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HOW DO MIGRATORY BIRDS FIND THEIR WAY?

Scientists are discovering that there is no single navigation system.

A

A bird may travel hundreds or even thousands of kilometres and still arrive in the same wintering area year after year. For a long time, people imagined that birds followed one perfect natural compass. Research now suggests something more flexible: migratory birds combine several sources of information, and the importance of each source can change during a journey.

B

During the day, some species use the position of the Sun as a compass. Because the Sun moves across the sky, the birds also need an internal body clock so that an afternoon Sun does not point them in the wrong direction. At night, certain species can use patterns of stars, while cloudy conditions make these sky-based signals less useful.

C

Birds can also respond to Earth’s magnetic field, which can provide a broad sense of direction when visual signals are weak. However, a magnetic compass is not the same as a detailed street map. As birds get closer to a familiar destination, coastlines, rivers, smells and other local landmarks can become more important. Experienced birds can therefore use information that young birds on their first migration have not yet learned.

D

This difference can be seen in displacement experiments, in which researchers move migrating birds away from their normal route. Adult birds often adjust their direction and head towards the usual destination from the new location. Young birds are more likely to continue in the direction they inherited, at least at first. Such results suggest that an inborn compass can begin the journey, while experience gradually adds a richer mental map. Wind and weather can still alter the exact path on any particular day, so successful navigation depends on flexibility rather than one unchanging rule.

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