The island of Guam has experienced a precipitous decline of its native forest birds, and several lines of evidence implicate the introduced brown tree snake (Boiga irregularis) as the cause of the range reductions and extinctions. The range expansion of B. irregularis correlated well with the range contraction of the forest avifauna. Exceptionally high predation by snakes on bird—baited funnel traps occurred in areas where bird populations had declined. Little or no snake predation occurred in areas with stable bird populations. Several factors have contributed to Boiga's success in decimating the avifauna on Gaum. Although most common in forests, B. irregularis has occupied a variety of habitats on Guam, and few effective barriers to its dispersal exist. Prey refuges are present only in urban areas, on concrete or metal structures, and in the savanna. Boiga's nocturnal and arboreal habits and an apparent keen ability to locate prey, make roosting and nesting birds, eggs, and nestlings vulnerable. Besides birds, B. irregularis feeds on small mammals and lizards. By including the abundant small reptiles as a major component in its diet, Boiga has maintained high densities in forest and second—growth habitats while exterminating its more vulnerable prey. This is the first time a snake has been implicated as an agent of extinction.