As brought out in the discussion of Birhor social organization, the band can be viewed as a loose union of families bound together by ties of kinship. This union easily splits or coalesces into smaller or larger units during the course of the year. These changes in composition of the residential unit are facilitated by fairly regular migratory movements of the yearly round.

This yearly round of the Birhor is not nearly so patterned as is found in agricultural populations, as there are a number of factors influencing their movement other than seasonal climatic changes. So far as economic activities are concerned, the three seasons for the Birhor are the wet season, June to October; the cold season, October to February; and the dry season, February to June. The relation of hunting, gathering, and ropemaking to these seasonal changes is described in the chapter on subsistence activities. This pattern of response to the distribution of economic resources leads to a general pattern of movement in which bands reach a maximum average size during the dry season. They may or may not remain in the same size unit during the wet season; however, they split into the smallest average size units at the end of the wet season.