May 04, 2006

Flight Mechanics

flapNest Update: Our eaglets are ten weeks old now, and we're currently waiting for their first flight. Nause, our oldest eaglet, will likely make the plunge first. Once the eaglet fledges, it will be gone from the nest for a little while, so we're looking for a sustained period where only one eaglet is visible, or we're looking for a clear shot of the eaglet leaving or returning to the nest. Once the eaglets fledge, they will continue to use the nest as home base for four or five weeks until they become independent.

Speaking of Blackwater Refuge eaglets, one of our cam watchers -- Woody Dawson -- sent me a photo he just took from our Wildlife Drive showing a nest that has three bald eaglets in it. In this great close-up, you can see two of the eaglets and the mother. The third eaglet was farther back in the nest.

Flight Mechanics

Now that flight is imminent for our eaglets, it's a good time to talk about how eagles fly.

A full-grown eagle has over 7000 feathers on its body and has a wingspan of 6-7 feet (upwards of 8 feet on the larger female). The eagle's large wingspan means it must use a great deal of energy to flap its wings. Consequently, bald eagles don't do that much flapping, but instead spend a great deal of their time soaring, which requires about a 20th the power needed for flapping.

Eagles soar or glide when they migrate, and also when they hunt and fish. Using thermals and updrafts, an eagle can soar continuously for hours while using little energy.

Eagles can fly to an altitude of about 10,000 feet, and during normal level flight, reach a speed of 30-35 mph.

Like all birds, eagles have very light bones; in fact most of their bones are hollow and contain only air. If you added up the total weight of their bones, it would be about half the total weight of their feathers.

flapEagles have several types of feathers to aid them in flight, including the primary and secondary wing feathers, tail feathers, and coverts.

The ten outer primary feathers are on the end of the wing; these feathers are stiff and narrow because they meet the air first and must provide wind resistance. These outer primaries are also notched and they can open like fingers, which can help a flying eagle to achieve lift and drag.

The inner primaries and secondary feathers are flatter and will overlap to provide a smooth lifting surface.

Body, plume and downy feathers are needed for body covering, streamlining, waterproofing, and temperature regulation. This graphic shows you individual samples of several different types of feathers for both the adult and immature bald eagle.

When an eagle wants to move fast, it can turn the front edge of its wings into the wind and cut through the air. If it wants to slow down or stop, it can spread its wings and drag them through the air to create a "braking" effect.

When eagles flap their wings, the power comes from the downstroke. If you remember, we mentioned in a previous log that when the eaglets are flapping around the nest and their feet get off the nest floor, the lift is created by a strong downstroke.

Woody Dawson was kind enough to loan me some of his fantastic bald eagle photos that he took in Alaska. In this series of shots, you can see how the eagle uses the downstroke to get its feet off the ground and its body into the air.

If you'd like to read more about eagle flight and eagle feathers, here are two informative articles: "Wings and Feathers" and "Bald Eagle's Quest for Flight."

Until next time,
Lisa - webmaster

Posted by Webmaster at May 4, 2006 07:54 PM