Birds on the Move : Migration Made Simple
By Sue Corcoran
Man has long been interested in the phenomena of birds that disappear in the autumn and reappear come spring . Aristotle thought that delicate birds morphed into hardier species and changed back only with the return of warm weather . After additional observation , he declared that small birds , notably swallows , burrowed into the mud at the first sign of winter , hibernating there until spring . His theories of transmutation and hibernation were the prevailing wisdom for over 2,000 years . Until the 17th century , it was unthinkable that small birds were capable of long flights .
Today we know billions of birds leave their homes every year and travel a significant distance in search of food and optimal nesting locations . More than half of the birds that breed in North America are migratory . Their migrations can cover thousands of miles , last for weeks , and encounter dangerous conditions . The routes they take ( called flyways ) and the places they stop for much-needed rest and refueling ( called stop-overs ) have been the same for centuries . Or so we thought .
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