Recall the theme song from the Rogers and Hammerstein musical Oklahoma. Gordon MacRae, as Curly, sang, "Oklahoma, ev'ry night my honey lamb and I sit alone and talk and watch a hawk makin' lazy circles in the sky." Those lazy circles of the hawks are the same lazy circles eagles make, and they're possible thanks to the updraft force of convection currents.
To understand convection currents, 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.
Thermals Come and Thermals Go
Thermals appear during morning or early afternoon hours after the sun rises and warms the earth. The earth heats unevenly; and when a surface grows hot enough, a thermal column rises into the atmosphere.Think of this air as vertical wind. If the surface under the thermal remains warm, the thermal remains in place. When thermal air cools, the effect falls apart.
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.
In 1983, two Arkansas rabbit hunters stumbled upon an injured juvenile eagle in Osceola. Taken to the Memphis Zoo Rehab program, and named after the Arkansas town, a veterinarian removed Osceola's damaged wing. While working with Osceola over the years, eagle rehab specialist and hang glider, John Stokes, harbored a dream. He wanted to fly with Osceola. Thirteen years being injured, Stokes secured Osceola into a special harness and the two went hang gliding.
- YouTube: Rodgers & Hammerstein's Oklahoma!
- YouTube: Mr. Temme: Convection Demos
- Aerospaceweb: Birds, Thermals & Soaring Flight
- PBS: Nature: Raptor Force: Thermal Versus Dynamic Soaring
- Journey North: Up, Up and Away: Thermals and Updrafts
- Journey North: How Birds Fly
- Geat River Road: American Bald Eagle Scientific Fact Sheet
- Dan Scheiman, Ph.D., Director of Bird Conservation, Audubon Arkansas
- Dan Scheiman, Ph.D., Director of Bird Conservation, Audubon Arkansas: How Do Birds Find Thermal Columns of Air?
- Osceola: The Hang Gliding Eagle
- Tom Brakefield/Stockbyte/Getty Images