Millions of birds of many different varieties migrate at the end of summer. With unfailing regularity they leave the regions where they were born to fly to warmer climates for the winter. The following spring they return to their breeding grounds.
Each year these migratory birds travel as much as 20,000 miles, finding their way back on time with extraordinary precision. Some travel as individuals.
Migration is triggered off by the length of daylight, which apparently affect the birds' nervous system. When the days get shorter the birds receive the signal to leave for their warmer winter grounds, and when the daylight increases to a certain level, they receive another signal to return home.
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