About the Marsh
Dyke Marsh is a freshwater tidal wetland consisting of 485 acres just south of Alexandria, Virginia, and north of historic Mt. Vernon. Located on the west bank of the Potomac River some 95 miles from the Chesapeake Bay, Dyke Marsh is owned by the U. S. Federal Government and is managed by the National Park Service as a part of the George Washington Memorial Parkway. It is an area of open water, cattail marsh, wetland shrubs and plants, and deciduous swamp forest.
In the early 1800’s earthen walls were built around the perimeter of the marsh in order to create more "fast land," land not inundated by high tides. The "dyked" area was then used to graze livestock and to grow crops. Over the years, the marsh has developed into a wide expanse of marsh vegetation, and tidal silts have provided a footing for marsh plants and the floodplain tree species of sycamores, maples and poplars. Today, the area is unique in that it represents the largest remaining piece of freshwater tidal wetlands in the Washington Metropolitan area. It is best known for its resident and migratory bird populations, the weekly bird walks that take place year-long, and for the extensive breeding bird survey conducted every summer.
Animals of the Marsh
Dyke Marsh provides habitat which supports a diverse collection of animals. Evidence of beaver activity is visible along the haul road and beavers, along with muskrats, can most often be seen in the early evening swimming in the marsh. Other mammal species observed include gray squirrels, cottontail rabbits, shrews, field mice, river otters, red fox, little brown bats, and whitetail deer. Marsh-dwelling fish include carp, bullhead, chain pickerel, shad, striped bass, and shiners. Reptiles such as snapping turtles and northern water snakes, and amphibians such as frogs, are also common.
To date, more than 360 species of plants have been recorded in Dyke Marsh. The dominant species in the marsh itself is the narrow-leaved cattail, which typically develops its characteristic flower spike by June. Other species associated with the tidal marsh include: arrowhead, arrow arum, pickerelweed, sweetflag, spatter dock, and northern wild rice. A result of human disturbance in the marsh has been the introduction of several exotic, or non-native, plant species. Exotic species are very opportunistic, growing in disturbed areas and oftentimes outcompeting beneficial native species. Portions of the floodplain forest are being overgrown with several exotic vines: porcelainberry, Japanese honeysuckle, and Asian bittersweet. In the marsh, yellow iris, common reed and purple loosestrife are slowly encroaching on the native species.
Saturday Morning Plant Walk in Dyke Marsh
Around 300 people came out on April 20 to learn about raptors.
To celebrate Earth Day, the Friends of Dyke Marsh, the Monarch Teacher Network, the National Park Service and the Raptor Conservancy of Virginia (RCV) set up an exhibit in Belle Haven Park from 10 a.m. to 12 noon near the bicycle path.
Kent Knowles and Gabby Hrycyshyn of RCV introduced visitors to birds of prey – a barred owl, an Eastern screech owl, a peregrine falcon hybrid, a red-shouldered hawk and a red-tailed hawk. RCV rehabilitates injured raptors and then releases those that can survive on their own.
For more information on their work, visit www.raptorsva.org.
Through conservation and education, FODM works to support raptors and other wildlife and their habitat.
FODMers Learn About Frogs and Bats
FODM member Deborah Hammer led a group of 35 on a frog and bat walk in the western part of the Dyke Marsh Wildlife Preserve on the evening of May 1. Deborah is a bat educator and rehabilitator. She lives nearby and has observed frogs and bats in this part of Dyke Marsh for the last 13 years. She is concerned that she is not hearing many spring peepers or seeing as many bats as she used to. “The sky used to be filled with them,” she commented.
Deborah told the group that frogs and toads need still water like ephemeral pools upland to lay eggs and hatch tadpoles. Bats need woodland trees upland of wetlands for nesting and marsh areas for hunting for food. She is concerned that development proposed for Westgrove Park to the west of Dyke Marsh could destroy or degrade the little nearby suitable habitat that remains. “It’s all interconnected,” she commented.
She offered many interesting facts, among them these:
■ The most common native frogs here are the green frog, bullfrog, spring peeper and southern leopard frog. Dyke Marsh is the southern leopard frog’s northernmost habitat.
■ Bats can live 20 years on average, are the only mammal that flies and can eat 3,000 insects a night.
■ There are 16 bat species in Virginia and 10 in Fairfax County.
The Marsh Wren -- Loss of Habitat, Loss of Birdsby Glenda C. Booth
It’s a little brown bird that lives in marsh cattails, bulrushes or cordgrass, and known for its calls. Some people hear a rusty hinge. Others say it’s a sputtering, bubbling trill. Other fans liken the call to a clattering sewing machine, a guttural rattle or a liquid gurgle, ending in chatter. It can be a loud little bird, especially on a spring night.
It is the marsh wren (Cistothorus palustris), once a common bird in the Dyke Marsh Wildlife Preserve. The Dyke Marsh Wildlife Preserve today supports the only known nesting population of marsh wrens in the upper Potomac River tidal zone.
National Park Service biologist Brent Steury wrote, “Marsh wrens are cute little brown-eyed birds not much over five inches long and weigh in at about half an ounce. Their dark brown cap rests atop a bold white supercilium, or eyebrow, that broadens as it extends from the base of the bill to the base of the neck. The body is chestnut, with a black, white streaked cape over the shoulders. The throat is nearly white and the belly pale buff. Sexes are identical. Active and noisy, they flutter rapidly among the cattails, often observed when perched with splayed legs, each foot tightly clasping a separate stem, bill agape in loud unmistakable song.” They usually hold their tails upright or often almost laying on the back, a distinctive trait.
Marsh wrens breed in fresh and brackish marshes, usually in areas of dense, reedy vegetation. They construct elaborate football-shaped nests with round openings by weaving grasses or cattail leaves in a circular manner anchored to reeds or cattails a few feet above the water. The male builds several “dummy nests” nearby, presumably to trick predators.
Seeing a marsh wren can be challenging, except during the breeding season, when males perform aerial displays before alighting on singing perches on the perimeters of their claimed breeding territories or when building one of their dummy nests. They often flit around furtively, popping up now and then to look around.
A Steady, Sad Decline
The Dyke Marsh Wildlife Preserve was once popping with marsh wrens. In the late 1800s, observers reported of hundreds of globular marsh wren nests affixed to reeds in the wetlands that lined the Potomac River, wetlands now largely gone.
Louis Halle, who biked from Washington, D.C., to Dyke Marsh, extolled this little bird in his 1947 Spring in Washington: “We heard the wrens this morning before there was light to see them. All over the marshes we heard them, singing in a steady chorus, each song a gurgling chatter, brief but repeated with hardly time for breath between. When it became light enough, we saw the singing wrens as far as the eye could reach over the marshes, carried upward on fluttering wings above the grass-tops by the very exuberance of their song, and sinking back again. The dots were bobbing up and down everywhere, like a natural effervescence given off by the marsh.”
In 1950, surveyors counted 87 singing males in Dyke Marsh. "Thirty-one territories were located in 1998 and 34 in 1999. The minimum estimated population size for 1998 was 38 (34 territorial males and seven breeding females); and for 1999, 48 (34 territories males and 14 breeding females)," wrote Sandy C. Spencer in her 2000 master’s degree thesis for George Mason University. (Spencer is now Supervisory Wildlife Biologist at the Patuxent Research Refuge.)
In 2007, surveyors found six established territories; in 2008, 10 breeding pairs; in 2010 and 2011, 12 established territories, but in these years, surveyors probably did not cover the same areas. In the 2012 survey, Larry Cartwright, head of FODM’s annual breeding bird survey, confirmed only two active nests and reported that surveyors saw or heard eight territorial males and possibly two to four more, conservative estimates he believes.
“The fate of marsh wrens and least bitterns remain in doubt at Dyke Marsh, but the trend suggests eventual disappearance for at least the marsh wren,” Cartwright contends.
As Dyke Marsh erodes away, a major factor in the marsh wren’s decline is loss of habitat. Spencer offered this guidance in 2000: “Available habitat comprises only 12 percent of the entire preserve, but actual use by marsh wrens for nesting territories was only 3.6 percent in 1999, because of very narrow habitat selection preferences (tall, dense, emergent vegetation adjacent to water). Protection of the remaining habitat from reduction or degradation due to expansion of invasive plants and erosion and restoration of lost habitat are strongly recommended to ensure persistence of this population over the long term.”
Volunteers Identify 16 Herptofauna, Two New Species in DMWP
Belleview Elementary School Students Learn All about Dyke Marsh
Learn More About DMWP’s Complex Biodiversity
To access DMWP information, visit the Biodiversity Database website.
Dr. Barrows wrote in the spring 2008 Marsh Wren, the newsletter of the Friends of Dyke Marsh, that there are thousands of “key players” in the wetland’s ecosystem, yet to be studied and recorded.
About invertebrates (including arthropods), BDWA quotes Professor Edward O. Wilson: "The truth is that we need invertebrates but they don’t need us. If human beings were to disappear tomorrow, the world would go on . . . . But if invertebrates were to disappear; I doubt that the human species could last more than a few months. Most of the fishes, amphibians, birds and mammals would crash to extinction about that same time. Next . . . the bulk of the flowering plants . . . and the world would return to the state of a billion years ago. . . ."
Observing reptiles and amphibians in the field is always challenging. The conditions have to be “just right” – warm enough (they are cold-blooded) to be outside, cool enough to justify the dangers of sunning, good chances of finding something to eat, etc. Usually, you will see nothing.
On a warm, sunny day in spring, I saw my first lizards in Dyke Marsh despite walking the trail often for six years. The first was eight inches long with a four inch body. It was a dark tan brown in color with very pale stripes. The head was orange. A second, smaller lizard was 4 inches long, black with bright white stripes and a bright blue tail. Both lizards had short legs, long tails and bullet-shaped heads. They appeared to slither over the ground, but could easily climb vertical wooden surfaces.
The ground-dwelling Five-lined Skinks (Eumeces fasciatus) range throughout the eastern half of North America and are most abundant on the coastal plain. As skinks mature, their stripes fade and their tails lose the bright blue coloration. The heads of mature males turn reddish orange during the breeding season.
Our area has isolated enclaves of several different types of lizards. Five-Lined Skinks are well known to be native to this area. But seeing them for yourself is a different matter. Despite visiting the same spot over a dozen times since, I have seen no lizards. This experience tells us something about field observation. A place we think we know extremely well can still surprise us.
-- Doug Wilson
Friends of Dyke Marsh, Inc. is a
non-profit §501(c)(3) organization.