1. This paper describes the formation of basic flocks, particularly the movement of the young from the dispersal to the final settlement in a restricted area, and discusses the factors relating with the association of members in a basic flock. The field work was carried out in the summer and autumn of 1970 at the Akasaka Palace, Tokyo, Japan. The study area of about 37ha consisted of mixed evergreen and deciduous broad-leaved forest. In the study area, virtually all of the birds were marked individually by colour rings and their previous history was known.
2. The formation of the basic flocks took place through a successive process; the establishment of summer and autumn ranges of young, and the final association of the young and adults in an area. The young were associated with each other to form the summer flock when the family flocks broke up. Most of the young established their summer range within a relatively short period, usually about a month, after having become independent of their parents.
3. Forming the summer flocks, the young gradually showed a preference for a particular part of their summer range. The number and members of young observed in the same area became more and more constant from this time onward. By the middle of October, most of the young established the autumn range within their summer range. It is considered that the young establish their summer or autumn ranges within the home range of their family flock.
4. The adults remained on or near their previous breeding territory even after the breeding, and then the young settled in the area as the autumn range. Consequently, the home ranges of the adults and the autumn ranges of the young overlapped extensively as a whole. And a basic flock was formed by these birds. Therefore, it is concluded that the formation of basic flocks are closely related with the period of the extensive overlap of home ranges between individuals, and that the members of a basic flock are primarily associated with each other by their site attachment.