Species that undergo simultaneous flight-feather molt are susceptible to mortality during the flightless period. Remigial molt was examined in non-breeding Western Grebes (Aechmophorus occidentalis) and Clark's Grebes (A. clarkii) using beachcast carcasses from California to Washington to determine w^rhere and when molt occurs and help identify regions of maximum risk. Molt occurred in 18% of individuals and in all age classes and months examined, including winter, not previously recognized as a significant period for Western and Clark's grebe molt. Molt was encountered in all regions, with the highest proportions in Oregon/Washington during fall (35%), and central (18%) and southern (17%) California during winter. The percentage of Western and Clark's grebes in molt collected during the first 3 days of an oil spill was greater than later in the response (27% vs. 12%), suggesting a greater vulnerability to oiling during this part of their life cycle. Oil spills and other mortality events can provide insights into avian biology not otherwise available, and knowledge of molting regions can minimize wildlife impacts of spills through prevention and response. Received 15 November 2012, accepted 6 August 2013.