You can see that they are adept at making their way through the marsh. They would be hard to find, except that they are almost continually singing. Males learn up to 200 song types, with which they have verbal duels with neighboring competitors. Females may also be attracted to males with rich repetoirs.
Females are attracted to males with rich territories. In some marshes, half the males mate simultaneously with two or more females. Males are so “zealous” that them build multiple nests, some times six dummy nests for every nest used by a female.
This extraordinary competition also leads Marsh Wrens to destroy competitors’ and other species’ eggs. Box sexes of Marsh Wrens destroy eggs that they find. Information and the quote in this post comes from Kroodsma and Verner 2014.