Thermal and Deflective Air Currents
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. Hawks
will circle (kettle) in the thermal
currents and climb as high as possible. 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. These currents don't rise high like the thermals and are
utilized by low flying, maneuverable raptors like sharpies, Cooper's
hawks, harriers, etc. This "accipiter wind" can provide
propulsion as well as lift, carrying the birds along like a surfer on
a wave. Later in the fall, as the days shorten, thermal activity
decreases and deflective activity increases.