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Satellite Telemetry Studies of Bald Eagles in Washington State

James W. Watson and D. John Pierce, Washington Department of Fish and Wildlife, Wildlife Research Division, 600 Capitol Way N., Olympia, WA 98501-1091 USA

Since 1994, movements of 19 bald eagles have been monitored through satellite telemetry in Washington state. Studies of 10 eagles captured during the nesting season have provided new information on juvenile and non-breeding adult movements, home range use, and long-range migratory movements to British Columbia and southeast Alaska. Wintering studies of 9 adults captured on the Skagit River identified breeding origins in Canadian provinces and Alaska. As of April 1997, 8 satellite platform transmitter terminals (PTT's) were no longer transmitting; average life of 5 expired transmitters expected to be 24 months was 23 ± 3 months. Two PTT's expired prematurely from unknown causes at 2 and 7 months, and another experienced electronic failure at 2 weeks. Of 2,962 locations received from the 19 PTT's, 62.5% were useful for identification of long-range movements with accuracy <10 km (class 0-3 locations). More importantly, 85% of 1,202 transmission cycles had at least one useful location. Preliminary analysis of home ranges of 4 adults assessed by satellite telemetry and compared to VHF telemetry locations collected on the ground suggests that PTT locations identify core areas of use, but greatly overestimate range size and provide little useful information on specific perch locations. PTT locations, however, were instrumental in locating territories of individuals on distant breeding areas that could then be surveyed locally. Nine nesting attempts of 5 adults with PTT's (3 female, 2 male) were unsuccessful; 3 failures were attributed to causes other than the transmitter. In contrast, 3 of 8 nesting attempts by adults bearing only VHF transmitters were successful. Potential concerns are interference of the vertical PTT antenna on female copulation. Recapture of one telemetered adult, and recovery of the carcass of another showed no skin abrasion or abnormal wear from teflon-ribbon harnesses, consistent with other research showing low-injury to eagles from backpack-mounted transmitters. Advantages of satellite technology over conventional VHF include the quality and quantity of information gathered; challenges include coordination of programmable cycles and PTT deployment when faced with the uncertainties of trapping success, recovery of downed PTT's, and relatively high cost/individual animal and associated risks for small budgets.

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