Navigation in migrating birds
Each year, billions of birds migrate between breeding and wintering grounds, some covering more than ten thousand kilometres. Many return to the same nesting site year after year. How they find their way has fascinated scientists for centuries, and research suggests that birds use several sources of information rather than one.
Experiments in the mid-twentieth century showed that some birds use the sun as a compass, adjusting for its movement across the sky with an internal clock. Night-migrating birds, in contrast, can orient themselves using the stars. In planetarium studies, young birds exposed to a sky that rotated around a different star learned to treat that star as north.
Birds also appear to sense the Earth’s magnetic field. When researchers placed small magnets on the heads of some species, their ability to orient was disrupted on cloudy days, when the sun and stars were hidden. The exact mechanism remains debated, but it may involve special proteins in the eye.
Finally, familiar landmarks and even smells seem to guide birds over the last part of the journey. Homing pigeons whose sense of smell was blocked had greater difficulty returning home from unfamiliar locations. Scientists now think that birds combine these cues, relying on whichever is most reliable at a given time.