Stable isotope analysis is often used to identify the geographic origins of migratory bird populations. While this method can accurately predict the provenance of migratory species, stable hydrogen isotope values measured in feathers (δ²H_f) can be variable within a site and may be influenced by differences among age class (second year vs. after second year), type of feather (primary vs. rectrix), year of sampling, species, and local hydrology. In this study, sources of variation in δ²H_f values were assessed in a wetland-associated Neotropical migratory bird, the Prothonotary Warbler (Protonotaria citrea), by comparing δ²H_f values among age classes, sexes, years and feather types in individuals breeding in eastern Virginia, USA. Age and year were found to influence δ²H_f values, with individuals in their second year having more depleted δ²H_f values (-62.43 ± 9.56‰) than individuals older than 2 years (-53.73 ± 9.04‰). Differences between primaries and rectrices were within the range of sampling error (-2.98‰), and there was no effect of sex. For wetland-associated songbirds, age-related differences in the dietary proportion of aquatic vs. terrestrial prey may provide an additional mechanism for differences observed in δ²H_f values between age classes. In studies that use stable isotopes to assign geographic feather origin, researchers should attempt to account for and propagate known variation in δ²H_f values in assignment models whenever possible.