Population dynamics of the eared grebe (Podiceps nigricollis) were investigated. In autumn, over 99% of North American eared grebes stage at Mono Lake, California, and Great Salt Lake, Utah, thus creating an opportunity to measure population trends. Aerial photocounts showed that numbers at both lakes fluctuated in parallel, from a total of 3.56 million in 1997, crashing to 1.60 million in 1998, and rebounding to 3.27 million by 2000. The decline was coincident with a major El Nino, during which hundreds of thousands of eared grebes starved in the Gulf of California. Thus, catastrophic events are evidently the major factors affecting proximate changes in population size; factors that set the ultimate limit to the population may be clarified through long-term studies at staging areas.