I studied population densities of forest bird communities in the Mariana Islands of Saipan, Aguiguan and Sarigan in order to evaluate hypotheses concerning seasonal shifts in populations, habitat effects on densities, inter-island differences in densities, social group size and underlying reasons for community structuring. With the exception of one species, I found no evidence to support the hypothesis that seasonal shifts occur in populations. Hence, this island system differs in this regard from mainland tropical forests. I also found, contrary to theory, no evidence that the presence of altered habitat permits populations to be greater than they would be in their absence, as disturbed habitat had far lower densities of most species than native forest. Furthermore, I found no support for the hypothesis that social group size differs seasonally as a consequence of differential breeding activity, which is again contrary to findings for mainland tropical forests. A high density of nectar resources on two islands appeared responsible for high population densities of the Micronesian Myzomela on them. Inter-island density comparisons showed strong evidence for there being unfilled niches on Sarigan, thereby making the island a favorable site for species translocations. Examination of the prehistoric composition of Marianas forest bird communities indicated that they once resembled in density structure those of the comparatively pristine Palau Islands. Removing ecologically similar but now absent species from the original Mariana communities likely resulted in reduced competition for resources in ways that increased niche breadth and, thus, populations. The wholesale loss of species in the Marianas has led to communities in which three ecologically versatile species now account for the vast majority of individuals in the community.