The "Bullpen" of Monteverde is a swale of emerald pasture fenced by tropical forest and studded by gigantic trees, their trunks twisted like taffy and their limbs draped in mosses, vines, and bromeliads.

Humans are lost in tree shadow like explorers in a Romantic painting. One towering patriarch, measured within the hug of outstretched arms, is more than 40 feet in circumference.

At dawn this splendor is an amphitheater of birdcall, avian cries echoing across to each other like cheering fans in a stadium. Conservation biologist Debra Hamilton DeRosier leads the One World Journeys team like a train of hobbits across the grassy arena, identifying each chirp and chatter like a mother picking out the voices of her children.

We speak in whispers, in hushed awe of the noise. It ranges from the distinctive call of the Gray-Breasted Wood-Wren to the lovely song of the Slaty-backed Nightingale Thrush.

"Monteverde has a magic energy that is different than other places," says DeRosier's naturalist friend, Victorino Molina. Locals spin whimsy about the cloud mountains as a UFO landing zone or a nexus of New Age ley lines of power.

There's just one problem with this idyll. The calls scientists hear are changing as deforestation combines with global warming to push lowland forest birds into the drying territory of the wet.

The Emerald Toucanet is beginning to probe the cloud forest. It preys on the chicks of Monteverde's most famous and beautiful bird, the Resplendent Quetzal.

The Three-wattled Bellbird - another tourist favorite because males make a distinctive "bong" sound - "is declining quite rapidly," DeRosier says. Deforestation is robbing it of the wild avocado trees it feeds on. "One place bellbirds used to go (on the Pacific coast of Costa Rica) is now a huge resort with golf courses." Consequently, 350 bellbirds were counted cramming into five hectares, or about 12.5 acres, of forest.

"That's way too many," says DeRosier.

As a result, DeRosier has joined other Monteverde residents in championing reforestation. While the Monteverde Conservation League estimates almost a million trees have been planted in the high-altitude cloud forest zone, planting is just beginning at lower elevations where bellbirds migrate. Using money donated by the British Embassy, the biologist has helped plant 7,500 wild avocado trees near the village of Los Llanos, and activist Mary McBurney has overseen planting of 10,000 more nearby.

The avocado, grape-sized instead of the kind bought in grocery stores, is a favorite food of birds and other wildlife.

The tree planting is an attempt to slow disturbing trends. The nightingale thrush we hear is joining the Blue-crowned Motmot, brown jays, and some varieties of warblers, finches, and wrens in moving higher as Costa Rica warms. Determining how much of this is caused by natural climate variation and how much by global warming is a challenging task, but the threat is unmistakable.

"The Bellbird and Quetzal are an economic resource for tourism," said DeRosier, "and if you make a living off of tourism, you have to put something back."

She sets an example. The 42-year-old biologist came to Monteverde in 1993 from her native Massachusetts and, like so many scientist colleagues, could neither bring herself to leave or easily support herself in Costa Rica. When not trying to save the birds, she runs a tourist gift shop to support her research. "I recently went to the United States for four months, thought about it, and basically decided this is who I am."

While the trends suggest mountain-top species may someday be squeezed out, much remains unknown. Bellbirds, for example, migrate from the Atlantic side of Costa Rica to the Pacific side and then up to Nicaragua in a complex loop, feeding at many altitudes. Only five nest sites have ever been discovered, and few females are seen. "There's a big question what's happening," DeRosier says.

Just as difficult is managing forest already set aside. The complex of preserves near Monteverde are governed by three private organizations and two government agencies, explained Allison Dennes, information coordinator of the Monteverde Conservation League. This confuses would-be donors and causes jurisdictional rivalry.

While farmers have enlisted in the effort to plant windbreaks, direct payments to encourage reforestation have run out. Nor is it easy to keep poachers out of the preserves. The League's unarmed forest patrol routinely faces threats from guns and machetes, said Paul Englander, a League assistant director.

Sometimes the threats turn real. A few days ago, guard Heiner Alvarado was shot and lightly wounded by a defiant hunter.

Monteverde, I realize, is paradise under pressure. Even while joining excited hikers aiming binoculars at a regal male Quetzal, gloriously blue-green and preening, I know his kind's survival will only be assured by more research and struggle.