Many birds fly extraordinarily great distances in search of warmer climate, food and favorable breeding grounds, a seasonal phenomenon we now know as migration. But to early scholars such a notion was inconceivable—how can a bird weighing only a couple of hundred grams survive the hardships of intercontinental travel over lofty mountains and vast oceans to distant lands with no apparent means of navigation, and then fly all the way back? Yet, scholars struggled to explain why some species of birds appeared and disappeared as the seasons changed.
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Source: amusingplanet.com