Ornithological surveys were carried out, from 1981 to 1986, in the hinterland of French Guiana, in order to find out the best possible location, and estimate the optimal size, of a national park in this rain forest area. The structure of the bird community, together with the structure of the assemblages of small foliage gleaners and of diurnal raptors, were studied in ten sites within the unbroken primary forest. The main results of these studies are briefly summarized : local and regional species richness, diversity, trophic guilds, population density, vertical and horizontal distribution, etc. In mature undisturbed rain forest, about 1% of the species are abundant, and up to 80% and more may be considered as rare. Any kind of large scale perturbation (either logging or hunting) leads to an increase of common species, and to a decrease of the rare ones, eventually leading to a reduction of richness, diversity, and equitability, as well as of the density and biomass of the bird community as a whole. Al least along the habitat axis, species niches are densely packed and often widely overlapping. There is a roughly equal trend among species toward specialization and generalism. Competition is likely to be reduced at least as much by low densities, uneven distribution and interspecific territoriality or avoidance, than by ecological segregation. Apparent coexistence is thus limited by low spatial overlap, whatever the similarity of the niches, and rarity of many species is a major condition of community richness. There are different ecological correlates of rarity according to specific responses to the strong constraints of this ecosystem (habitat heterogeneity, food seasonality, interspecific competition, predation pressure). The relatively low level of food availability for insectivores is investigated. The numerous tree fall gaps and other disturbances (flooding, army ants, ...), randomly distributed, and colonized by fugitive species, are other important factors maintaining species diversity. Rain forest bird communities are very sensitive to human disturbance. Any kind of logging or hunting must be excluded from reserves designed for the conservation of «pristine» bird communities. From our estimates in Guiana, the minimum critical area of such reserves, including long term viable populations of every species (when isolated), may probably be over 10 000 km2.