十万个为什么·科学启蒙30篇（2）
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Why Do Birds Migrate Thousands of Miles Each Year?
为什么鸟类每年要迁徙数千英里？
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Birds migrate to find better food, warmer weather, and safe nesting places.
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They use the sun, stars, Earth’s magnetic field, and even landmarks to navigate.
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Young birds often follow experienced adults on their first long journey south.
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Some species, like the Arctic Tern, fly over 40,000 miles round-trip each year.
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Their bodies prepare for migration by storing extra fat as fuel for long flights.
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Scientists track migrating birds using tiny GPS tags attached to their backs.
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Changes in daylight length trigger hormonal shifts that begin migration behavior.
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Not all birds migrate—some stay year-round if food and shelter remain available.
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Wind patterns and weather fronts strongly influence when and how high birds fly.
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Conservation efforts protect stopover habitats where tired birds rest and refuel.