関東地方における秋期のヒヨドリの渡り‐齋藤（１９３５‐１９４３）の観察記録の解析

Abstract

In the plains of Kanto district, Japan, flocks of the Brown-eared Bulbul Hypsipetes amaurotis often undertake a southern autumn migration. From 1934 to 1942 Mr. Genzaburo Saito made daily observations throughout the annual autumn migrations at Chiba City, Chiba Prefecture, and recorded values of several migration parameters. I analyzed his data to clarify the migration status c. 70 years ago and to assess whether the status differed from the present one. Results showed the annual number of migrating individuals to vary from c. 400 to c. 1,700 with a median of c. 900 and c. 14 individuals in a flock. Flights began in late September and finished in early November, with most flocks flying in October. Comparison with the data of Yamaguchi (2004, 2005) revealed the values obtained at Chiba City c. 70 years ago do not differ greatly from those occurring at the present time. Therefore, even though part of the H. amaurotis population has begun to breed in the plain regions in west-southern districts in Japan over the past 70 years, the migration status has remained unchanged. During each migration period, the number of migrating birds showed at least two peaks. The mean time at which peaks occurred differed between days, suggesting that the site at which each flock had started to fly was respectively different. Flocks arrived from the northwest and departed in a southeasterly direction, and the variance of the direction was very small. As this direction is parallel to the coastline at Chiba City, the birds may fly along the coastline towards the southern part of Boso peninsula where Chiba City is located. However, some flocks may visit forests en route, and remain there for a time. Therefore, autumn migrating flocks of H. amaurotis are thought to show a flexible behaviour responding to environmental conditions.