Migrating Bluebirds

How do bluebirds know when it's time to fly south? They often disappear from our place in August when the weather is still warm.

—Mary Drogos, Glenview, Illinois

George: Birds rely on the light to tell them when to migrate. As the sun in the northern hemisphere moves south and the hours of daylight dwindle, it triggers an impulse within every migratory bird. This impulse ends only when the bird arrives at a wintering ground far to the south.

The birds instinctively follow ancient paths of migratory flight that are permanently chiseled into the face of the Earth, such as coastlines, major rivers and mountains.