The initiation in 1971 of regular, year-round bird censuses and reproductive studies on three sites of the Analysis of Ecosystems, U. S. International Biological Program (IBP) provided the means and incentive to overcome the disadvantages of studying temporal changes as just outlined.... The data thus provide a basis for discussion of both within- and between-community variation in a number of community characteristics. In this paper we will stress overall levels of numbers and biomass, temporal changes in them, and the factors that might be responsible for them.