Bald Eagle Migration Update: April 21, 1997
"Here are the latest data for both our eagles. It looks like #N 98 just decided to head back north. Her April 16th location puts her approximately back where she was in early April, before the cold arctic front moved into the area with temperatures in the teens and wind chills near zero. As you recall, we were unable to tell which day # N 98 moved because of the faulty transmitter. Have a look at today's data and see if it helps your analysis:
Satellite-tracked eagles have given us clues about distances eagles migrate in a day. Over the years, Peter Nye's eagles have traveled 100 miles per day on average, and one eagle moved 225 miles in a single day. Imagine the energy it would take you to travel such distances. To an eagle, saving energy during migration is crucial. After all, the eagle doesn't know where it will find its next meal.
As Peter Nye has explained, strong uplifting winds and thermals to are ideal for eagle migration, because they let eagles soar and glide. Eagles avoid "powered flight" (flying by flapping their wings) whenever possible--and for good reason: According to research by eagle biologist Austin-Smith, it takes 20 times more energy for an eagle to fly by flapping its wings than to fly by gliding! (Soaring is the same as gliding, but soaring means flying upward in a spiral.)
How do eagles manage to save so much energy? They take a free ride on air currents. A "thermal" is one example. A thermal is a bubble of air which rises from the ground. Thermals form as the sun heats the earth, and the air near the surface warms and expands. Since warm air is lighter than cold air, the warm air rises and a thermal is formed. For an eagle, a thermal is like an elevator which carries the eagle effortlessly high into the sky. Without even flapping its wings, the eagle defies gravity and rides upward--sometimes as fast at 10 miles per hour. The eagle then glides for several miles to the next thermal without flapping its wings at all. In fact, according to observations by eagle biologist Jon Gerrard, on a good day an eagle may use powered flight only to reach the first thermal at the beginning of a day's migration--and at the end of the day to get to a good roosting place. Compare that to a hummingbird which beats its wings 75 times per second when in flight.
Air currents in a thermal can be very strong and turbulent. Ask any pilot, and you're sure to hear stories about planes hitting thermals unexpectedly. Pilots watch for the cumulus clouds which often form at the top of a thermal, but sometimes there is no visual clue at all. Because thermals and other air currents are strong during the day, ornithologist think this may explain why many "powered fliers" often fly at night.
Is flight really free? Eagles do get a free ride when riding thermals, and can migrate great distances with ease. But it does require energy for an eagle to travel. This is a basic law of nature: All work requires energy. How would you answer this question?
Challenge Question # 8
Now that we're challenging laws of nature, what are your thoughts about this:
Challenge Question # 9
How to Respond to Journey North Bald Eagle Challenge Question # 9
How to Respond to Journey North Bald Eagle Challenge Question # 8
Challenge Question # 8
The Next Bald Eagle Migration Update Will be Posted on May 5, 1997.