(birds)

The Secret Life of Redstarts

The key to a big-picture understanding of songbird populations may lie in the details, from the social to the sexist to the subatomic.

by Don Stap

Coming down out of the hills above the southwest coast of Jamaica in the predawn darkness, we turn east and drive with the shoreline to our right. Far out on the bay, black waters hold a medallion of hammered silver cast down by a full moon. It is a 20-minute drive to Luana Point, where Pete Marra, a biologist with the Smithsonian Environmental Research Center, has spent much of the past decade studying the population dynamics of American redstarts.

This small, flashy warbler (the male is glossy black with orange patches on its sides, wings, and tail) is doing fine throughout most of its breeding range. It is not a precipitous population decline or any other such big-picture concern that drives Marra's research. His is detailed, close-up work: How does one local population of redstarts fluctuate over time, and why?

On our first day in the field, it is Marra's plan simply to walk through the study plots and get a feel for how many redstarts are here. We've hardly begun looking when a male with a leg band flits through the branches above us. Minutes later we hear the chip notes of a second redstart, and Marra spots it: another male. We walk 50 yards or so, then stop, listening for call notes and scanning the tops of the mangroves. "This bird is very skittish," Marra says. "Let's stay quiet." The redstart he is looking for was first banded in 1994, and it has returned to this same group of mangroves year after year. Redstarts, like a number of other songbirds, are territorial not only on their breeding grounds but on their wintering grounds as well.

After a few minutes, Marra decides to move on. We've already edged into territory where another redstart is active, and as we continue, we come across one bird after another.

Soon the mangroves give way to logwood and acacia trees. We enter a forest where the ground is a few inches higher and a hundred yards farther from the beach. Dry leaves crunch underfoot. Where openings exist, the ground is grassy, but elsewhere the understory is dense. Redstarts are fewer and farther between. In an hour we see only one—a female—compared with 6 birds in less than 30 minutes in the mangroves. Perhaps that's because the morning is an hour older and the redstarts are less active. Then again, the change was sudden, coinciding with the change of habitat.

Marra, who has been coming to Jamaica 2 or 3 times a year for 10 years, has spent thousands of hours at this site trying to analyze those kinds of observations. Years ago he and an assistant used a compass, a machete, and a length of rope to divide the area into 50-meter grids. This way the boundaries the birds squabble over—branches and thin air—could be measured as precisely as suburban subdivisions.

Making sense of the comings and goings of a small shuttlecock of a bird that never seems to sit still long enough to be observed (a foraging redstart changes perches every two seconds) is one of the great challenges in bird studies. Consequently, the small-town lives of forest-dwelling songbirds have remained largely a mystery to us. Their births and deaths, pairings and divorces, narrow escapes from the cold shadows of sleek wings and sharp talons, have gone unnoticed. And a crucial question remains unanswered: How do basic ecological factors—weather, disease, food availability, predation, and habitat—influence the year-to-year ups and downs in a local population of a forest-dwelling songbird?

In recent years debate has swirled over what is causing the apparent decline of so many of our songbirds. Some say it's rapid deforestation rates in the neotropics, which began to increase at the same time (the mid-1980s) that populations of tropical-forest–dependent species began to decrease. This argument, however, is based on deductive reasoning.

The male redstarts, slightly larger than the females, were grabbing the best, food-rich habitat for themselves; female redstarts had to settle for what was left over.

Marra prefers the slow accretion of facts through observation and experiment. His is painstaking work that can be traced back to Richard Holmes, under whom Marra completed a Ph.D. at Dartmouth. In 1969, when Marra was still in grade school, Holmes walked into a beech-maple forest in New Hampshire's White Mountains and began asking questions about the bird communities there. The work he set in motion is the most detailed, longest-term study of bird population dynamics in North America. "The least flycatcher was the most numerous species in the area when we started," Holmes told me. "But now it's completely absent. The redstart population has declined, too." This, he believes, is a result of changes in the forest itself.

Hubbard Brook Experimental Forest, the site of the New Hampshire study, was logged for spruce in the late 1800s, and then logged again for selected hardwoods in the early 1900s. When Holmes first began working there—in an area dominated by beech, maple, and yellow birch—the leafy crowns of the trees formed an even canopy. Over the years, though, as trees toppled over from natural causes, sunlight seeped into the forest through the openings created. Shrubs and saplings sprang up. Today it is a more mature forest, with an open canopy and a mixed understory. Least flycatchers, which prefer an open understory, have left, while black-throated green warblers and ovenbirds, which nest in dense shrubs, have increased in number.

Natural cycles affect bird populations as well. Holmes has witnessed several outbreaks of caterpillars over the years, after which bird populations rose dramatically, a result of increased nesting success due to the abundant food supply. On the other hand, predation of nestlings by squirrels and chipmunks may increase when their food is either scarce or abundant. Every three or four years, for example, beech trees produce unusually large crops of nuts; squirrel populations then grow, leading to more animals preying on eggs and nestlings. In one study, 8 of 11 redstart nests protected by squirrel baffles fledged young, compared with only 10 of 42 nests without baffles.

The Hubbard Brook studies have produced 130 scientific articles. Tom Sherry, a biology professor at Tulane University and Holmes's principal partner, says most evident is how much the population changes year to year: "It fluctuates enormously, and there's no one cause for it. It's clear, though, that what goes on in the summer with migratory birds does influence long-term population trends. Some people have argued that it's mainly events on wintering grounds that influence long-term trends, but we've been able to show that summer is a factor."

In the mid-1980s Holmes and Sherry expanded their studies to Jamaica. When Marra first began working here, the study site was composed primarily of the mangroves, but he soon extended it to compare redstarts wintering in mangroves with those in logwood-acacia areas.

The two habitats differ in a number of ways, but what most interests Marra is that the mangroves remain moist much longer during the winter, while the logwood forest dries out. To show me why this is important, Marra points to the base of a mangrove encircled by pneumatophores—stalklike aerating branches that rise vertically from the tree's roots. When Marra brushes them with his foot, a small cloud of flies rises suddenly, then settles again. Flying insects are the staple of the redstart's winter diet. Because mangrove forest remains moist longer than logwood forest, it contains more living vegetation, and so supports more flies. For redstarts, an abundance of flies raises real estate values.

Marra discovered that the mangroves held nearly twice as many males as females, and that the ratio was reversed in the drier logwood forest. Earlier research had described this phenomenon—males and females occupying different winter habitat—in about a dozen other songbirds, but only a study of hooded warblers tried to explain it. The authors suggested that the males and the females had an innate preference for different habitats. Marra's instincts told him otherwise. What he thought he found was a case of male chauvinism.

The male redstarts, slightly larger than the females, were grabbing the best, food-rich habitat for themselves; female redstarts had to settle for what was left over. To prove his case, Marra ran an experiment in which he removed male redstarts from several plots in the mangroves and females from the logwood areas. The results—females moved in and took over the vacated mangrove sites, while few birds bothered to recolonize the logwood territories—indicated that the birds were competing for the best habitat.

Making sense of the comings and goings of a small shuttlecock of a bird that never seems to sit still long enough to be observed (a foraging redstart changes perches every two seconds) is one of the great challenges in bird studies.

The consequences extend much further. By weighing birds in Jamaica in November and again in March, Marra has observed that the birds wintering in the mangroves are in better physical condition. He also knows they are the first to leave the area and head north. "We've known for a long time that males arrive on the breeding grounds before females," Marra says. "And that those males that arrive earliest have their pick of the best territories. Availability of moist forests in Jamaica may be related to breeding success in the summer."

But how could Marra know if the redstarts from the mangroves, which left first, were really the first to arrive on their breeding grounds? The answer is in the air. The invisible world of subatomic particles holds a telltale marker than can link a redstart in New Hampshire with the type of forest it wintered in 2,000 miles to the south. Carbon atoms are found in two forms—those with six neutrons in their nucleus to match the six electrons spinning around it (called carbon 12) and those with a seventh neutron (called carbon 13).

These two isotopes of carbon occur in the carbon dioxide found in the air. When carbon dioxide is absorbed by plant tissue during photosynthesis, the rate at which carbon 13 is diffused from the air into the plant tissues differs depending on the type of photosynthetic pathway present in the plant, and also on the amount of moisture that the plant perspires through its leaves. In short, plants retain different amounts of the carbon 13 isotope, depending on their habitat. Plants in moist forests (such as mangroves) retain less carbon 13; plants in dry forests (logwood) retain more.

Marra examined insects from both the mangrove and the logwood forests. Those that had eaten the vegetation in the mangrove forest contained less carbon 13. In 1995 and 1996 Marra collected blood from the insectivorous redstarts in the two habitats and tested it for carbon-isotope levels. Those from the mangroves had less carbon 13.

Then, in the spring of 1997, Marra netted redstarts as they arrived at Hubbard Brook. Carbon isotopes remain stable for six to eight weeks, so blood sampled after migration would still reveal whether a bird had been feeding in wet or dry forests that winter. The results confirmed his theory. Early-arriving birds at Hubbard Brook had lower levels of carbon 13 than late arrivals. "This," says Marra, "means that the quantity of moist forests may limit the number of birds that can arrive early in the spring."

The pattern was clear: From air to leaf to insect to redstart, carbon 13 left a trail, like a watermark no one noticed until it was held up to the light. And that trail leads to better understanding of the world of migratory songbirds.

Don Stap, who teaches creative writing at Central Florida University, is working on a book about birdsong to be published by Scribner.

© 2003  NASI

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