Males and females have similar plumage. This species has a black crown, white superciliary stripes, warm-brown upperparts with faint black bars, a black and white striped triangular region on the upper back, buffy sides and breast, whitish below, and dark barring on the tail. Marsh Wrens have brown eyes, pale brown feet and legs, and a brownish bill. Males are larger than females, though body size varies geographically. Timing of pair formation is variable depending on population and year.
Males begin singing within one to two days of their arrival on the breeding grounds.
In migrant populations females usually arrive on breeding grounds seven to ten days after males and then mate with males a few days after their arrival. When a female approaches a male's territory he flies over to her and sings. The eastern and western groups of Marsh Wrens migrate to different wintering grounds, the eastern birds migrate to the Atlantic and Gulf Coasts, and the western birds migrate to Mexico and Texas. During the breeding season males are highly territorial, defending areas from 60 to 10,000 m 2 . Territory size is probably positively correlated with the quality of the male and inversely related to quality of the territory.