Migratory Mystery of a Secretive Sparrow Comes to Light

January 5, 2005 by Russ Greenberg

Brian Olsen takes the measurements of a female swamp sparrow captured in a mist net in a marsh at the Woodland Beach Wildlife Area in Delaware. Olsen is conducting a survey for the National Zoological Park on the success rate of swamp sparrow nests in the marsh. This bird was measured, banded and released. © John Barrat

Staring intently at a tangled clump of salt hay and marsh elder growing in Delaware's Woodland Beach Wildlife Area, Russell Greenberg is waiting for the "chip, chip, chip" of a female coastal plain swamp sparrow. It's mid-May and the start of nesting season for these secretive, rusty brown birds.

Years ago in a different marsh, Greenberg, director of the Migratory Bird Center at the Smithsonian's National Zoological Park, discovered that each time a female swamp sparrow leaves her nest-about every 15 minutes-she sings the "chip, chip, chip." By glimpsing the bird at the instant she sings, a seasoned ornithologist like Greenberg can usually locate her well-concealed nest.

Today, he is helping graduate student Brian Olsen, from Virginia Polytechnic Institute, find and mark coastal plain swamp sparrow nests as part of a National Zoo study to document how many nests are lost to high tides, raccoons and other perils of the salt marsh.

With binoculars and rubber hip waders, Greenberg has been patiently observing swamp sparrows in the wild for more than 20 years. Recently, his knowledge of these birds, combined with a bit of cutting-edge scientific sleuthing, paid off. In March, he and Peter Marra, senior scientist at the Smithsonian Environmental Research Center on the Chesapeake Bay, solved a mystery that has clouded scientists' understanding of the coastal plain swamp sparrow since the discovery of this subspecies some 50 years ago.

Each September, after a summer of laying eggs and raising chicks in the wetlands of New Jersey, Delaware and Maryland, the coastal plain swamp sparrow disappeared somewhere south. No one knew its migratory destination or had seen it during the winter months.

Seconds after hearing a female swamp sparrow's nest departure call, Russell Greenberg locates her nest in dry marsh grass at the Woodland Beach Wildlife Area. © John Barrat

"Here was an entire North American taxa for which no one knew where they wintered," Greenberg says. "That's kind of incredible."

Through analysis of the hydrogen and carbon isotopes in the male sparrow's cap feathers, Greenberg and Marra were able to "trick the birds into revealing where they winter," Greenberg says.

Isotopic signature

By hand-rearing a number of coastal plain swamp sparrows at the National Zoo, Greenberg confirmed that, like other swamp sparrows, males of the coastal plain subspecies sprout a reddish patch of feathers on their heads in February or March. This plumage badge appears just as the birds are getting ready to head north to their breeding grounds.

"It's just a little rusty cap," Greenberg says, opening a field guide during an interview at the Migratory Bird Center to a picture of the sparrow.

Later, Greenberg collected samples of the rusty cap feathers from wild sparrows he had caught in Delaware salt marshes. He and Marra sent the feathers to Matt Wooller of the University of Alaska at Fairbanks for analysis using isotope-ratio mass spectrometry. This scientific technique yielded what Greenberg calls an "isotopic signature" of the feathers.

The research is based on the knowledge that birds, like all creatures, are what they eat. Elements such as oxygen, carbon and hydrogen that occur naturally in soil, air and water are absorbed into the body tissues of animals through eating, breathing and drinking.

A swamp sparrow nest containing 4 eggs. © John Barrat

The number of neutrons in an element's nucleus can vary by geographic location. For example, a carbon atom found in one region of North America may have six neutrons. In another habitat, a carbon atom may have seven or eight neutrons. By mapping these variations geographically, isotopic signatures found in animal tissues can be linked to specific regions.

Marra likens isotope readings from tissues to "snapshots" of where a bird has been spending its time. He has been using isotope analysis to draw connections between wintering and breeding populations of American redstarts, another species of migratory songbird.

Deuterium pattern

Feathers are an especially useful tissue for tracking male coastal plain swamp sparrows because, once the sparrows sprout their rust-colored caps, those feathers stop growing. No longer metabolically active, the feathers effectively freeze the isotopic signal the birds have picked up from their winter habitat.

Deuterium, a stable isotope of hydrogen, is the particular signal Greenberg and Marra "tuned in" during their coastal plain swamp sparrow study. Scientists have long known that rain in southern latitudes contains more deuterium than precipitation farther north. This pattern of deuterium variation has been mapped.

Greenberg and Marra were able to match the isotopic signature of male coastal plain swamp sparrow cap feathers to latitudes of an area stretching from Charleston, S.C., to Beaufort, N.C.

Similar analysis of carbon isotopes in the feathers suggested that the birds stuck to coastal marshes and did not venture inland during the winter. In March, Greenberg and Marra traveled to North Carolina and, for the first time, found and photographed the coastal plain swamp sparrow in its winter habitat. They located 12 coastal plain swamp sparrows in three different North Carolina sites. All the birds were in marshes on the mainland bordering Pamlico and Albemarle sounds.

It does at first seem odd that these birds only migrate such a short distance, Greenberg notes. "Also, the migration takes place, we think, quite early. So they do not seem forced down by bad weather.

"Although the distance is small, the climate of the coastal Carolinas is very different" from coastal New Jersey, Maryland and Delaware, Greenberg continues. "It is really the first place where winter freezes don't occur regularly. For a sparrow that pokes around in the mud to feed, unfrozen ground is probably a premium."

Extended studies

The dark back, black forehead and dark eyestripe of the coastal plain swamp sparrow distinguish it from other swamp sparrows. This photo was taken at the Bombay Hook National Wildlife Refuge in Delaware. © Gerhard Hofmann

Locating their winter home is just one of several studies the National Zoo's Migratory Bird Center is carrying out on this subspecies. Researchers also are examining how the coastal plain swamp sparrow has adapted to life in the salt marshes. Most swamp sparrows live inland, in freshwater marshes and swamps.

The coastal plain subspecies has a number of physical traits that set it apart from its inland cousins. It shares these traits with other North American sparrow species that live in salt marshes. "There's a whole suite of differences" between freshwater and saltwater swamp sparrows, Greenberg says. "For example, salt-marsh swamp sparrows all tend to be very gray." Their plumage is probably a camouflaging adaptation to the gray and black muds usually found in salt marshes.

Coastal plain swamp sparrows, like other salt-marsh sparrows, also have large kidneys-probably because they need to flush more fluid through their bodies to eliminate the salt they ingest from brackish water.

In addition, salt-marsh sparrows have longer, thinner bills, which are better for eating invertebrates. Inland sparrows, with short, broad bills, eat more seeds, which are in short supply in salt marshes.

For years, the Migratory Bird Center has worked with experienced volunteers from the Delmarva Ornithological Society to census populations of coastal plain swamp sparrows during their breeding season.

Greenberg notes that the subspecies has disappeared from some sites where it once bred. "Their geographic range is small, and ecologically, they're restricted," he says. Greenberg and his colleagues estimate as few as 25,000 breeding pairs of coastal plain swamp sparrows exist. It's a situation "of conservation concern," Greenberg says.

With the discovery of the coastal plain swamp sparrow's winter range, an important chapter of this bird's life history has been revealed, Marra says.

“To fully understand the ecology of this subspecies, and to devise appropriate measures to protect them, we need to know where they are year round. This knowledge is critical.”