In a study of adult Common Eider (Somateria mollissima) females in northeastern England, the annual mortality rate increased by 1 percentage point every 4 years and doubled over the study period (1962–1995). All age classes of adult females experienced years of low mortality, but in years when the overall mortality was high, the oldest age groups suffered three times the mortality rate incurred by the younger females. Over 58% (n = 95) of all deaths of adult females each year occurred in a 60-day period starting in late May, just after the majority had hatched their eggs. Some of this mortality could be linked to the failure to build up large enough food reserves to meet both the costs of producing the eggs and then the stress of starving during the 26-day incubation period. However, this “capital” breeding strategy and variations in the availability of food in the period before laying did not totally explained the large year-to-year fluctuations in mortality, which varied by up to 33% between consecutive years; it also did not explain the variation in age-related deaths. The causes of variations in the mortality rates of Common Eiders may be related to compromised immunity developed during incubation, increasing the risk of death from viruses and helminth parasite infections.