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Because of the cold climates of the northern parts, birds migrate to the South to spend the winter. |
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The arrival of birds which have wintered in the South is a sign to northerners that Spring has come. |
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Birds come back to the North one way or another every year in spring¬time, as all people who live there know. |
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When migratory birds come back to the North from the South, people know it is a sign that Spring is over. |
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It is the coming of Spring which lets migratory birds know it is time to head back to their nesting grounds in the North. |