Male Marsh Wren nestlings taken from eastern and western populations that were raised in standardized conditions still developed population specific song and neuroanatomy. Western populations have higher densities and higher rates of polygyny than eastern males, suggesting that larger repertoires and more complex songs may have been a selective advantage for western males. Patterns of singing are dependent on the males mating status and on the extent that he helps feed the young. Unmated males sing continuously throughout breeding season and mated males only decrease singing if they help feed nestlings. Only females incubate eggs and brood nestlings.
Females primarily feed young, although male feeding occurs in some populations.
Young are fed small insects. Habitat Marsh Wrens use a variety of wetland habitats. Cattails and bulbrush is the vegetation that dominates most Marsh Wren habitat. This wren occurs in salt and brackish marshes in addition to freshwater sites. Conservation Habitat degradation is the greatest threat to Marsh Wren vitality, although some migrants are killed every year in collisions with communication towers and other structures.