For 7,000 years, coastal plain swamp sparrows have been adapting to life in tidal marshes.
By Howard Youth
“Many ornithologists dream of going to South America to describe a new bird,” says scientist Russell Greenberg, who directs the Zoo’s Smithsonian Migratory Bird Center (SMBC). “But here we have this very distinct bird within a stone’s throw of some of the most important ornithological institutions in the world, and almost nothing was known about it—right here at home.”
Meet the coastal plain swamp sparrow, which nests only in tidal wetlands of the mid-Atlantic region. This subspecies was found in 1948 in Vienna, Maryland, and formally described in 1951. Over the years, subtle differences in plumage color inspired ornithologists to designate different sparrow subspecies, including the coastal plain swamp sparrow. “It turns out that with this bird,” says Greenberg, “what coloration first showed naturalists represents a whole suite of differences,”
Unraveling the mysteries of this localized subspecies has been a labor of love for SMBC scientists and other ornithologists, as well as citizen scientists. For the past 25 years, Greenberg has studied various aspects of the songbird’s natural history. He considers the bird an element of “the Galapagos of North America—an archipelago of tidal marshes where we see evolution in action.” Like Darwin’s finches on the famed Ecuadorian isles, coastal plain swamp sparrows are small songbirds with a big survival tale to tell.
From an evolutionary perspective, the coastal plain swamp sparrow hasn’t taken a very long time to evolve into the distinctive subspecies we see today. Its wetland habitats appeared following the recession of glaciers. That means this bird has likely been isolated from inland populations for about 7,000 years. Since then, it has been adapting to a saltier life. “The question there is: Can you make it, or will you not make it?” says Greenberg.
Salinity is a great challenge for birds and other creatures, and Greenberg’s favorite sparrow is definitely living on the edge. “The coastal plain swamp sparrow is taking its first baby steps toward being a truly salt marsh bird, just getting its feet wet in the tidal marsh,” he says. They are found only in freshwater or in brackish areas that are only half as salty as the ocean. In contrast, seaside and saltmarsh sharp-tailed sparrows can live in marshes that are regularly doused with full-strength seawater.
In the process of trying to make it in a harsh tidal marsh environment, coastal plain swamp sparrows developed different behavior and coloration, likely the result of natural selection. One way to grasp the distinctiveness of this subspecies is to contrast it with another widely distributed subspecies, the southern swamp sparrow. It nests in different habitat, usually freshwater wetlands such as cattail marshes or alder swamps. The coastal plain swamp sparrow has a longer nesting season, larger bill, a different song, darker plumage, and larger kidneys, likely an adaptation for excreting salt. It also lays on average one less egg per clutch.
Greenberg describes this feathered phenomenon as an Amish quilt of rust, black, and gray. It’s not flashy, but those colors help the bird blend with its grassy, bushy, muddy wetland habitat. (A number of diurnal small mammals sharing the sparrow’s habitat are also darker than their inland relations.)
Big Bills and Sparrow Songs
Why do coastal plain swamp sparrows have larger bills? Males also have larger bills than females. Why? “The embarrassing thing,” says Greenberg, “is after studying them for so long, I just don’t know. The most likely cause is a difference in feeding behavior, such as probing tidal mud. But sexual selection and even temperature regulation are possible answers as well.”
Greenberg and his colleagues first documented the larger bills, and other ornithologists have since studied the bird’s song. Ornithologists now think that the big bills may have something to do with the differences in song between subspecies. In other swamp sparrow populations, females seem to favor territorial males that sing an optimal combination of quick and broad frequency trills. Pulling off such a stunt requires a fast series of sounds that keep the bill opening and closing rapid fire. A larger bill doesn’t favor this, or as Greenberg puts it, “It’s a drag on performing.”
Coastal plain swamp sparrows have quite a different song, one that is lower frequency and less musical. But coastal plain swamp sparrows have a more varied repertoire. “There is a big ecological force working on coastal big-billed birds,” says Greenberg. “This drives song evolution to be quite different. So different, that they don’t respond very much to the songs of other subspecies.”
Along its relatively new course toward life in a saltier setting, the coastal plain swamp sparrow has come to depend upon specific plants for successful nesting. The female builds the nest, usually of a fine-bladed marsh grass called salt hay. Nests are usually placed within live clumps of salt hay but fastened to branches of a low bush sitting within the clump.
Ten inches seems to be the preferred nest height. That places the nest above wet ground, making it less likely to flood, but is low enough that grasses and shrubs screen it from predators’ view. Salt hay and a favored shrub called high tide bush grow in relatively undisturbed habitats. Around Baltimore, in the New Jersey Meadowlands, and in many other degraded wetland areas, the invasive common reed overwhelms these plants, turning many damp acres into monoculture where the sparrows can’t nest.
Even when vegetation suits them, nesting in the seldom-flooded higher marsh has its risks. Greenberg says coastal plain swamp sparrow nesting attempts fail 75 to 80 percent of the time, compared with 50 percent documented in the similar southern swamp sparrow. He suspects the drier wetland edge where the sparrows nest allows easy access for predators, both from the marsh and adjacent farmlands. These include rice rats, rat snakes, opossums, and foxes. Although not predators, marsh wrens often enter sparrow nests and puncture their eggs. Scientists plan to study nest predation in greater detail.
These threats likely drove the coastal plain swamp sparrow to have a longer nesting season (up to a month longer) and to lay fewer eggs per clutch than its inland relatives. If nests fail, parents will attempt nesting up to five times in a nesting season. By laying fewer eggs than inland populations, coastal plain swamp sparrows may hedge their bets, saving energy for future nesting attempts.
Evolution in Action?
Breeding in high marsh habitat away from other swamp sparrows, this subspecies is likely on its way to becoming a full species some day. Is this indeed evolution in action? To find out, Greenberg and his colleagues did what plant biologists call a “common garden experiment.” They raised chicks of both the inland southern swamp sparrow and their coastal cousins in the same surroundings. They found that the birds kept their distinctive markings and bill-size differences when raised outside of their natural habitat. The results confirmed the suspicion that the birds’ plumage and beak differences were genetic.
Interestingly, the Zoo’s Center for Conservation Genetics had earlier studied “neutral” genes that do not code for characteristics involved in adaptation to tidal marshes. The survey showed no real differences between the populations. This suggests that the subspecies split very recently and that the differences we see are the result of natural selection working very hard on tidal marsh birds.
Sparrows’ evolution may help us understand global issues. Brian Olsen, a research assistant professor in the School of Biology & Ecology and the Climate Change Institute at the University of Maine did his doctoral dissertation on coastal plain swamp sparrows and has worked with Greenberg studying these birds. He thinks research on the sparrow’s adaptations is relevant to concerns over how wildlife will respond to human-caused environmental changes occurring on the planet. “Coastal plain swamp sparrows are a case study in how a species can change niche, something that we are asking many species on the planet to do right now,” he says.
Life on the Edge
From a conservation standpoint, coastal plain swamp sparrows live on the edge both literally and figuratively. “Their marshes face every environmental problem facing mankind,” says Greenberg. These include habitat loss and degradation, invasive plants, pollution, and the growing threat of climate change.
A particular risk is the expected increase in storm surges as well as the long-term threat of sea-level rise. “They only live in a little tidal band a few kilometers to a few hundred meters wide,” Greenberg points out. They are gone from the Philadelphia area and Staten Island, vanishing from the Meadowlands, and fading away in many areas around the upper Chesapeake Bay.
The picture in Delaware, however, shows more promise. There, Greenberg, his colleagues, and local birders surveyed breeding areas between 2000 and 2009 and determined that populations appeared stable. Greenberg believes this is because many Delaware River wetlands remain in good condition and fall under some degree of protection.
But these areas are not the only ones used by sparrows. They winter elsewhere. “At the Smithsonian Migratory Bird
Center, we say you can’t understand a bird if you study it at just one time of year,” says Greenberg. Not long ago, there were no winter records for coastal plain swamp sparrows. To find them, Greenberg and his colleagues enlisted the help of Mat Wooller, a scientist at the Alaska Stable Isotope Facility.
In April 2004, they sent Wooller crown feathers collected on the sparrow’s breeding grounds, small bits of evidence that would provide big clues about foods eaten on the wintering grounds. Wooller analyzed the ratios of different elements found in the feathers, and gathered clues that led to pay dirt. The carbon ratio indicated that coastal plain swamp sparrows winter in a coastal habitat, while the hydrogen ratio gave a fix on a promising latitudinal range.
In March 2005, Greenberg and colleagues from the Smithsonian Environmental Research Center searched coastal areas of North and South Carolina. Immediately, they found coastal plain swamp sparrows skulking at the edge of brackish marsh, between pine woods and open, saltier marsh. The bird’s winter range appears to stretch from southern Virginia to South Carolina, with its core in coastal North Carolina.
The story isn’t over. The bigger-bill mystery and other questions provide fodder for future study focused on a rapidly evolving bird that we can call our own. “People often mention black ducks and great blue herons when they speak of the Bay,” says Greenberg. But they are not localized specialty birds like coastal plain swamp sparrows. “Here’s one of the few unique and truly endemic birds in our mid-Atlantic region. It’s like duck pin bowling or blue crabs, only more distinctive.”
—HOWARD YOUTH is a freelance writer specializing in conservation issues.
Sidebar: At the Zoo
Want to see coastal plain swamp sparrows in D.C.? You’ll find 19 in the Bird House, in an enclosure near the Bali mynah and keel-billed toucan. Recently, two females laid eggs in nests they’d built. The eggs did not hatch, but the birds may try again. When breeding activity picks up, males sit on obvious perches and belt out their songs. The Smithsonian’s National Zoo is the only institution to exhibit these birds, which were originally part of an experiment conducted by the Smithsonian Migratory Bird Center. Keepers feed the birds a diet of live insects, pellets, greens, and some seed.
If you have a comment about Smithsonian Zoogoer magazine, please email it to us.Smithsonian Zoogoer 40(1) 2011. Copyright 2011 Friends of the National Zoo. All rights reserved.