Every year billions of birds undertake journeys that would exhaust the most determined human traveler. Arctic terns fly from one polar region to the other and back, covering tens of thousands of kilometers, while tiny hummingbirds cross the Gulf of Mexico without a single rest. Migration is driven by the search for food and safe nesting grounds as the seasons change. Birds navigate using a combination of the sun, the stars, the Earth's magnetic field, and familiar landmarks such as coastlines and rivers. Before departure many species eat heavily, nearly doubling their weight in fat to fuel the trip.
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