Migration remains a natural mysteryBy Terry Plowman
More than 1,500 miles north of Cape Henlopen, a Canada goose lifts into flight, urged to fly southward by a mysterious biological clock.
By the time it lands in a Sussex County cornfield two weeks later, it has accomplished one of the most amazing, yet least understood feats performed by an animal in nature -- instinctive navigation.
It is a mysterious annual process that is successful over thousands of miles of trackless ocean and earth, through day and night, even through low- and no-visibility conditions.
How do they do it?
Despite decades of scientific research, the complete answer has yet to emerge. But ornithologists are piecing together evidence of an impressive set of skills which migrating birds use to find their way.
Studies of homing pigeons and migrating waterfowl show that they navigate using a combination of physical landmarks, the position of the sun and stars and even the earth's magnetic field.
While scientists have guessed for years that migrating birds use the sun and stars as a compass reference, only recently have they discovered an unusual deposit of magnetic tissue that they believe is sensitive to the earth's (and maybe even the moon's) magnetic field.
According to Dr. Charles Walcott, a prominent pigeon researcher and Director of the Ornithology Lab at Cornell University, scientists have found that some fibers in pigeons' optic nerves as well as a sort of magnetic facial nerve are responsive to the earth's magnetic pull.
The iron ore-like deposit consists of magnetite, a highly magnetic mineral which has also been found in honey bees. Its location in the bird's face and near its eyes makes it a natural navigation system.
While this magnetic tissue has been confirmed only in homing pigeons, scientists believe other migratory birds have it as well.
Many species of fish also migrate, apparently following their food sources as well as temperature patterns in the water, according to Stewart Michels, state Fish and Wildlife environmental scientist. Research also indicates that, like waterfowl, they may use geomagnetic forces to assist their navigation, Michels said.
But despite all the research, scientists are still not sure how migrating birds combine all these natural skills to so accurately and consistently find their way across thousands of miles.
"It seems that birds from different areas use different combinations of these skills, just like someone who grows up in the city would use the skyscrapers and street numbers to find their way, and someone from the country would use natural landmarks," Walcott said.
Birds also appear to use different skills depending on visibility.
In the daytime they determine which way is south "boy-scout style," Walcott said, by looking at the location of the sun. They also use large physical landmarks such as rivers when they can see them. At night they use the position of the stars, and under poor visibility conditions they apparently rely on their built-in magnetic compass to assist their flight.
Whatever the navigation method, about 36,000 Canada geese and a record 100,000-plus ducks will find their way to Delaware this winter, according estimates by state waterfowl officials.
And like clockwork, they will lift off in the spring, bound for the breeding grounds of Canada and the Arctic.