How can birds know when to migrate?

Flying on a straight course, "Sandhill" cranes head north to their summer home. Each spring, the cranes fly from Mexico and the southern part of the United States to Canada and Alaska. In the fall, they again head south. Large numbers of birds make similar round- trips every year. They migrate mainly to find food, better weather conditions, and places to breed and to raise their young/babies.

Perhaps you, like some scientists, have wondered how migrating birds know just when to come and go. Scientists believe that changes in the hours of daylight are the most important signal. As days grow longer in the spring, the increased periods of the sunlight cause changes in the birds' bodies. The birds eat more to store fat for fuel. They grow restless. Soon they take off for their summer homes. the same thing happens in the fall, when days start getting shorter. Changes in the weather and a shrinking food supply may also help the birds know when it's time to move on.

Besides knowing when to migrate, birds seem to know exactly where they're going. Most follow the same routes every year. To find their way, they use their keen senses to respond to a variety of clues. These include landmarks, such as rivers and mountains, and the position of the sun and the stars. scientists continue to study other factors, as well, that may help the birds navigate on their long journeys.