http://www.dailymail.co.uk/sciencetech/article-3713227/Secrets-birds-soar-revealed-Spirals-wind-help-creatures-catch-thermals-fly-great-heights.html

These graphs show how an untrained glider (left) takes random
decisions and descends, while the trained glider (right) learns to use spiraling
patterns in regions of strong ascending currents, as observed in the thermal
soaring of birds and gliders. The colours indicate the vertical wind velocity

They wrote in their study, published in the journal Proceedings of the National
Academy of Sciences: ‘torque and vertical accelerations’ appear to be the cues
that most effectively guide the most efficient soaring path of birds through
thermals, rather than differences in temperature.

‘Temperature was specifically shown to yield minor gains,’ they continues,
adding: ‘a sensor of temperature could then be safely spared in the
instrumentation for autonomous flying vehicles.’