It has been hypothesized that populations that are strongly connected between two periods of the year (i.e., individuals that breed in similar locations and also spend the nonbreeding season in similar locations) will be most vulnerable to population perturbations. Using stable-hydrogen isotopes in feathers and data from the North American Breeding Bird Survey, we examined this hypothesis for a vulnerable songbird, the Cerulean Warbler (Dendroica cerulea). Cerulean Warblers exhibit a parallel migration system, whereby western breeding populations are generally connected to southwestern wintering sites and eastern breeding populations are generally connected to northeastern wintering sites. As predicted, breeding populations that exhibited the strongest degree of migratory connectivity with a specific wintering region were also those populations that experienced the most severe declines over the past 40 years. Our results suggest that the strength of migratory connectivity should be an important factor when making resource-allocation decisions for the management and conservation of migratory species.