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Following the food

Why Do Birds Migrate?

Birds fly thousands of miles because the food supply moves with the seasons.

1 Food disappearsas winter comes2 The bird fliesto where foodis3 It returns whenfood comes back
Enormous effort, because the alternative is starving.

  1. Step 1 of 5

    Northern summers are extraordinarily productive.

    Northern summer Huge food supply, longdaysNorthern winter Almost nothing to eat

    Long days and insect explosions make high latitudes brilliant places to raise young — for about three months a year.

  2. Step 2 of 5

    Then the food vanishes completely.

    Summer insect supplyabundantWinteralmost none

    Insects die off, fruit is gone, water freezes. A bird that stays has to survive months on nothing.

  3. Step 3 of 5

    Flying south is the cheaper option.

    Stay or go?Migration:exhausting andriskyStaying: nofood at all

    Migration is exhausting and dangerous, but starving is worse. Species migrate only where the journey costs less than the winter would.

  4. Step 4 of 5

    They navigate by several systems at once.

    Sun position by dayStar patterns by nightEarth's magnetic fieldCoastlines and rivers

    Birds use the Sun, star patterns, landmarks and Earth's magnetic field. Losing one cue rarely stops them, because they have backups.

  5. Step 5 of 5

    Some journeys are almost unbelievable.

    Arctic tern, one yearabout 70,000 kmSwallow, one wayabout 10,000 km

    The Arctic tern flies from Arctic to Antarctic and back each year — around 70,000 km, and over two million kilometres in a lifetime.

    Many small birds fly the whole journey at night and navigate by stars they have learned as chicks.

The short version

Birds migrate because seasonal food supplies move, and they navigate with several overlapping systems.

Try it yourself

Note the first swallow you see each spring and compare with last year. Arrival dates are remarkably consistent.

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