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 (δDp). Variance in deuterium associated with the year of interest, the long-term isotopic database, and in feathers (δDf) 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 δDf values at two spatial scales. At scales >500 km, annual differences accounted for 0–45% of variation in δDf 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 δDf values of Mallard broods at two Canadian prairie sites. Variability in δDf among Mallards from one small wetland complex in prairie Canada was best explained by variability in δDp 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 δDf 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.
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