Migration of Birds

Birds migrate the farthest of any creature in the animal world. The ruby-throated hummingbird, which weighs only 1/10 of an ounce, briskly travels 500 miles across the Gulf of Mexico. The record belongs to the Arctic tern, which travels 11,000 miles each way. Not all birds migrate, however; the crow stays in the same place all year round.

Migration is usually tied to the change of seasons. Birds often leaves as winter comes, when food becomes scarce. They go to warmer places to spend the winter. In the spring they come back to mate. But the link between climate changes and migration is not always clear. For example, around the equator, where there are no seasons, many birds still migrate. In this case, it is possible that migrations are tied to periods of rain and drought.

To track down the places where birds go when they migrate, scientists put lightweight rings around their legs. They then catch the birds all over the world and can draw migration maps. Very light radio transmitters are also used to pick up the birds movements. The results of these studies show that migration tend to go from north to south and south to north.

Species that breed in the north tend to migrate to the south and the other way around. For a given group of birds, the distance flown and the points of departure and arrival are always the same. For instance, the Kirtland’s warbler always nests in the pine woods of north central Michigan. Near the end of the summer it heads southeast toward the Bahamas, where it arrives in mid- September. In April, it flies over Florida, Georgia, and South Carolina on its way back to Michigan. It arrives in May.

Some birds fly at night and use the starts to guide them. Mallards in lllinois head in the correct direction on clear nights, but become confused when the stars are hidden by clouds. Other birds, such as starlings, use the sun to stay on the right course.