In autumn, >99% of North American Eared Grebes (Podiceps nigricollis) stage at Mono Lake, California, and Great Salt Lake, Utah, creating an exceptional opportunity to measure population trends in a common and otherwise widely distributed species. Aerial photocounts showed that numbers at both lakes fluctuated in parallel, from a total of ∼3.56 million in 1997, then crashing to 1.60 million in 1998. The decline was coincident with a major El Niño, during which hundreds of thousands starved in the Gulf of California. The population rebounded to 3.27 million by 2000, illustrating the species' potentially high survivorship throughout the year. Catastrophic events are evidently the major factors affecting proximate changes in population size. Factors that set the ultimate limit to the population can be clarified through long-term studies at staging areas.