Population Trend Estimation of Three Threatened Bird Species in Japanese Rural Forests: the Japanese Night Heron Gorsachius goisagi, Goshawk Accipiter gentilis and Grey-faced Buzzard Butastur indicus

Abstract

The population trends of the Japanese Night Heron Gorsachius goisagi, the Goshawk Accipiter gentilis and the Grey-faced Buzzard Butastur indicus were estimated based on records of the number of individuals kept in Japanese zoos since the 1960s. Because no special effort is made to collect these species, and because they seldom breed in captivity, their population changes in zoos were assumed to reflect that of the wild populations. In order to reinforce the zoo data, we collected local records of the population trends of these three species in Niigata Prefecture, Tochigi Prefecture, Cape Irago of Aichi Prefecture and the Miyako Islands of Okinawa Prefecture. The results indicated that at all investigated localities, except Tochigi Prefecture, the Japanese Night Heron and the Grey-faced Buzzard had decreased in numbers since the 1960s, whereas the Goshawk had increased in number nationwide. There is, however, a possibility that the increase in the Goshawk population is an overestimation, and it is necessary to investigate its wild population trend in detail. The most likely cause of the declines of the Japanese Night Heron and the Grey-faced Buzzard is habitat degradation, both in quality and in quantity. To date, these two species have not been the targets of any concerted conservation effort, but they must henceforth be accorded this status.