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Mapping Golden Eagles

Since 1994, Dr. Grainger Hunt has used an airplane to monitor the survival and movements of golden eagles fitted with backpack-style radio transmitters near Livermore, California. Each transmitter is designed to emit a unique radio frequency signal allowing the identification of specific individuals. The pulse rate of a transmitter also provides information on the status of the eagle. A slow steady pulse is heard when the eagle is alive while a fast steady "mortality" pulse indicates the transmitter has been motionless for more than four hours suggesting that the bird may be dead.

Dr. Hunt listens for these signals while flying a regular survey route several thousand feet above the terrain. The plane is equipped with an antenna on each wing that receives a signal from the direction the wing is pointing and a receiver inside the plane that scans for the frequency of each transmitter. When Dr. Hunt hears a signal, he locks the scanner onto that frequency. A "switch box" that allows each antenna to be operated independently is then used to determine which antenna is receiving the strongest signal. Dr. Hunt then turns the plane in the direction of the signal until both antennas begin to receive the signal at a similar strength. As he approaches the eagle's location, the signal becomes stronger. When he is directly above the eagle, the location is recorded using a Global Positioning System (GPS), a small device that uses satellites to accurately determine the latitude and longitude of the plane's position. This information is later analyzed using Geographic Information Systems (GIS) technology, a computer program that superimposes the eagle locations on maps containing other information such as vegetation cover, land use, roads, and man-made structures. Recently developed computerized maps of topographical conditions and other landscape features are of particular use in identifying the characteristics of habitats used by the eagles.

Mapping software courtesy of MAPTECH