Silent skies: migratory bird populations decline worldwide.

A magnificent natural phenomenon is slowly vanishing right above our heads. The great North-South bird migration, which stitches the skies each autumn and spring, is steadily subsiding. Scientists blame the decline of migratory birds on habitat loss throughout the Americas.

About 250 bird species, including such songbirds as the dainty Cerulean warbler, shorebirds like the buff-breasted sandpiper, and soaring raptors like the broad-winged hawk, escape the harsh northern winter by flying south of the U.S. border--to Mexico, Central and South America and the Caribbean. These flights are incredible feats of wing power. Like runners carbo-loading before a race, migrant birds eat a lot before their flights south, often doubling their body weight.

The record for long-distance flying is held by the Eskimo curlew, a shorebird that navigates from Alaska to Tierra del Fuego, a distance of 10,000 miles. Other birds are marathon flyers. The blackpoll warbler weighs in at under an ounce, even after its pre-flight feeding frenzy, yet in 72 hours manages to fly up to 2,000 miles from Maine to northern Venezuela without food, water or even a minute's treetop respite.

Because they're always on the move, migratory birds are extremely difficult to track. But recent biological studies present grim evidence: Migratory bird populations are declining. Among the familiar and beloved species at risk are the wood thrush, northern oriole, golden-cheeked warbler, scarlet tanager and whippoorwill. Conservation biologist George Powell has long been convinced that there's trouble in the skies.

"We still don't know the habitat needs of migrant birds," warns Powell, director of conservation biology for the Rare Center for Tropical Conservation in Philadelphia. "Different kinds of migrants need different kinds of habitat."

As biologists scramble to learn all they can about migrants, people continue to destroy the birds' forest homes. Latin America has the highest rate of deforestation in the world; Central America, including Mexico, loses about two million acres of forest annually.

Imagine the eastern wood-peewee that wings its way from New Jersey to Colombia every September. It arrives exhausted and famished to the same area it spent the previous winter, expecting to find a bug-rich, water-drenched, protected forest, not a sterile, chemical-laden banana plantation or barren cattle pasture.

Meanwhile, in the North, a major threat to bird nesting sites is "forest fragmentation," or large forest tracts that are broken up by housing developments, roads and shopping malls. In addition, virgin forests have almost completely disappeared. Less than one percent of the forest in California's Central Valley remains--nearly all of the migratory bird species once abundant there have disappeared as well.

In the Mississippi Valley, thousands of acres of bottomland forest have now been cleared and farmed. And trees grown in single species "monocultures" for wood and paper provide dismal bird habitat. Shorebirds, too, have suffered as seaside development has eaten up their habitats. Waterfowl like the blue-winged teal have been hard hit because developers and farmers have drained thousands of acres of wetlands.

Fortunately, it's probably not too late to save migratory birds. Says Powell, "We have to change people's attitudes and identify habitat protection priorities."

These are the goals of several campaigns now underway throughout the Americas. In a rainforest on Costa Rica's northeast coast, for example, ornithologist Daniel Hernandez is snaring migratory birds with barely visible "mist" nets, gently weighing them, and attaching a tiny coded band to each bird's leg before he sets it free. He hopes other American scientists will capture his tagged birds and send him details about where and when they were snared a second time.

"Ninety percent of migratory bird studies are conducted in their breeding areas in the States or Canada," Hernandez explains. "But we also need research in the tropics so we can determine which habitat areas must be saved."

An Idaho-based conservation group called the Peregrine Fund is studying migrants in northern Guatemala's undisturbed rainforest and in forests recovering from slash-and-burn farming. According to project director David Whitacre, the project works closely with local villagers, many of whom had no idea that their forests were winter homes for North American travellers.

And in Chiapas, Mexico, biologist Rosa Maria Vidal directs a conservation organization called Pronatura, which is working with subsistence farmers, teaching them the importance of preserving forests and woodlots. Pronatura's message, delivered through folk tales, puppet shows and theater, must be translated into the seven indigenous languages and dialects of Chiapas.

While most of Mexico's and Central America's great forest expanses have been razed, new research from the Smithsonian's Migratory Bird Center shows that some migratory species, like the Kentucky warbler and yellow-bellied flycatcher, can thrive in patches of forest, and even in heavily shaded coffee and cacao plantations. The study suggests that habitat priorities should include saving forest remnants, planting bands of trees along rivers and streams and promoting agriculture that requires protective shade from tall trees.

In order to link North and South migratory bird conservation crusades, the National Fish and Wildlife Foundation launched the "Partners in Flight" program four years ago. This campaign unites conservation efforts by U.S. federal, state and private programs, and is now bringing Latin American conservation organizations into the loop. "These birds are global citizens," says Partners in Flight director Peter Stangel, "so our efforts have to cross political boundaries."

Can all this conservation triage prevent a silent spring? Biologist Powell thinks that, while it is unlikely that migratory birds will face mass extinction, their declining numbers still translate into an international tragedy. He names the hooded warbler as a migrant he would sorely miss. "It's just a wonderful, cheerful, brightly colored harbinger of spring, one of the most beautiful creatures you can see," he says. "It seems to need primary forest, so it's going to be in trouble, if it's not in serious distress now."

Even with continuing loss of American forests, the hooded warbler will probably survive, Powell explains--but when people go out into the bottomlands of the Atlantic Central states, most won't hear this warbler's wonderful song. "Unfortunately, 99 percent of us won't even know what we're missing."

Contact: Smithsonian Migratory Bird Center, National Zoological Park, Washington, DC 20008/[phone]; The National Audubon Society, 700 Broadway, New York, NY 10003-9501/[phone].