I took this shot at Rattlesnake Springs in New Mexico. Although it was nearly 10:00 AM on this cloudy day, hundreds of vultures where still roosting in the trees. What were they waiting for?
Vultures are very energy conscious, and would much rather glide on rising air currents than actually flap their wings. As a result, they will often spend a cloudy morning sitting in trees, waiting for the sun to come out and produce enough heat to produce thermals, rising columns of air. These give them a free ride to the sky. Vultures are so expert at gliding that they can often fly for hours without flapping their wings.