The thousands of sandhill cranes congregating on the sandbars and islands of the Platte River seem completely unconcerned about the gathering storm.

In south-central Nebraska, a few miles south of Grand Island, the prairie sky stretches nearly unbroken in all directions. On this sunny evening in late March, it's easy to see that a change of weather is coming. Clouds fill the sky from southwest to northeast, building from wispy to billowing to anvil-shaped.

I am less than a mile from the Crane Trust's Nature and Visitor Center, and the viewing blind is a small, insubstantial building — just plywood and clear plastic — specially designed for viewing the cranes, but not a good place to ride out a bad storm. The winds build. Windows, walls and doors bang, rattle and shake like Dorothy Gale's house during the opening scenes of "The Wizard of Oz." But the great masses of cranes just outside the blind don't seem to care a whit.

Every night during the migration season of spring, storm or not, the birds return to the braided watery strands of the Platte to bed down for the night. In the hour before darkness, they soar back to the waterway in tight, linear formations like World War II fighter squadrons. Twenty or 30 at a time, they glide down, banking into the wind for their descent onto sandbars. Rapidly, this small section of the river fills with a huge assemblage of cranes, nearly one atop the other, until there's no more room on the sandbar for latecomers.

As the sun sinks farther below the flat horizon, the weather outside the blind turns worse. The interval between the lightning flashes and the thunder claps, previously a rather benign 15 "Mississippis," shrinks to a disquieting four or five. We crane watchers decide that, with the increasing difficulty of seeing well in the gloom and the likelihood of a very wet walk back to our cars if we delay much longer, it's probably time to go.

Each spring, half a million sandhill cranes stop in this area on their way to summer breeding grounds in Canada, Alaska and Siberia. They winter over in a wide expanse of territory that includes New Mexico, Texas, Florida and Mexico. While the cranes disperse over a huge area in summer and winter, the route north is an hourglass-shaped flyway, and nearly all of the cranes stop on the Great Bend of the Platte River in March and April to rest and eat.

Such a concentration of large birds in a relatively tiny area makes this spot among the world's most spectacular animal migration sites, on par with those of the Serengeti wildebeest, Pacific baleen whales and Alaskan caribou. And it's less than a day's drive from the Twin Cities.

Each day during crane season, the elegant long-necked birds feed in cornfields that lie adjacent to the Platte. A drive along the county roads paralleling the river is sure to provide excellent views of large flocks feeding on leftover grain. They do this from sunup to sundown, when they again return to the river to sleep.

It's fun to watch the cranes hop around in the fields, but the true highlights are the massive early-morning departures of flocks from their river roosts and the subsequent late-evening returns.

Two primary viewing spots

In addition to the Crane Trust Nature and Visitor Center near Grand Island, the other prime spot for viewing the birds is the Rowe Sanctuary near Kearney. The sanctuary has three large viewing blinds strategically located to provide excellent bird views.

As with the Crane Trust, viewings from the blinds are scheduled twice daily during March and early April, once at dawn and again at sunset.

Along with a cup of coffee, crane watchers are given instructions and a short introduction to the cranes' story by staffers and trained volunteer guides. Then the guides take their charges on a quarter-mile walk over an easy, flat path to the blind.

On the early morning that I went to the Rowe site, the weather was comfortably crisp. My companion — Kearney Visitor Bureau Director and veteran crane watcher Roger Jasnoch — told me that the day's 40-degree temperatures were relatively balmy.

"It's important to dress in layers. This is Nebraska, so the weather changes very quickly. I've been here in rain, snow and deep cold. But the cranes don't care, and neither do most of the visitors," he said.

After guides Ed Berglund and Gordon Maupin led our group of 30 crane watchers to the blind in the darkness, we took positions at the windows, one per observer. It was difficult to see anything in the low light, but the noise was amazing. The cacophony of bird calls hit at least 65 decibels. Maupin told us to cup our hands over our ears to concentrate the sound.

"Hearing the birds is as wonderful as seeing them," he said.

He was right. The call of the sandhill crane is unique and difficult to describe in words.

"It's not really a honk or a trumpet," said Jasnoch. "It's maybe more of a vibrating, rattling sort of bugle call."

I asked the bird watchers around me to describe the sound. Every one of them had difficulty. "It's sort of like a long croak," said one. "I've been told it sounds like a French-style 'r' trilled in the throat," said another. To me, it sounded like a chorus of long, loud spring peepers.

The sound emanates from the cranes' very complicated throat anatomy.

"The crane's trachea is shaped something like a saxophone or French horn," Jasnoch said, "and this anatomical arrangement makes for an amazingly loud call. A single bird can be heard two miles away."

Being in close proximity to 30,000 birds, as we were in the blind, made for an unforgettable auditory experience.

Sun rises on cranes

As the sun began to rise, we could see the great huddling masses of birds more clearly. The black figures on the river became gray, and I could start to see the birds' red-capped heads through my binoculars. Soon the noise rose in intensity and the roosting cranes began to take off, leaving the river to spend the day in nearby fields, fattening up for their journey north.

The birds take off in groups called lifts, some of which can be immense. A bit after 8 a.m., with the sun fully up, an eagle flew past our area, spooking the remaining cranes and provoking a massive, chaotic and incredibly noisy lift. In unchoreographed unison, thousands of birds flew off the Platte in one enormous, whirling flappage. Birds filled the sky, swirling up and around into a tornado of panicked motion.

While large lifts comprising hundreds of birds are common, this massive, eagle-induced one was unusual. Jansoch, who has lived in central Nebraska all his life, told me that the lift was among the biggest he'd ever witnessed. It was one of the most impressive moments in nature I've ever experienced.

Large-scale animal migrations like the one I was witnessing in Nebraska are a grand spectacle of nature.

"The saddest words in our language," Maupin said, "are 'you should have seen.' I've heard people say, 'you should have seen the herds of bison, you should have seen the clouds of passenger pigeons, when such things existed.' Well, happily, the cranes remain, and as long as we protect their habitat, they always will."

William Gurstelle is a travel writer based in the Twin Cities. Read more of his work at www. PeregrinationNation.com.