Ever notice an eagle soaring in circles without so much as a wing beat? The circular pattern is actually a clue that the bird couldn't pull this little trick just anywhere. These birds are using thermals, masses of warm air rising as invisible columns. The air rises faster than the light-weight free-loader sinks, and so the bird is carried effortless skyward.
But why would birds do this? Simple—the basic rule for predators is that they must get more energy from their prey than they use capturing it, and soaring is a wonderful hunting technique. A free ride means no expenditure of energy, and with their incredibly sharp eyes, they can spot their prey scampering across the ground from high above and rapidly respond to this unmistakable invitation for lunch.
Birds often actively seek thermals, and one way they find them is to
watch for other birds circling and rising. But as one ornithologist learned, some
don't distinguish a circling helicopter from a circling bird. How frustrating to
watch a helicopter obviously using a thermal that you just can't catch! 
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Kodi R. Jeffery, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
When conditions are right, moisture in the upper portions of rising thermals condenses to form clouds. Each of the clouds seen here is formed at the top of a thermal.