In the moments before daybreak, as braids of river, silty sandbars and tufts of prairie emerge from darkness, thousands of gray-feathered sandhill cranes are chattering as softly as a snore. A young eagle, a predator, flies upriver, and it is as though the Earth has moved: Several hundred cranes rise in a swarm, their 6-foot-wide wings working mightily, their voices clamoring in alarm. A minute passes, then two, and the frightened cranes descend -- long legs dangling, wings held open for a parachute landing -- to the Platte River to regain a place among those that did not budge because it is not time. Time, on this river in the middle of Nebraska, comes in many dimensions, but none as epic as the spring ritual that has occurred for thousands of years, when migrating cranes stop at the Platte to fatten up before continuing toward the Arctic. Snow geese arrive before the cranes each February, and a rich array of songbirds -- the Nashville warbler, Harris's sparrow and gray-cheeked thrush among them -- continue to visit into May. But it is the cranes, a half-million or more of which make the Platte their home until early April, that attract a human migration. Visitors from around the world gather in the predawn dark at spots along a 50-mile stretch of the Platte between Grand Island and Kearney. They silently shuffle along the riverbank and take their places on viewing platforms or, at Audubon's Rowe Sanctuary, in well-built blinds.

When the cranes awaken, the birds' long necks arcing as parents call out to offspring, males search for females, and all prepare to fly to the fields for a day of feeding, while the humans witness and wonder.

Erna Pihlar, who was born in Slovenia but for decades has made her home in Omaha, watches as two birds dance.

"I'm calling them ballerinas," she says. "It would be nice to have Darwin here, so he could tell me exactly what is going on."

She listens as the cranes chorus.

"Their voices," Pihlar says, in a whisper. "It reminds me of an elephant, but more fine. It is prehistoric."

Birds dine on corn, mice

When settlers arrived in central Nebraska not even two centuries ago, they found a Platte River that came to be described as "a mile wide and an inch deep." Development and dams have since thinned the flow, narrowed the river, and turned many sandbars into thickly wooded riverbanks. No good for cranes.

So it is really only in the short stretch east and west of the Audubon site, where conservationists clear sandbars of growth to offer cranes some safety against coyotes and other dangers of the night, where the big birds roost today.

Cranes love the Platte River Valley because of the food they find here. As the sun rises, the cranes take off, in pairs and pods, and fly low toward the fields. Those heading north soon cross Interstate 80, two ribbons of pavement that perfectly intersect the north-south Central Flyway of the birds. The interstate serves another kind of migration, of course, as tractor-trailers and automobiles hurtle between the Pacific and Atlantic oceans. Many birds wander in cornfields only a few hundred feet from this flow of east-west traffic. Others continue to quieter corners along country roads.

Each crane bobs its long beak into the earth in search of bugs, mice and more. Mostly, though, their diet has shifted with settlement, from wild grass seed to corn. The birds eat an estimated 3 million pounds of unharvested corn each spring.

Most cranes that pass through the Platte region are lesser sandhill cranes, though some greater sandhills are found among them. Each lesser sandhill crane stands 3 to 4 feet tall. Its head is featherless, topped with a red blaze of skin. Its long neck leads down to a well-feathered body, about 5 or 6 pounds when the crane arrives from a winter home, whether in Texas, New Mexico, or Mexico, but a pound or more heavier after weeks of eating in the fields.

Wander and watch

It is best, if following cranes for a day, to do as they do and wander.

Route 30 leads east and Route 11 north, and everywhere along the way things answer the shifting seasons. Young steers gather around hay in the mud field of a feed lot. Rail cars wait beneath storage elevators. A farmer plows a still-chilled field, readying it for another planting of corn. In the town of St. Paul, near noon, Rotarians sit to enjoy a buffet of chicken-fried steak at the Sweet Shoppe.

Two sheriff's deputies, gray-haired brothers who seem to stand a mile tall, read a list of the household items at an upcoming farm auction and chat with a pastor who tells them about an outdoor sunrise service. They all wonder if the forecast for weekend snow will hold. And they are glad that the next day, Friday, will bring a bit of spring first, and a strong south wind.

The arrival of a south wind can tempt cranes well-enough fed and rested. When the warm wind rises, the birds ride thermals thousands of feet into the sky, circling, as if to say goodbye before soaring toward the Arctic.

The next day, then, may be the day, but first the cranes prepare to sleep another night on the Platte.

People find sanctuary, too

People gather in the glorious wash of dwindling light, and chatter during the walk to the Audubon blinds. The cranes wait in fields near the river's edge, and in the blind, volunteers from across the country guide tours and savor the natural rhythms of the Platte.

"It is a sanctuary for people, too," says Phil Mesner, a volunteer for more than a decade. "It is a safe, safe feeling."

The sun nears the western edge of the big, blue sky, and cranes appear above the river, at first only wisps far to the north, then in ever-growing flocks. They sweep low and settle.

The sun burns an orange older than the Earth. Within minutes, a sandbar 30 feet across is covered with calling cranes.

The temptation is to marvel at the scale: Tens of thousands of cranes carrying on an endless journey that traverses seasons and latitudes, linking frozen tundra to Mexican marsh, and millennia past to today.

But look, there at the edge of an almost empty sandbar, two cranes stand face to face. One lifts its neck with a quick call to the other. The second crane takes a few quick steps and jumps a foot above the ground, wings wide, as if to say, "Ta-da!"