Swamp sparrows live in a variety of wetland habitats. Unlike most swamp sparrows, which live in freshwater habitats, the coastal- plain subspecies lives in tidal wetlands, where freshwater and seawater mix and the mud is gray rather than brown. Coastal-plain swamp sparrows differ from all other populations of swamp sparrows in having plumage that is gray brown rather than rusty brown. DNA analysis indicates several important genetic differences between swamp sparrows that inhabit tidal marshes and other subspecies of swamp sparrows. Therefore there must have been genetic - selection pressure on swamp sparrows in tidal marshes to become darker and grayer.

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