The natural forest cover of Java is now reduced to small, very isolated fragments which remain the only suitable habitats for a number of endemic species. The aim of this survey was to assess the distribution and population size of the ten forest Falconiforms, including the endangered Java hawk eagle, Spizaetus bartelsi. Five of the main forest reserves, ranging from 5 to 500 km2, were studied. Among the ten species, four are restricted to low altitude forest (below 1500m) and are now very local because of its disappearance. Six species are making wide use of secondary habitats around forest reserves and are thus less susceptible to forest alteration and fragmentation. All the forest raptors here show some attributes of the so-called 'syndrome of insularity', i.e. wide distribution and habitat niche as well as rather high densities. So sizable populations can survive in relatively small patches. But there is a positive correlation between the size of the reserve and the abundance index of all species. The two forest hawk eagles (S. bartelsi and Hieraaetus kienerii) may have minimum home range sizes of about 20-30 km2 and may not survive in forest patches smaller than 20 to 100 km2. Their present population of at least 8 to 10 pairs per reserve may be considered as the minimum critical size for a viable population. Reserves of less than 300 km2 cannot include viable populations of every forest raptor. So additional well protected reserves must be added to the present network, to provide a better prospect of survival for all the species.