Prior to the expansive study on the ecology of the bald eagle in Arizona conducted by Biosystems Analysis, Inc.
(Hunt et al 1992) little to nothing was known about the movements of juvenile eagles. Most bald eagles nesting
in inland Canada and Alaska migrate south into the contiguous United States in winter. Similarly, eaglets banded
in the Great Lakes region migrated south into the Midwest and southern Mississippi Valley states, and also into Texas.
During the 1930s and 1940s, over 1000 nestlings in Florida were banded in one of the first studies of the migration
of bald eagles native to the southern portions of the United States. Bands were recovered throughout the states
east of the Mississippi River, but most recoveries showed the eagles moved northward up the east coast, some all
the way to Canada.
By following radio transmittered juveniles (hatch year birds), Biosystems Analysis, Inc. provided the first
glimpse at the migration patterns of juvenile bald eagles in Arizona. Using airplanes to track the eagles,
it was determined that the juveniles migrated north. Birds were detected in such locations as the mouth of
the Klamath River in northern California, Yellowstone Lake in Wyoming, and Swan Lake in Manitoba, Canada.
To understand the migratory timing of the Arizona juveniles we must first realize that their strategy evolved
during pristine times when food distribution and abundance may have differed from today (Hunt et al. 1992).
As to what food supplies would have been available to inexperienced juveniles in the northern latitudes in the
summer, the possibilities include:(1) winter-killed ungulate carcasses emerging during snowmelt; (2) fish kills
at thawing high-altitude lakes; (3) nesting and molting waterfowl; (4) the spawning salmon and other fishes;
and (5) the calving seasons of elk, bison, pronghorn antelope, and other ungulates. Together, these might
have acted as a selective force to maintain the adaptation among southern eagles to migrate northward.
The movements of 2-4 year eagles are also not well known.
In an effort to further supplement the post fledgling movement data obtained by Biosystems Analysis, Inc.
during the spring of 2002, Arizona Game and Fish Department biologists attached satellite platform transmitter
terminals (PTTs) to eight juvenile bald eagles from breeding areas on the lower Verde River. The juvenile birds
selected for this study came from six breeding areas downstream of Bartlett Reservoir near the eastern limits of
metropolitan Phoenix; Box Bar
, Ft. McDowell
, and Rodeo
The PTTs were manufactured by North Star Science and Technology. Two of the units are solar powered and will
give us an opportunity to follow the year to year movements and migration of sub -adult eagles up until and
possibly beyond sexual maturity. Four additional solar units will be used in 2003.
The backpack-mounted PTTs are attached to seven to nine week old young using a non-abrasive Teflon ribbon harness.
The PTTs are programmed to transmit every other day for the first eight days and once a week thereafter (10 hours per day).
PTT transmissions are read by National Oceanic and Atmospheric Administration satellites, processed by Service Argos,
Inc, and provided to biologists from the Arizona Game and Fish Department and US Bureau of Reclamation every 4 days
via e-mail. At least two of these weather satellites are operational at any one time. They are on polar orbits
at about 850 km above the earth's surface. They are not stationary, and fly over the north and south poles,
orbiting the earth once every 102 minutes.
Each transmission is assessed for the accuracy of the location, adding those that are believed to be accurate
within one kilometer to a GIS database. The database is then used to generate maps
for each eagle. Although
the accuracy will not be as precise as that achieved during the Biosystems Analysis, Inc, study, the PTTs will
negate the labor intensive and potentially dangerous need for an airplane and pilot.
Our study population is a relatively isolated population of desert nesting bald eagles, almost exclusively
confined to Arizona. In 2002, there were 41 occupied breeding areas in the state. Paradoxically, the densest
concentrations of eagles is found on the lower Verde River within easy driving range of residents of the
Phoenix metropolitan area in Maricopa County.
A map of the study area including information about individual nesting sites can be found