Reading passage
Every year, billions of birds leave their breeding grounds and travel enormous distances to spend the winter in warmer regions. This movement, known as migration, is one of the most spectacular events in the natural world. The Arctic tern holds the record: it flies from the Arctic to the Antarctic and back each year, a round trip of more than 70,000 kilometres. Over an average lifetime of thirty years, a single tern may travel a distance equivalent to three return journeys to the Moon.
Birds migrate primarily to find food and safe places to raise their young. In the far north, the long summer days produce an explosion of insects, ideal for feeding hungry chicks. When winter approaches, however, daylight shrinks, water freezes and insects disappear, so the birds must move south or starve. Migration therefore allows a species to exploit two different environments, each at the time of year when it is most productive.
How birds find their way over such vast distances has fascinated scientists for centuries, and research has revealed that they use several different compasses. By day, many species navigate by the position of the sun, correcting for its movement across the sky with an internal clock. At night, some songbirds orient themselves by patterns of stars, while young birds making their first journey appear to follow an inherited map of directions and distances, needing no teacher to show them the way.
Perhaps the most surprising discovery is that birds can sense the magnetic field of the Earth. Tiny particles of magnetite, a natural magnetic mineral, have been found in the beaks of pigeons, and specialised proteins in the eyes of robins seem to react to magnetic direction. Experiments in which small magnets were attached to birds confused their sense of orientation, providing further evidence that a magnetic compass guides them when neither sun nor stars are visible in the sky.
Migration also demands extraordinary physical preparation. In the weeks before departure, many birds eat constantly and almost double their body weight, storing layers of fat that serve as fuel for the journey ahead. The bar-tailed godwit, for example, flies more than 11,000 kilometres from Alaska to New Zealand without stopping, a non-stop effort of about eight days. To lighten the load, some migrants even shrink their digestive organs before take-off, since these are not needed during flight.
The journey itself is full of danger. Storms can blow birds far off course or exhaust them over open water, and predators such as falcons wait along traditional routes. Human activity adds further hazards: tall buildings, television towers and wind turbines kill millions of migrants each year, while artificial light confuses birds that navigate by the stars, drawing them into cities where they collide with glass. Climate change is also shifting the timing of insect hatches, so some birds now arrive at their breeding grounds too late to find abundant food for their chicks.
Because many migratory species are declining, scientists are racing to understand their journeys in greater detail. Modern tracking devices, some weighing less than a gram, can be fitted to a bird like a tiny backpack and record its exact route across continents and oceans. The data have revealed surprising stopover sites, for instance single coastal wetlands where hundreds of thousands of birds pause to refuel. Protecting these few critical places, conservationists argue, may matter more than trying to safeguard the whole of a long migration route.
International agreements now encourage countries to cooperate, since a single bird may breed in one nation, rest in a second and winter in a third. There are success stories: the recovery of several goose populations shows that hunting controls and habitat protection can work when governments act together. Nevertheless, with fewer insects, a changing climate and ever-expanding cities, the long-distance traveller faces an uncertain future, and the great spectacle of migration can no longer be taken for granted.