GIBBON, Neb. -- Along the Platte River in the chilly moments before dawn, the silence is broken by a morning call.
The chorus builds as the sun rises, and there they are: hundreds of thousands of sandhill cranes awaiting some signal from their sentinels that it's time to wake up and face the day.
Suddenly, they stir and lift off all together, resuming a ritual of one of the world's great natural migrations.
"I describe it as the sounds of a chorus of angels, none of whom could sing on key, but all trying as hard as they can," says Paul Johnsgard, a retired professor at the University of Nebraska who's been coming here as a witness for 52 years.
"It's almost like watching ballet in slow motion, because the wing beats are slow and they move in such an elegant way," he says. "Both in the air and on the ground. There's nothing awkward about them. It's just poetic."
Nebraska is a way station for the cranes -- and a magnet for bird lovers. The world-renowned naturalist Jane Goodall has visited for the last 13 years.
"I wasn't quite prepared for the absolutely unbelievable, glorious spectacle of all these thousands of birds coming in," she says. "It's just unbeatable, and it's really peaceful."
The shallow Platte is perfect for the cranes, because they sleep standing in the water to avoid predators. This is familiar turf to them -- it's where they pick out mates or discourage rivals.
The nearby cornfields are where they fatten up for three weeks before resuming a 7,000-mile trip that starts in Mexico and ends as far north as Siberia.
And this migration has been going on far longer than there has been a Nebraska.
"We don't know when it started," Johnsgard says. "They stop where their parents and grandparents and great-grandparents stopped."
As dusk approaches, the cranes are stuffed and ready for their river bed. On cue, back they come, settling in for another night in Nebraska.
"You can't help but love a crane," Johnsgard says. "You really can't."