To an eagle, saving energy during migration is crucial. After all, the eagle doesn't know where it will find its next meal. As Peter Nye has explained , strong uplifting winds and thermals to are ideal for eagle migration, because they let eagles soar and glide. They take a free ride on air currents. A thermal is a bubble of air which rises from the ground.
Thermals form as the sun heats the earth, and the air near the surface warms and expands.
Since warm air is lighter than cold air, the warm air rises and a thermal is formed. For an eagle, a thermal is like an elevator which carries the eagle effortlessly high into the sky. The eagle then glides for several miles to the next thermal without flapping its wings at all. Compare that to a hummingbird which beats its wings 75 times per second when in flight. Air currents in a thermal can be very strong and turbulent.