The decline of the Little Egret Egretta garzetta in the rice-field dominated landscape of Higashimatsuyama City, central Japan

Abstract

埼玉県東松山市周辺でのコサギ Egretta garzetta の減少 During the 1970s, the Little Egret Egretta garzetta was a common species in the rice field dominated landscape in and around Higashimatsuyama City, central Japan. Based on field data collected from1970 to 2015, I investigated seasonal and annual varia- tion in the densities of various heron species including the Little Egret, along with the abundance indices of major prey species (fish and shrimp), and the breeding density of the Northern Goshawk Accipiter gentilis (a possible egret predator) in the area. While the Little Egret declined over the study period, both the Great Egret Ardea alba and Grey Heron A. cinerea increased. There was no evidence of a reduction in fish or shrimp, but goshawks increased during the 1980s and 1990s following the first observation of breed- ing in the area in 1982. The reduction of Little Egrets seems to have coincided with the increase of Northern Goshawks in this area. Some indications were found of goshawks having hunted Little Egrets, and as the egrets forage in open farmland and as goshawks prefer to hunt over the same habitat, I suspect that predation by Northern Goshawks may have been a factor in the local decline of the Little Egret. Further studies are required to eliminate other possible factors that may have contributed to the reduction in Little Egret numbers, such as local variation observed in the abundance indices of fish and shrimp, which may have been caused by increases in the numbers of Great Cormorant Phalacro- corax