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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Strengthen Weaken
None of the genetic differences that have been identified in the genomes of coastal-plain swamp sparrows and freshwater swamp sparrows affect plumage color.
Mud in tidal marshes tends to be grayish because of the presence of iron sulfide, whereas freshwater mud is browner because of the presence of iron oxide.
Some species of birds that live in tidal marshes do not have gray plumage.
The diets of both coastal-plain and freshwater swamp sparrows can change significantly from season to season.
Baby birds of the coastal-plain subspecies and baby birds of a freshwater swamp subspecies, all raised on an identical diet under controlled conditions, grew plumage similar in color to that of their respective parents.

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