Science fiction writers have been using UAVs for surveillance for a generation or more. In his excellent 1980 science/fantasy novel Changeling, Roger Zelazny describes small, autonomous surveillance craft called 'tracer-birds' that were also able to detect and ride air currents:
The dark birdforms dotted the mountaintops like statues of prehistoric beasts, wings outspread. Had there been an eye to observe them, it might not even have noted their minute, tropism-like pursuit of the sun across the sky as they recharged their batteries for the night's flight.
...they stirred, almost simultaneously, as if shaken by a sudden breeze. They began to flex their wings.
Soon, one by one, they dropped from the heights, caught the air, rose, found their way, found their patterns, resumed their journey...
(Read more about Roger Zelazny's tracer-birds)