The use of hydrogen isotopes to delineate origins of migratory birds relies on predictable relationships between levels of deuterium (δD) in tissue and those predicted from long-term precipitation (δD_p). Variance in deuterium associated with the year of interest, the long-term isotopic database, and in feathers (δD_f) of waterbirds occupying small wetlands is an issue confronting isotopic comparisons. We examined the factors that influence isotopic variation among feathers of known-origin Mallards (Anas platyrhynchos) from breeding locations in north-central North America. We evaluated temporal variability in δD_f values at two spatial scales. At scales >500 km, annual differences accounted for 0–45% of variation in δD_f values of feathers from the North American Boreal Forest, Peace Lowlands, Aspen Parkland, and Prairie ecoregions; annual variability was greatest in the Aspen Parkland. At scales <50 km, differences among and within years contributed similarly and together accounted for 93–95% of variation in δD_f values of Mallard broods at two Canadian prairie sites. Variability in δD_f among Mallards from one small wetland complex in prairie Canada was best explained by variability in δD_p values that represented the period of feather synthesis through the beginning of the previous growing season >12 months earlier. These annual and seasonal shifts in δD_f values may help to explain why assigning origins of wetland birds on the basis of deuterium levels can be more uncertain at finer spatial scales, particularly in regions of complex wetland hydrology.