How do eagles glide, without flapping their wings?

If you watch an eagle’s movements through binoculars, you will see that even though the bird appears to be gliding lazily, it is actually very active. Its outer wing feathers and tail feathers are constant motion, catching up-drafts of warm air, and steering the brid through moving air currents.

To understand convection currents/ air current, think of a pot of water. As the water close to the burner warms, it rises to the top and boils. At the same time, cooler water on top moves downward to replace the rising hot water. Convection currents in air work similarly. As the air closest to the earth warms, it rises in a column called a thermal. Cooler air outside the thermal column is forced down.

image source: wikimedia commons

Eagles fly into thermals, using them to conserve energy while migrating or looking for prey. How eagles find thermals is unclear. Once inside, they stop flapping but keep their wings extended. Their tail feathers open like fans, and tapered feathers on the wing edges spread apart; both actions enhance airflow. Without flapping their wings, the eagles will descend -- but inside the thermal, the rate of descent is slower as the lighter, hot air pushes vertically. Staying aloft requires forward motion. To remain inside a thermal column, eagles navigate in circular paths, steering with tails and wings. Thus they create lazy circles in the sky. Many circling birds within a thermal is called a kettle of eagles. When one thermal cools, eagle within will either catch a new thermal or take up flapping to stay aloft.