The Southern Swamp Sparrow (Melospiza georgiana georgiana) breeds in northeastern North America in montane, freshwater marshes and fens. Its close relative, the Coastal Plain Swamp Sparrow (M. g. nigrescens), breeds in northeastern North America, but in coastal salt marshes. Coastal Plain Swamp Sparrows are darker than Southern Swamp Sparrows. Darkly colored feathers are more resistant to bacterial degradation by bacilli, which are unusually salt-tolerant. We tested whether the difference in feather color of the pale montane Southern Swamp Sparrow and the dark Coastal Plain Swamp Sparrow could be an adaptive response to differences in the occurrence and activity of bacilli in habitats that differ in salinity. Southern Swamp Sparrows were caught and sampled in cranberry fens in western Maryland, whereas Coastal Plain Swamp Sparrows were sampled in salt marshes on the western shore of the Delaware River, just where it broadens into Delaware Bay. The number of birds with feather-degrading bacteria in their plumage was significantly greater among Swamp Sparrows in salt marshes than among those in freshwater fens. The number of colonies of feather-degrading bacilli per bird was also higher for salt-marsh Swamp Sparrows than for those from freshwater fens. We conclude that the dark plumage of Coastal Plain Swamp Sparrows evolved to resist feather-degradation by salt-tolerant bacilli that occur more frequently and abundantly in their plumage than in the pale plumage of the Southern Swamp Sparrow.