Why do so many specie of birds coexist in forests? Generally there will be three different answers to this question. The first answer is concerned with the diversity of vegetation structure: existence of various habitats for different species of birds. The second is concerned with primary production of forests. The high productivity of forests ensures a steady supply of resources necessary for survival and breeding of diverse bird species. The third is concerned with the complexity of biotic interactions among different species of organisms. The complex interactions consequently function to maintain the populations of diverse bird species in forests. Ecologists are required to explain the community and/or ecosystem process by organizing these three viewpoints: habitat, productivity and biotic interactions. The following three papers are contributions from speakers at a symposium held at the 2000 Annual Meeting of Ornithological Society of Japan. They review ecology of birds in forests or of forests with birds, sharing the viewpoints.