Eating on the Run
The return of mature forests in the East, good news for most wildlife, may be bad news for this scrub-loving bird
On ocean beach with its buggy, wave-cast seaweed is a place where you would expect to find brown and gray shorebirds such as sandpipers and plovers poking around for food. But biologist Ted Simons has an indelible image in his mind of a spring morning some years ago when the white sand of East Ship Island, a few miles off the Mississippi coast, was alive with the colors of famished songbirds that had literally fallen out of the sky after a grueling flight across the Gulf of Mexico.
"It looked like someone had strung Christmas lights on the shore," says Simons. "Scarlet tanagers, rose-breasted grosbeaks, orchard orioles, indigo buntings and a tremendous variety of warblers were hopping all over the place." Beach fleas and other tiny sea-edge animals normally aren't on the menu of forest-nesting birds. But as Simons explains, the migrants had run into bad weather or head winds. Their energy reserves were at zero, and they were gorging on whatever fast food was available.
"They were the ones that made it," adds Simons, a scientist with the Cooperative Fish and Wildlife Research Unit at North Carolina State University in Raleigh. "A lot of trans-Gulf migrants starve to death on the wing and drop into the sea if they have to fly in turbulent air a good part of the way." Others arrive on the beach too exhausted to recover.
Simons' story illustrates the amazing plasticity of the billions of small landbirds that travel the skyways between breeding grounds in North America and wintering areas in Latin America: They can alter their foraging behavior on a moment's notice to meet the energy demands of migration, even brunching on the beach. In another example, ecologist Jeff Parrish recently discovered that many insect-eating species suddenly switch to a fruit-rich diet to fatten up in a hurry for the fall trip. "Eating berries is a lot more efficient than chasing insects," he says. In spring, warblers often arrive on the north shore of Lake Huron before the appearance of their favorite food, caterpillars. The birds feast instead on early hordes of midges, aquatic insects that hatch in the warm, near-shore shallows.
These kinds of findings come from a small but growing cadre of biologists working in a relatively new field of study called stopover ecology. Though songbirds in general are doing well, marked population drops in some locations in recent years among neotropical migrants give the research an air of urgency. Not only are the birds losing nesting and wintering habitats, places for feathered multitudes to rest and refuel en route are also in increasingly short supply, most notably within 50 miles of the coast. That is where migrants concentrate before and after long over-water flights. That's also where demographers place three out of four Americans by the year 2010, a huge increase from the current ratio of one in two.
"The importance of habitats where migrant landbirds can safely and rapidly replenish their depleted fat stores has been largely overlooked," says University of Southern Mississippi biologist Frank Moore. And the loss of such sites, he emphasizes, can have a domino effect: There will be more competition for food in the remaining areas; young and less-experienced birds may be pushed into poorer habitat where mortality will be higher; and songbirds that move on with lower than usual fat stores will have a smaller margin of safety for surviving problems at the next rest stop or final destination. If, instead, a migrant lingers too long to refuel, it could arrive late on the breeding grounds and have trouble claiming a territory or a mate.
So while scientists investigate every aspect of songbird life on the run--from the migrants' fat conditions to how long they stay in an area--a parallel effort is under way by conservationists to identify critical stopover sites and find ways to preserve or restore them.
Much of the focus is on the Gulf of Mexico's northern coast, where great waves of spring migrants arrive almost daily between mid-April and early May. Sidney Gauthreaux, a Clemson University biologist, has monitored these clouds of passerines on National Weather Service radar screens in Louisiana for three decades. He once calculated that 30,000 songbirds cross each mile of coast between Lake Charles, Louisiana, and Corpus Christi, Texas, every hour for five hours during such a wave, with the larger and faster fliers--such as thrushes, tanagers and kingbirds--reaching the coast about mid-morning.
That's 150,000 birds per mile along a 300-mile front, a total of 45 million tired and hungry travelers that have been aloft since shortly after sunset the previous evening. They have flown 600 miles or more from departure points as near as Mexico's Yucatan Peninsula or as far away as Colombia. Although narrow barrier islands like East Ship, scattered from Florida's panhandle to the Mexican border, may be the first land the migrants see, they mainly use the islands as emergency landing strips. On the 8 days out of 10 during spring migration that the birds ride favorable winds and have fat reserves left, they overfly the islands and gulf shore, choosing instead to make their first rest stop in forested expanses about 30 miles inland.
A bird that arrives fatigued and fat-free after a difficult flight, however, doesn't have the luxury of looking for the best rest stop. On East Ship, part of Gulf Islands National Seashore, when researchers netted and banded more than 12,000 migrants in a three-year study published in 1991, they found that most of the birds had no visible fat. "The keel of the breastbone protruded on a lot of birds, especially grosbeaks and thrushes," says coauthor Simons. "They had even metabolized their flight muscles." Yet biologists recaptured very few of the birds. Most of the migrants apparently left the 617-acre island and its uninviting mix of habitats--marsh, dune, scrub, relic dune and pine forest--after a quick rest and meal to find more suitable refueling sites on the mainland.
Migrants arriving with empty tanks on the marshy coast of southwestern Louisiana have a better option: wooded uplands called cheniers that parallel the shore to a distance of 12 miles inland. Chenier is a French word that means "place of oak," and live oaks and hackberries dominate the plant community of the old beach ridges--along with red mulberries and honey locusts, trees bearing fruit or flowers favored by a variety of songbirds.
Cheniers are regularly used by at least 63 species of neotropical migrants. Banding studies also show that spring transients often stay a day or more in these slivers of high ground, where insect densities are higher than on barrier islands and birds can gain weight at a faster rate. (Stores of fat in songbirds average 30 to 50 percent of body mass before the birds embark on a long-distance migration, compared with 3 to 5 percent for a nonmigrating bird. And migrants are capable of replenishing their energy reserves at a rate approaching 10 percent of mass a day.)
While state and federal wildlife agencies have acquired 100,000 acres of marshlands in the Louisiana coastal plain to benefit waterfowl, only one chenier and pieces of two others are protected. And their few hundred acres belong to conservation groups. All other cheniers are in private hands, and they have seen moderate to heavy disturbance from residential and industrial development, oil and gas exploration, and agriculture.
Indeed, more than 95 percent of the cheniers are grazed by cattle, according to biologist Wylie Barrow of the National Wetlands Research Center in Lafayette. Because cattle eat new growth, there is little or no regeneration of the trees that form the forest canopy and subcanopy, where many migrants forage; most of the understory vegetation, used by species like the gray catbird and Canada warbler, has been lost; and grass has replaced leaf litter, where various thrushes and the ovenbird like to rummage. "The landscape suggests an urban park," Barrow says.
Though forest birds tolerate some habitat degradation at stopover sites, they have a strong preference for access to well-developed understory, which holds a variety of food. An ongoing study by Barrow and colleagues is finding that migrants using the cheniers "rank" alternate habitats shortly after they arrive. More than 60 percent of the researchers' color-marked birds switched from disturbed study areas to adjacent control plots where vegetation was largely intact. Early arrivals like the black-and-white warbler predictably choose the best habitat, but as the season progresses and the woods fill with birds competing for food, the migrants increasingly use disturbed sites.
Restoring degraded cheniers by planting native vegetation should be a conservation priority, Barrow maintains. "Any rehabilitation effort will depend on the willingness of cattlemen to reduce grazing pressure," he says, while also pointing out that ranchers have left a lot of canopy trees to shade their cows, and the cheniers might otherwise have been cleared.
Meanwhile, Simons has created a "virtual bird" and a checkerboard computer landscape, based on satellite data, to show how the loss of stopover habitat can affect the number of trans-Gulf migrants that regain enough energy to reach their breeding grounds. He models the birds' movements using data about prevailing winds; wing shape and body size; the birds' amount of fat; and details of habitat they encounter.
Gulf Coast forests, of course, aren't the only critical stopover habitat. Western species crossing the inhospitable Chihuahuan Desert count on reaching the shelter of the middle-Rio Grande bosque before their energy stores are depleted. But human numbers in the Rio Grande basin are expected to double by the year 2020, and scientists say the continued loss of streamside habitat could jeopardize the future of some bird populations.
Appreciation of that risk is still very new. Scant attention was paid to the needs of fall migrants facing a long oceanic flight until Parrish's eye-opening study as a Brown University graduate student in the mid-1990s. Working on Block Island off Rhode Island, Parrish examined fecal samples to discover that most of 69 bird species had switched from eating insects to eating mostly fruit. Hermit thrushes, red-eyed vireos and yellow-rumped warblers gorged almost exclusively on the berries of such native shrubs as bayberry, northern arrowwood and pokeweed. When researchers removed all of the fruit from test plots, three out of four migrants fed in berry-rich habitats rather than in the stripped areas.
"People often think of coastal scrub as trash habitat," says Parrish, now an ornithologist with The Nature Conservancy's new Wings of the Americas program, "but the importance of these fruit-rich areas during migration needs to be considered in land-management and conservation decisions." The simple step of saving or planting native shrubs in the backyard, he adds, will help provide migrants with places to fuel up.
The Nature Conservancy's ambitious goal is to identify critical breeding, wintering and migration habitats and build an intercontinental network of refuges for birds, using its own vast system of 1,600 preserves as the foundation. But as Parrish concedes, migration is such a sweeping phenomenon that acquiring even the most important stopover sites between the Gulf of Mexico and the northern forests is an impossible dream.
In the end, the fate of birds that fit in a human palm, but which perform feats beyond our capability or imagination, likely rests in the hands of individual landowners willing to provide bed and breakfast for guests who will thank them with a song.
Les Line's small woodlot in the Taconic Hills of New York has a dense understory and fills up with migrants every May.