The demographic consequences of oil spills or other mortality events for breeding seabird populations depend on the age and sex composition of the affected population. Differential wintering by sex and age classes occurs in many migratory bird species, and can have important conservation implications. We examined the age and sex ratios of Western Grebes Aechmophorus occidentalis and Clark’s Grebes A. clarkii that died in three mortality events in southern, central and northern California. The most significant of these events for grebes was the Ventura Oiled Bird Incident (VOBI), an oil spill in southern California in 2005 in which approximately 2500 grebes were killed. The other two events examined included a harmful algal bloom in the Monterey Bay of central California in 2007, and the Kure oil spill in Humboldt Bay, northern California in 1997. Age and sex of birds was determined by examining gonads and the bursa of Fabricius and assessing chromoboxhelicase-DNA-binding sex genes. Bursal involution was evaluated against gonad maturity to determine its value in ageing grebes. While we found bursal involution to be a useful age estimator, some observed patterns warrant further study in grebes. Age and sex ratio patterns varied among the three mortality events and between species, and indicate some degree of differential wintering or local partitioning. For example, Western Grebes killed during VOBI were predominantly adult (79%) with no differences in overall sex ratio; immatures had a male-biased sex ratio. Opportunities to examine demographic patterns in oiled wildlife are frequently overlooked but can be critical to fully understanding and predicting impacts of oil spills, and to directing recovery activities. Because oil spill events tend to be unselective, they also offer a rare opportunity to assess demographic parameters in wintering seabirds.