DECLINING MARTIAL POLEMAETUS-BELLICOSUS AND TAWNY AQUILA-RAPAX EAGLE POPULATIONS AND CAUSES OF MORTALITY ON FARMLANDS IN CENTRAL NAMIBIA

Martial Polemaetus bellicosus and tawny Aquila rapax eagles declined over six years (1983-88) from three to one pairs and seven to two pairs, respectively, in a 840-km2 study area on commercial farmlands in the central savanna region of Namibia. Potential historic populations were estimated as five and 19 pairs, respectively. Both eagle species bred successfully, fledging 0.42 (n = 12) and 0.58 young/pair/year (n = 24). By the end of the dependence period the breeding success had dropped to 0.25 and 0.42 young/pair/year, a post-fledging mortality rate of 40% and 23% for martial and tawny eagles, respectively. The radio-tagged carcasses of five eagles of each species were recovered. Four martial eagles had been shot and one drowned, and all five tawny eagles were poisoned by strychnine set in baits by farmers for mammalian predators. Three farmers were responsible for the eight eagle deaths whose sources were located, and these were up to 18 km (mean 11.5 km) from the birds' respective nests. No recruitment took place into the tawny eagle population and, because of the extensive use of poisons, it is unlikely that birds survive to the end of their first year of independence. The only recruitment to the martial eagle population was that of an immature female to replace a bird shot 17 months previously. This study demonstrates that relatively few farmers can have a major influence on populations of large birds of prey over large areas.