Conspecific brood parasitism (CBP) is an alternative breeding tactic that occurs in many brood-tending animals and can have important fitness effects for both host and parasite. We use protein fingerprinting of egg albumen to distinguish the eggs from different females and to estimate the frequency, pattern and tactics of CBP and other forms of mixed maternity in a Hudson Bay population of common eiders (Somateria mollissima sedentaria). Mixed clutches, containing eggs from more than one female, occurred in 31% of the 86 nests studied that progressed to clutch completion. Other females than the host laid 8% of the eggs. In 11 (41%) of the mixed clutches another female laid before the host started laying, corroborating the hypothesis that takeover of nests started by other females accounts for many of the mixed clutches in this population. Our results also indicate that traditional non-molecular methods of identifying foreign eggs may considerably underestimate the frequency of mixed clutches.