There are two ways birds use air currents to travel.
Thermals are updrafts of air that hawks use to gain altitude.
Thermal currents are produced by uneven heating of the air at ground level and start early when the cool nighttime air is hit by the morning sun. The currents intensify as the sun gains strength and continue till early afternoon. When the air cools to a point at which it can no longer lift an object, the hawks peel off and continue gliding. In this way, given a good day with lots of thermal activity, they can fly hundreds of miles without flapping their wings. Deflective air currents are simply the result of a strong wind hitting the side of a mountain and being deflected upward.