Animal migration explained: how birds, whales and butterflies find their way
TUREFF• •0
Every year, billions of animals make journeys that seem almost impossible: small songbirds crossing deserts and seas, whales travelling between feeding and breeding grounds, and butterflies flying thousands of kilometres to places they have never been. How they navigate is one of the most fascinating questions in biology, and scientists have answered parts of it while other parts remain open.
Why animals migrate
Migration is a trade-off. Moving costs energy and is dangerous, but it lets animals use seasonal resources in more than one place.
- Food: many birds breed where summer brings a burst of insects and long days for feeding chicks, then leave before winter shortages.
- Breeding: some whales feed in cold, productive waters and give birth in warmer waters.
- Climate: escaping cold or drought can be the difference between surviving and not.
Two problems: direction and position
Scientists separate navigation into two tasks. A compass tells an animal which way is south. A map sense tells it where it is relative to its goal. Many migrants clearly have a compass; how much of a true map they have, especially on their first journey, is still being studied.
How birds find their way
Research has shown birds use several compasses that back each other up.
- The magnetic compass. Many bird species have been shown to sense the Earth's magnetic field. Experiments with migrants in cages show they orient in particular directions even without a view of the sky.
- The sun compass. Birds can use the sun's position, but because the sun moves across the sky, they need an internal clock to interpret it.
- The star compass. Night-migrating songbirds learn the pattern of stars around the point the sky rotates about, which in the Northern Hemisphere is near the Pole Star.
- Landmarks and smell. Coastlines, mountain ranges and rivers help, and homing pigeons are thought to use smells as part of their map.
These cues are not used in isolation. Studies have found that birds calibrate one compass against another, for example checking their magnetic compass against the sunset. Young birds of many species make their first migration alone, guided by an inherited direction and timing, and refine their route with experience.
How monarch butterflies navigate
The monarch butterfly of North America makes one of the most famous insect migrations, and no single butterfly completes the whole round trip; it takes several generations. Research has shown that migrating monarchs use a time-compensated sun compass: they read the sun's position and correct it for the time of day. Surprisingly, experiments found that clocks in the antennae are essential for this correction. Studies also suggest a light-dependent magnetic compass that may help when the sun is hidden.
Insect migration is not only an American story. Painted lady butterflies and many moths travel between Africa and Europe, and radar studies have revealed vast numbers of insects moving high overhead unseen.
How whales cross oceans
Humpback whales and grey whales are among the mammals with the longest migrations, travelling between polar feeding grounds and tropical or subtropical breeding areas. How they hold such straight courses across open ocean is less well understood than in birds, partly because whales are so hard to study. Researchers have proposed that they may use some combination of the Earth's magnetic field, the sun and stars, underwater sound and ocean features, but this remains an active area of research rather than a settled answer.
Threats along the way
Migrants depend on a chain of places: breeding sites, wintering sites and stopovers in between. Losing any link can affect a whole population. Problems include habitat loss, light pollution that disorients night-flying birds, collisions with buildings and power lines, and changing seasons that can put arrival times out of step with food supplies.
How you can watch and help
- Watch migration hotspots such as headlands, estuaries and islands in spring and autumn, when birds gather before or after sea crossings.
- Record what you see in a national bird or butterfly recording scheme; arrival and departure dates help track change.
- Reduce outdoor lighting during migration seasons.
- Make windows visible to birds with patterns or decals.
- Support wetland and coastal reserves that provide vital stopovers.
Have you seen a migration in action, such as the first swallows or a flock heading out to sea? Share it in the comments.