The Marsh Wren is a small wren, ranging from 10.4 and 14.0 cm in total length and weighing between 9.0 and 14.0 grams. 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. Juvenile plumage is similar to adult plumage except there are fewer black markings on back, the superciliary stripe is faint or absent, and wings are only faintly barred (Kroodsma and Verner 1997).
The breeding range of the Marsh Wren (Cistothorus palustris) extends west from the Atlantic coast to Nebraska and North from southern Illinois to northwestern. In the western United States C. palustris is a year round resident. There are also breeding and resident populations along coasts of the southern Atlantic and Gulf states. The wintering range extends south from the southwestern states of New Mexico, Arizona and Texas and into Mexico; Florida is also a wintering ground for Marsh Wrens (Kroodsma and Verner 1997).
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. The habitat of the winter range is similar to that of the breeding range (Kroodsma and Verner 1997).
Marsh Wrens are polygynous, but the percentage of males mating with more than one female varies among populations (e.g. in Georgia 5% of males had more than one mate, while 41-54% of males in Manitoba had more than one female). 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.
Males build numerous, single-opening dome-shaped nests on their territory prior to female arrival. Female choice is dependent upon territory quality and nesting status of other females (i.e. females tend to prefer unmated males). When a female approaches a male's territory he flies over to her and sings. If she enters his territory the male will show her several nests he has constructed, and if she chooses to mate with the male she may line a nest with strips of grass, small stems, cattail downs, feathers and rootlets or she may build an entirely new nest (Kroodsma and Verner 1997).
Not all populations of Marsh Wrens are migratory and tendency to migrate is dependent upon age, sex (older males are more sedentary while younger males migrate more often) and immediate winter conditions. 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. Timing of arrival on breeding grounds varies with location (e.g. in Missouri males arrive in the first week of May, while in Massachusetts males begin to arrive in late April). Departure for wintering grounds begins in most populations in August and continues through October (Kroodsma and Verner 1997).
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. Territory quality is a good indicator of reproductive success of the resident male. Breeding females have smaller "sub-territories" around their nests that they vigorously defend against intruders. Males defend territories early in the breeding season (before egg laying begins); males chase each other through disputed areas of the territory and sing energetically during the encounter.
Marsh Wrens destroy eggs and nestlings of conspecifics as well as heterospecifics such as Red-winged Blackbirds (Agelaius phoeniceus) and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus). Marsh Wrens may consume the contents of the eggs that they destroy. Yellow-headed Blackbirds show aggressive responses when confronted by Marsh Wrens or exposed to Marsh Wren song. Male Marsh Wrens destroy eggs of conspecifics in other territories and have been seen to attack their own eggs if the female is removed from the nest. Marsh Wrens reduce competition for food in the area surrounding their nests by destroying the nests of neighboring birds (Bump 1986; Kroodsma and Verner 1997).
Marsh Wrens are known for their loud gurgling song. Only males sing. Marsh Wrens learn their song from adults; they begin imitating songs at about 15 days of age. Learning continues throughout their adult life; it has been shown that adult males imitate songs presented to them on a tape recording or by a live tutor.
Marsh Wren song consists of several introductory notes, a trill of repeated syllables, and usually one to several concluding notes. Songs usually last about 1-2 seconds with a highest song rate of about 20 songs per minute. Males usually sing for extended periods in the early morning and sometimes throughout the night. Males and females use a variety of calls including a nasal buzz given during nest building and a melodious, rolling warble used by paired males and females when they come into contact during visits to the nest.
There are two major styles of singing behavior in eastern and western Marsh Wren populations. "Western males use harsh and grating, broad-band sounds, often coupled with loud tonal notes; songs of eastern males are more liquid, without the harshness" (Kroodsma and Verner 1997: 9). Kroodsma and Verner (1987) found that eastern males sing less complex songs and had smaller repertoires than western males. Western males are also capable of learning more songs than eastern males and devote more brain tissue to the region controlling vocal behaviors. Kroodsma and Canady (1985) found a genetic basis for the difference in songs of the eastern and western groups. 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. High rates of singing continue for mated males because they are trying to attract a second female (Kroodsma and Canady 1985; Kroodsma and Verner 1987; Kroodsma and Verner 1997).
Only females incubate eggs and brood nestlings. Females primarily feed young, although male feeding occurs in some populations. Young are fed small insects. Fecal sacs are removed from the nest and consumed by the female or carried away by the male (Kroodsma and Verner 1997).
Marsh Wrens mainly eat invertebrates, especially insects (e.g. bees, ants, wasps, beetles, and moths) and spiders as well as aquatic invertebrates in freshwater marshes. Foraging occurs on or near the marsh floor; Marsh Wrens glean insects from stems and leaves of vegetation and the water's surface. Some birds may forage in thickets or shrub patches that occur near the marsh (Kroodsma and Verner 1997).
Conservation concerns may prevent development of wetland habitat.
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. Destruction of marshes and wetlands used by Marsh Wrens as breeding or wintering grounds results in a great reduction in numbers in the area. Marsh Wrens readily colonize newly created or restored wetland habitats, however, resulting in increasing population sizes in some areas (e.g. high numbers of Marsh Wrens were found in marshes created by dam construction along the Colorado River) (Kroodsma and Verner 1997). Marsh wrens are a species of special concern in Michigan.
There are two distinct groups of Marsh Wrens in North America and 14 subspecies. The two groups are divided into eastern and western populations largely on the basis of vocal characteristics, although there are differences in color, pattern, size and molt (Kroodsma and Verner 1997).
Michelle Lesperance (author), University of Michigan-Ann Arbor, Terry Root (editor), University of Michigan-Ann Arbor.
Bump, S. 1986. Yellow-headed Blackbird nest defense: aggressive responses to Marsh Wrens. The Condor, 88: 328-335.
Kroodsma, D., R. Canady. 1985. Differences in repertoire size, singing behavior, and associated neuroanatomy among Marsh Wren populations have a genetic basis. The Auk, 102: 439-446.
Kroodsma, D., J. Verner. 1997. Marsh Wren (*Cistothorus palustris*). A Poole, F Gill, eds. The Birds of North America, No 308. Phildelphia, PA and Washington, D.C.: The Academy of Natural Sciences and The American Ornithologists' Union.
Kroodsma, D., J. Verner. 1987. Use of song repertoires among Marsh Wren populations. The Auk, 104: 63-72.