital' breeder, producing eggs from stored resources. These traits are expected to influence the occurrence of conspecific brood parasitism (CBP), which is particularly common in waterfowl. We analysed CBP in an eider population in the central Baltic Sea 2001-2002, using non-destructive egg albumen sampling combined with protein fingerprinting. This technique greatly increases the detection of parasitic eggs compared to more traditional methods. Parasitic eggs occurred in 20-22% of 164 nests studied, 6% of 754 eggs being laid by other than the host female. Parasitism increased with nest density, was rather evenly distributed over the laying season, and occurred both early and late in the laying sequence of the host. Protein fingerprinting showed that host females laid up to seven eggs, more than previously reported. Among 33 parasitised nests 22 had one parasitic egg, nine had two and two had three. In all but one case all parasitic eggs within a nest were laid by the same female. Although colonial breeding facilitates CBP, it is less frequent in this eider population than in several other diving ducks. Possible contributing reasons are the relatively small clutch size and start of incubation after egg 2 or 3, limiting the time window for successful parasitism.