Conspecific brood parasitism is an alternative reproductive tactic in which parasitic females lay eggs into the nests of other females of the same species who carry out the subsequent parental care. Conspecific brood parasitism is reported to be common among birds, however detailed information about rates of parasitism based on molecular identification of alien eggs at the population level is still scarce. Here, we used protein fingerprinting of egg albumen to identify eggs laid by parasites in a breeding population of Common Pochard Aythya ferina. Among 42 nests sampled during 3 field seasons, 162 eggs out of 432 (37.5%) were classified as parasitic. Conspecific brood parasitism occurred in total in 39 nests (93%). Average clutch size was about 10 eggs. However, after exclusion of parasitic eggs, the clutch size decreased to about 6 eggs. The number of parasitic females ranged from 1 to 7 and the mean number of parasitic females per clutch was 2.53 ± 0.28. We found a significant negative correlation between the number of parasitic eggs within a clutch and the timing of breeding — parasites laid most of the eggs before the estimated peak of initiated nests. In addition, number of parasitic eggs was negatively related to number of host eggs in the clutch. This observation implies that cost of excessive parasitism might cause clutch size adjustment in the host but other possible explanations are discussed.