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Washington, District Of Columbia (AFP) Aug 29, 2013
Whooping cranes learn how to migrate by following elders in their midst, suggesting that social influence has a larger bearing than genetics on the birds' behavior, scientists said Thursday.
The large, white birds are endangered in the wild of North America, with just one native population of about 250 in Canada that spends winters along the Gulf Coast of Texas.
But a growing captive-bred group in the northeastern state of Maryland has provided researchers with an unparalleled set of genetic and travel data to study, answering the critical question of whether the birds are programmed by nature to make their way south or if their behavior is learned.
"The knowledge is transmitted from older to younger birds," said study co-author Thomas Mueller, a biologist at the University of Maryland.
"Migration becomes more and more efficient as these birds age and that takes place over many years."
Without an experienced crane to follow, young birds strayed much farther from their intended path, said the findings in the US journal Science.
The longer the birds practiced, the better they became at sticking to a straight route.
The research was based on eight years of data from a population of 73 birds that were bred in captivity.
Humans play a key role in getting the migration started.
The chicks are born in the spring and soon begin learning to follow a light aircraft that is at first driven along the ground by a human wearing a white-suit disguise in order to prevent the birds from growing accustomed to humans in their midst.
Later, the aircraft is flown as the birds gain strength.
Eventually, in the fall, the young birds will follow this aircraft all they way south.
The cranes make their own way back to Wisconsin after the winter, and fly back southward in their own pairs or groups without the aircraft after the first year.
Scientists studied the birds' behavior beginning the first year after the human-led flight.
Their movements were tracked by satellite transmitters, radio telemetry and people observing them from the ground.
Researchers found that groups that included a seven-year-old adult were much better able to stick to a straight path for the 1,300 mile (2,000 kilometer) journey.
"We had groups that were only just juveniles, for example. They did significantly worse than if there was an older bird in the group," Mueller told AFP.
One-year-old birds that did not follow older birds veered an average of 60 miles (97 kilometers) from a straight flight path.
But when the one-year-old cranes traveled with older birds, the average deviation was less than 40 miles (64 kilometers).
Researchers said the experience of elders might have helped them recognize landmarks and keep the group on track.
"We've long known that learning is important in this species. But we were surprised by the degree to which the learning continued for many years," said co-author Sarah Converse of the US Geological Survey.
"Eight-year-old birds are better than six-year-olds, and they are better than four-year-olds, and so on," she said.
"Also, we were impressed by the importance of cultural transmission of knowledge."
Gender, group size and genetic closeness to other birds in the group had no impact on helping the birds stick to the straightest flight path.
The whooping crane, or Grus americana, is the largest bird in North America. It stands about five feet tall and can live for 30 years or more.
The birds were hunted heavily in the 1800s and beyond, and have lost much of their wetlands habitat. Barely more than a dozen were left by 1941. The population is now slowly rebounding.
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