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April is Cambodia’s driest month, yet the skies have cruelly poured rain during the entire hour-long drive east from Siem Reap, compounding the risk of colliding with speeding buses and errant livestock wandering two-lane National Highway 6. The real challenge begins once driver Thuy Chhoam turns south onto an earthen causeway stretching into a skillet-flat grid of rice fields. For the next half-hour we struggle through the mire, advancing less than three miles. My bird-mad guide, Sang Mony, remains unperturbed. Somewhere in these vanishing grasslands lurks the critically endangered Bengal florican, a stout, long-legged bird that is the world’s rarest bustard species.

Chhoam eases his 4WD Pajero around enormous potholes and through the pudding-thick mess. Suddenly we fishtail, sliding perilously close to the causeway lip. Mony stares intently out the window, searching as always for birds. “Black drongo,’’ he says calmly, as a dark blur wings low above the morass. A common-as-a-crow fork-tailed bird, but my conscientious guide doesn’t want me to miss any of Cambodia’s magic.

Not long ago Cambodia was an unrelenting nightmare: the Khmer Rouge, the killing fields, the unexploded ordnance scattered across the countryside. But with the collapse of Pol Pot’s rebel movement in the late 1990s, the kingdom slowly grew more stable and secure. In 1998 Cambodia counted 290,000 foreign tourists; 10 years later, 2.1 million. The undeniable attraction is Angkor Wat, the monumental ruins of a civilization that dominated Southeast Asia a millennium ago. Lying just three miles north of Siem Reap, the ancient temple city’s exquisitely carved lintels and bas-reliefs and its vaulting lotus-bud towers are a paragon of exotic world travel.

Cambodia also boasts some of the largest wilderness areas remaining in Asia. The brutal Khmer Rouge regime, which in 1975 drove the entire population into centralized slave-labor camps, had an unintended environmental effect: huge swaths of diverse habitat, including such birding hotspots as the Northern Plains’ savannah and the Tonle Sap’s seasonally flooded forest, essentially became uninhabited. “There were enormous expanses with very few people,” says Mark Gately, Cambodia program director for the Wildlife Conservation Society (WCS). “Wildlife was basically left to prosper.’’

While peacetime land conversion and illegal logging and poaching have since exacted an undeniable toll, the Oklahoma-sized country still supports wild elephant herds, tigers, leopards, freshwater dolphins, and more than 550 recorded avian species. Among them are such large, charismatic, and endangered birds as the Bengal florican and the sarus crane, the world’s tallest flying bird. Neighboring Thailand and Vietnam have their own riches, but not in such concentrations, nor in landscapes that blend such awesome archaeology with authentic adventure. At the top of my bucket list are two of the avian world’s rarest, most elusive species: the giant ibis and the white-shouldered ibis. Any chance of a sighting will eventually require a four-day quest into one of Southeast Asia’s most remote and rugged corners.

We start our journey with a slog into the grasslands north of Tonle Sap lake, a crucial refuge for the florican, a two-foot-tall black-and-white bird with a three-digit global population. More than half survive in this shrinking prairie, which is swiftly being converted to commercial farming. In response to outside operators who have built dams for deep-water rice fields that flood the bird’s habitat, the Cambodian government set aside more than 100 square miles in Siem Reap and neighboring Kampong Thom provinces as Integrated Farming and Biodiversity Areas. Working with the WCS and BirdLife International (National ���Ϲ���’s global partner), the IFBA program promotes traditional agricultural practices such as grazing and burning that, paradoxically, maintain suitable florican habitat. The WCS also pays villagers $15 for each florican nest containing an egg ($20 if the nest contains two), and another $15 if it hatches. The windfall—a month’s earnings for a subsistence farmer—protects nesting females.

Far from desolate, this near-treeless land is one of Cambodia’s 40 Important Bird Areas (IBAs). Mony quickly finds a strutting male florican 150 yards away. With its long neck held erect, the elegant bird flaps its massive white wings and lifts 10 feet straight up. It briefly hovers like a helicopter before dropping back into cover. During our 90-minute stakeout, three other male floricans make the same breeding-season display.

The remarkable air show sustains us during the bruising, 100-mile drive into the Northern Plains, a vast mosaic of savannah and dry forest in Preah Vihear Province that once rivaled Africa for wildlife. Here, in the 1950s, biologist Charles Wharton found abundant elephant tracks and herds of large ungulates, including banteng, Eld’s deer, and even kouprey. The kouprey, a type of forest ox now considered extinct, and elephants were shot out during the Pol Pot years; Khmer Rouge guerrillas fought here until the late 1990s. As we lurch along an unsealed road, Mony informs me that “a little bit” of land mines remain. Right on cue we pass a Cambodian Mine Action Centre camp near Koh Ker, a 10th century Angkor capital recently opened for tourists.

Thirty miles later we arrive in Tmatboey, whose 237 families would still languish in grinding poverty but for their natural setting: flat grasslands shaded by old-growth hardwoods and pocked by trapeangs, or seasonal waterholes. To preserve this landscape, ideal habitat for the elusive giant and white-shouldered ibis, the WCS established a simple community-based ecotourism project in 2004. Tmatboey’s subsistence farmers pledged to stop converting more forest into rice paddies in certain areas and to cease poaching. In return, the group built a rustic, solar-powered complex with four duplex cabins catering to adventure-minded birders, while the nonprofit Sam Veasna Center trained locals as guides and lodge staff. Members of 33 families now work directly for the project. To benefit other villagers, the WCS developed “Ibis Rice,” an initiative that purchases the crop at a fair-trade price from farmers who abide by the conservation rules. The wildlife-friendly brand is then packaged and sold to top-end restaurants, hotels, and gift shops in Siem Reap and Phnom Penh. This year 43 Tmatboey farmers harvested 57 tons of Ibis Rice. The village committee also receives a $30 community fee from every birder looking for an ibis, and uses this revenue to improve Tmatboey’s infrastructure and help pay for nest identification and protection. There is much upgrading to do. Beyond the reach of cell phone towers and power lines, Tmatboey is as off-the-grid as it gets: raised huts made from wooden planks; a few shops stocked with goods sold by itinerant peddlers; candle or battery power at night.

Dawn is the best time to spot ibis, so we’re in the truck by 5 a.m. and plowing through foot-deep mud—the result of a stupendous overnight downpour—two miles west of the settlement. Either ibis would be an exceptional encounter. Each species’ estimated global population numbers only a few hundred birds, owing to hunting, deforestation, and wetlands drained for agriculture. The giant ibis, a dark-gray bird topping 40 inches, and the white-shouldered ibis, which measures 30 inches and has a distinctive pale collar, are also extremely wary, shifting nests and roosts at the first hint of human disturbance.

In the mid-afternoon heat we hike muddy forest trails to another potential site one mile south of camp. Our local guide, Yin Sary, scouted white-shouldered ibis here yesterday; he cautions that they often relocate roosts after heavy rains. But if anyone can find these evasive birds, it is this 43-year-old farmer.

With a masterful eye, he scans the woodland, directing my gaze to dozens of colorful birds, including the iridescent purple sunbird and the greater yellownape woodpecker, a handsome olive bird with a dashing yellow crest. Sary also describes a 2004 incident a few miles away, where he fended off a nighttime leopard attack. He mimics the predator’s growl and shows me the scars the big cat left on his arm and chest. “The leopard is still there,’’ he tells me. “I’ve seen tracks in the paddy, one kilometer from Tmatboey.’’

But the father of four says he no longer hunts; wildlife generates revenue his village would otherwise never enjoy. “Foreigners pay $30 to see the ibis,’’ says Sary, who also earns $5 every day he guides. “I never imagined this.’’

It’s still more than 90 degrees Fahrenheit in the shade at 5 p.m. when we reach the recent ibis roost, a dead 60-foot-tall tree. We stake out 100 yards to the west and await the skittish birds. Gregarious flocks of Alexandrine parakeets break the silence, while swooping red-rumped swallows hunt for insects. Mony focuses a hoopoe in his scope; I’m so taken by its jaunty crest I don’t notice a white-shouldered ibis landing 10 yards away. Almost immediately, the restive ibis takes wing. With barely more than 400 birds in existence, I’ve squandered a perfect sighting.

“We wait a while,” Mony whispers.

Half an hour later the drone of cicadas is cut by an unearthly bugling shriek. Not one, not even two, but four white-shouldered ibis skim the canopy and land in the roost, silhouetted against a rising, nearly full moon. The birds fall silent, scanning the surrounding forest and cautiously inspecting the branches sagging beneath their weight. After 10 minutes the quartet suddenly flies off into the gloaming.

In the morning we resume our search for the giant ibis. The daunting floodwaters have subsided, allowing us to hop a creek and walk a mile of mucky paddy dikes to a marsh bristling with old, dead trees. But the giant ibis has once again changed roosts. I’m beginning to think it is just a myth. We double back to another possible site, spotting other large, wondrous birds, including a crested serpent eagle, which preys on snakes and lizards. A changeable hawk-eagle  dives so close that I can feel the whoosh of its broad, beating wings.

Sary stops, listens, nods. Somewhere to the east, lost in the molten sunrise, comes a faint, trumpeting taunt: a-leurk a-leurk. Giant ibis.

We trek through open forest, passing a resin tapper riding an oxcart. Tmatboey’s mature hardwoods are also a woodpecker’s dream. With 16 different recorded species, this forest has some of the highest woodpecker diversity on the planet. Chief among them: the greater flameback, a bird distinguished by its rust-gold mantle and fire-red crest, and the great slaty, a battleship-gray beast that, at 20 inches, is the Old World’s third-largest woodpecker. Striking, but not my grail bird.

On our last morning we speed-walk the forest to several other possible giant ibis roosts, but the only big birds perched in trees or hunting the marsh are woolly necked storks. Still, I’m glad to know the giant ibis has made a stand here. During the 2003–2004 breeding season, the WCS found five nests; by 2010–2011 the total had almost quadrupled, with 19 nests and 32 successful fledglings, despite a drought. For a bird with an estimated population of just 200 adults, this is heartening news. Multiplying ibis also portends better fortune for Tmatboey. “If the habitat is here, and we can protect the birds, the foreign tourists will come to see the village,’’ says Sary. “We want it to be like this forever.’’

More than any other Asian nation, Cambodia is sustained by water. For at least a millennium it has relied on an annual miracle: a flood that irrigates the kingdom’s abundant rice fields and overfills the Tonle Sap, Southeast Asia’s largest lake and one of the world’s most productive freshwater fisheries. The beneficiaries include some of the region’s rarest wading birds, which breed, feed, and nest in the safety of this unique ecosystem.

Every summer monsoon, the flood-stage Mekong River overflows into a 72-mile-long tributary, pushing upstream and spilling into the Tonle Sap’s shallow basin. Barely a yard deep in May, the Tonle Sap rises more than 30 feet by September, while its surface area swells from 900 to more than 5,000 square miles and the shoreline advances as much as 20 miles inland. The nutrient-rich floodwaters deposit silt and also nurture more than 200 fish species. My destination, Prek Toal, a 154-square-mile IBA along the lake’s northwest shore, supports the largest remaining stork, pelican, and ibis colonies in Southeast Asia.

If Humphrey Bogart ran bird tours on the African Queen, they’d look a lot like this: branches of the half-drowned riverside forest slap at our boat’s pilot house, while the captain periodically dives overboard to clear water hyacinth clogging the propeller. As we push up the winding channel, we pass beneath a treetop nest holding a curious gray-headed fish eagle chick. Common and black-capped kingfishers skip ahead of the boat like neon-blue tracers. I am forced to duck when an Amur falcon strafes the deck, relentlessly diving after birds our chugging boat flushes from the bushes.

I’m accompanied by Yuleng Ly, 34, a lifelong waterman and reformed hunter who is now one of Prek Toal’s 35 rangers. Like most of the staff, Ly once raided this very forest during the winter breeding season for the eggs of waterbirds, which were then hatched and fattened for slaughter as a holiday delicacy during the mid-April Khmer New Year. His current job is “long-term work,’’ Ly explains. “Hunting, you get big money only at one time of the year. Here, you get $5 every day you work.”

At Platform No. 5, a crude ranger station in the heart of the IBA, we tie up at the base of a squat tree and clamber 40 feet up bamboo ladders to a rickety, treetop lookout. As far as I can see, the trees are ornamented with spot-billed pelicans and painted storks, Asian openbills and black-headed ibis, lesser and greater adjutants.

This is only one of Prek Toal’s 16 colonies, Ly tells me. Since ranger patrols began in 2001, bird numbers have soared: Oriental darters grew from 482 birds to 10,874 in 2010, while spot-billed pelicans rose from 1,400 to 2,950 birds—giving Prek Toal the world’s largest colonies for these species. There has also been dramatic growth of species such as the painted stork (from 2,000 to 4,838) and the Asian openbill (1,200 to 27,690). As a Belgian ornithologist once noted, this place is a veritable “bird factory.”

Among this bounty I’ve seen, there’s still a superlative species missing: the so-called “bird of heaven.” For thousands of years the people of Southeast Asia revered the eastern sarus crane as a celestial messenger that collected the souls of the dead bound for paradise. By the 1980s the birds had mostly disappeared, victims of hunting and the illegal wildlife trade. Then, in 1998, Cambodian biologist Sam Veasna rediscovered the crane in a marshland 50 miles northwest of Siem Reap.

This wetland has a story nearly as tragic as that of Veasna, who died of cerebral malaria the following year. It owes its existence to the Khmer Rouge, who used forced labor to transform an ancient Angkor-era causeway into a massive, L-shaped dike stretching 12 miles. Mony estimates as many as 20,000 people perished during the uncompleted project, which was abandoned after Vietnam’s 1979 invasion. The forgotten reservoir became a sanctuary for wildlife, including the endangered Eld’s deer and the regal, five-foot-tall crane. In 2000 Veasna’s find led to the creation of the Ang Trapeang Thmor Sarus Crane Conservation Area, a 50-square-mile IBA.

Chhoam heads north along an unsealed reservoir road flanked by fallow rice fields and a village populated by former guerrillas. Spying a cluster of cranes in the distance, we swing off the causeway to close within a half-mile of the flock. Through Mony’s scope I can make out the scarlet heads of nearly 20 sarus cranes as well as a half-dozen Eld’s deer.

On the ride back to Siem Reap, Chhoam takes a road through the scrubland, stopping so I can inspect an Angkor-era stone bridge built to span a muddy, 50-foot-wide stream. More than 800 years old, this corbelled arch was originally part of a royal road from Angkor Wat to what is now Thailand. While Mony scans the sere landscape for birds, I’m already contemplating another trip. This road leads to Banteay Chhmar temple, a 12th century masterpiece of face-towers and bas-reliefs, including a lintel featuring two sarus cranes. And there’s always the giant ibis, out of scope but never far from mind.