A MONUMENTAL MIGRATION
Every spring, more than 600,000 sandhill cranes move through Nebraska's Platte River Valley.
TORU FUJIOKA
The spring migration of millions of waterfowl and more than 600,000 sandhill cranes through the Platte River Valley is only one example of avian migrations worldwide.
But the mysteries of migrating birds have never been solved.
"It's true we may not know their secrets unless they tell us," said Ron Johnson, a professor in the University of Nebraska-Lincoln School of Natural Resources.
But scientists have found some solid evidence and possible explanations.
"Migrating birds are looking for nutritious food, a suitable environment for breeding and safe places to raise their young," said Mark Vrtiska, a waterfowl biologist at the Nebraska Game and Parks Commission. He said some birds do not migrate if their habitat meets their needs.
Although the southern hemisphere is warm, most birds migrate to the North in the spring.
"It doesn't mean it's cold for them," Johnson said.
Birds may migrate because of abundant food and greater land mass in the North, he said.
Vrtiska and Johnson said sandhill cranes migrate at almost the same time each year largely because of changes in natural phenomena such as weather, tides, length of day and water temperature. Sandhill cranes wait for a south wind to take off from wintering grounds in the South and then fly toward breeding grounds in the Arctic.
To fly so far, the cranes must store energy, so they pause for several weeks in the central Platte Valley to build up body fat. Vrtiska said about 90 percent of the cranes' diet is wasted corn, which provides minerals and proteins they need for breeding. Cranes can gain up to 20 percent of their weight in the weeks they spend in Nebraska.
"Diet is a main reason why migratory birds fly through Nebraska," Johnson said.
He said young birds learn a migration route while flying with their parents, but other navigational tools may also help them stay on their course. For guidance, the birds may use such landscape features as rivers, coastlines and mountain ranges. They may also watch stars and the sun. The birds may even sense Earth's magnetic field with tiny grains of a mineral, called magnetite, in their brain. Their keen hearing may also help.
"Some birds - pigeons, for example - may hear such things as big storms or wind blowing across a valley hundreds of miles away," Johnson said.
While migrating, most birds fly at altitudes under 3,000 feet, but bar-headed geese have been observed flying above 29,000 feet, as high as Mt. Everest.
To save energy, Vrtiska said, migrating birds try to soar and take a free ride on thermals, or updrafts of air caused by solar heating. Some birds appear to save energy with assistance from a tailwind.
The majority migrate at night. Johnson said one of the most convincing explanations for this is that "when they land, they need to see to forage for food."
Flying at night may also help birds avoid predators and enjoy cooler temperatures.
"Birds aren't perfect navigators," Johnson said. "They sometimes get lost because of huge storms, winds or fog."
In addition to harsh weather, migrating birds have to overcome human-made barriers, Vrtiska said. Tall buildings, electric wires and wind-power generators may confuse them.
Sandhill cranes migrate 6,000 to 7,000 miles twice a year, flying north in the spring and south in the fall.
With a life span of 25 to 30 years, a sandhill crane may travel up to 420,000 miles in its lifetime.
