During the summer season there are barn swallows in almost all parts of North America. They build nests and raise their young there. But early in the fall the swallows leave their summer homes. They fly south for the winter. Some go as far south as Argentina. A barn swallow's winter home may be several thousand miles from its summer home. In the spring the swallows make the long journey back to the north.
Many other birds spend the summers in one part of the world and the winters in another. The traveling of birds between their summer homes and their winter homes is called bird migration.
The champion bird traveler is the arctic tern. This bird flies every fall from the far north to the far south. It returns every spring. Its fall and spring journeys together may be 22,000 miles—almost as long as a trip around the world at the equator.
Some birds make remarkable nonstop flights. The golden plover may fly over the Atlantic Ocean for more than 2,000 miles without stopping for food or rest. The tiny hummingbird crosses the Gulf of México in a 500-mile nonstop flight.
Birds run into many dangers as they migrate. Storms, electric wires, lighted buildings, and hunters are some of them. But migration is a help to the birds that make the journey safely. It lets them live the year round in the kind of region they are best fitted for. And it is a big help in getting food.
We do not see as many birds migrating as you would expect. Many birds fly at night and rest during the daytime.
Birds get ready for their journeys by eating a great deal. They go over their wing feathers. Some birds, moreover, gather into big groups before they start.
No one knows how migration came about or why some birds migrate and others do not. There are other mysteries about migration. No one knows how birds find their way. Probably landmarks such as rivers and mountains and coasts help. Air currents and the earth's magnetism may help, too. No one knows what starts birds on their spring and fall travels. Perhaps the amount of daylight they get each day lets them know when it is time to start.
Scientists are learning more and more about bird travels. Until a few years ago, for instance, the winter home of the chimney swift was unknown. In the fall the swifts flew out over the Gulf of Mexico, and no one saw them again until spring. Now it has been discovered that they spend the winters in Peru.
