Spatial synchrony is a common feature of mammalian population dynamics that appears to be caused by different processes in different systems. We sampled 60 sites across >900 km in northern and central Ontario, Canada, from 2001 to 2004 to assess spatial and temporal trends in abundance and population growth of small mammals. We tested alternative predictions for 3 causes of synchrony: dispersal, predation, and correlated environmental perturbations (the Moran effect). During 25,680 trap nights, Myodes gapperi, Tamias striatus, and Peromyscus maniculatus were the most commonly captured species. Populations of all 3 species fluctuated markedly during the 4 years of sampling, but fluctuations appeared to occur over a relatively small spatial extent (<200 km for all species). No pairwise combination of species exhibited positive interspecific synchrony, suggesting that nomadic predation was not synchronizing declines among species. Our data were most consistent with the dispersal hypothesis or a Moran effect caused through synchronous food crops.