American Bald Eagle: Rising High on Thermals
Wingspan: 7 - 8 feet
A bald eagle makes it look as if it takes no effort at all for it to rise to great heights above the earth's surface. This skill is a combination of two things: the differences in how quickly different parts of the earth's surface heat up and the broad wings of the eagle.
During the day, as the sun shines down on the surface of the earth, some places warm up faster than others. This is because some places absorb more of the sun's heat than others. For example, land warms up more quickly than water and a parking lot heats up faster than a grassy field.
Different temperatures on the earth's surface cause the air above these surfaces to also have different temperatures. Air over land heats up more quickly than air over water. When the warm air over the land is next to the cooler air over the water, forces are set up that make the warmer air rise above the cooler air. This rising part of the air is called a thermal. A bald eagle's wings are specially adapted to take advantage of these rising thermals of air. Look carefully though, turkey vultures also like to glide on the updrafts and may look like eagles from far away.
When an eagle first leaves its perch, it uses its broad wings and powerful flying muscles to get higher. Once an eagle is up in the air, its wide wings allow it to stay up with less work. When an eagle comes across a thermal, it can use this rising hot air to really climb. By circling over the middle of a rising thermal, a bald eagle can just spread its wings, allowing the warm air to lift the eagle to heights up to 3 miles above the surface of the earth.
You can watch bald eagles soaring high this winter. Head for places with open water where the birds can catch fish like the Wisconsin River at Sauk City. Some communities sponsor special winter bald eagle watching days.