Abstract
Long-term monitoring of the Mountain Hawk Eagle Spizaetus nipalensis since the early 1980s has shown that its breeding success in various parts of Japan has rapidly decreased in recent years. We examined if any changes in forest vegetation within the eagle home ranges caused this reduced breeding success. The breeding success of three pairs of S. nipalensis in western Hiroshima prefecture was monitored from 1981 to 2005. The presumed environmental factors responsible for breeding success such as vegetation structure, abundance of major avian and mammalian prey, incidence of direct human disturbance, age of pairs, weather conditions, and aggravation of the nesting environment were also monitored. Two of the three pairs exhibited a great reduction in breeding success after the late 1980s. Their low reproductive successes were associated with significant changes in forest vegetation such as an increase in old-aged conifer plantations and a decrease in deciduous forests and young conifer plantations. The third pair of S. nipalensis did not show any marked reduction in breeding success; however, forest vegetation within their home range did not change significantly and was consistently predominated by deciduous forest. Though the forest vegetation within a pair home range shifted from a predominantly young conifer plantation to an old well-grown one, breeding success for that pair did not fall appreciably. This was probably due to greater prey abundance in the appropriately managed forest. A conifer plantation well managed by thinning, pruning and tending would not only support prey population, but would also provide appropriate hunting grounds for S. nipalensis. To improve breeding success, we propose that at least 400 ha of suitable forest vegetation should be sustained for every breeding pair by conducting management practices that ensure well groomed artificial forests.
