Bald Eagle

Bald Eagle

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Western Bald Eagle Migration Update: March 10, 1999

Today's Update Includes:

Today's Satellite Data from WESTERN Eagles

Field Notes from Biologist Jim Watson

To: Journey North
From: Jim Watson

Hello Students:
"Anticipation" is still the key word here in Washington as we wait to see where our eagles migrate to. That is part of the enjoyment of being a biologist after spending several long days in the winter trapping eagles; I can sit at my desk and let technology do the work of locating my birds.

Even with the advantages of using satellite telemetry to track eagles from Washington, problems can arise. For example, this week I received word from British Columbia about another one of my telemetered eagles from last year that was "recovered". From the locations I had been receiving from this eagle I figured that the transmitter was stationary. I checked the latitude/longitudes on consecutive locations and it showed no movement--even when 3-4 days between readings. I haven't talked to the person who recovered the bird, but I am most anxious to know if the bird is alive, and if not, if there is any way to know why it died. Did the transmitter in any way contribute to its death?

That brings up the point I want to has to be used cautiously in wildlife biology in order to be effective. Here are some things to ponder:

Transmitter and harness used for satellite tracking.

Aside from the obvious concern for the individual eagle, why else would I want eliminate any effects of a transmitter on a bird's behavior or health?

Challenge Question #9
"Think of one way that a transmitter might affect an eagle's health or behavior if it were: 1) too heavy; 2) too large (surface area); or 3) the harness too tight or loose."

(To respond to this question, please follow the instructions below.)

Eagle with Satellite Backpack
(Click on face of image to enlarge.)

As you can see from this picture, the transmitter is worn on the eagle's back--almost like a backpack. These backpacks are known as "PTTs", or "platform transmitter terminals". Each PTT package weighs approximately 100 grams (3 1/2 oz.), which is less than 2 percent of the body weight of an average bald eagle. How would it feel to carry such a backpack?

Challenge Question #10
If you wore a backpack that weighed 2% of your body weight, how heavy would your backpack be?

(To respond to this question, please follow the instructions below.)

In spite of the challenges of using this technology, it continues to provide fascinating information. A telemetered eagle which I captured last year JUST arrived this week on the Skagit River--the time of year when most eagles (and salmon carcasses) have left. The eagle had remained for the entire winter near his breeding area. My guess is he won't stay long...but here we go again...anticipation....

Stay Tuned,

Jim Watson
Washington Department of Fish and Wildlife

Spring Fever in the West?
Discussion of Challenge Question # 4
Last week we asked, "What are two factors Jim Watson says may contribute to an eagle beginning to move around the wintering area, after remaining in the same general location for much of the winter?" You are right if you said prey abundance and the urge to migrate! Look back at the February 24, 1999 update to review Jim's discussion.
Discussion of Challenge Question # 5
We asked, "As the crow flies, what's the total distance between all the places where the satellite received readings from Eagle #18? Explain why the EAGLE actually may have flown further."

The eagle probably flew more than the 216 mile "total" shown below, because we have only 8 readings in a full month. Satellite data gives us only a snapshot of eagle behavior. This is something biologists must always keep in mind when analyzing their data, being careful not to make assumptions.

Distance Between Data Points


















































We also asked, "What is the name of the river Jim Watson says Eagle #18 is visiting?" and the 7 grade students at Iselin Middle School responded: "We believe Jim Watson is talking about the Columbia River which the Eagle #18 is visiting."

Almost! He is very near the Columbia River but to the east, on the Spokane River. a tributary of the Columbia River, but just. Let's see he'll continue to stay on the east side of the Cascade Mountains before he migrates northward.

How to Respond to Today's Challenge Questions

Please answer ONLY ONE question in EACH e-mail message.

1. Address an e-mail message to:
2. In the Subject Line of your message write: Challenge Question #9
(or Challenge Question #10)
3. In the body of EACH message, answer ONE of the questions above.

The Next Bald Eagle Migration Update Will be Posted on March 24, 1999.

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