In general, the coastal plain swamp sparrow tends to be less rusty-brown than the inland swamp sparrow. The plumage tends to have more gray and black and the bill is a tad larger.
Specific areas to note the differences are the:
The crown of the coastal plain swamp sparrow has a bit more black and a bit less rust than the inland bird. The nape is grayer and the back feathers have more black with less buff and rust coloration.
The rump of the coastal plain bird is a bit more olive with more extensive black centers to the feathers. The lower back is gray and olive and the tail feathers are bordered with a grayish-brown. The inland bird has a rustier tail and uppertail coverts with less black and a more medium-brown lower back.
Overall, the wings of the coastal plain bird are much less rusty, a bit darker with more gray and black in the back and less buff feather edging.
The differences in bill size and shape are subtle but the coastal plain swamp sparrow has a longer bill with a heavier base while the inland bird's bill appears more delicate.
The birds pictured above were photographed in early March on the wintering grounds—the differences in plumage are not quite so dramatic during the breeding season, however, from June to August, their ranges do not overlap.