Males are larger than females in size but the size of the bill remains the same in both sexes.
Reed-warblers are widespread from Europe through Australasia, with as many as 10 species presently represented in the tropical Pacific. Most birds found on the island of Saipan occur in thicket-meadow mosaics, forest edges, reed marshes, and wetlands. On Aguiguan, reed-warblers inhabit formerly disturbed areas vegetated by groves of trees and thickets three to seven feet tall, that may be similar to some habitats utilized on Saipan. On both Guam and Pagan, the species is reported to have inhabited freshwater wetland and wetland-edge vegetation almost exclusively. The population of nightingale reed-warblers on Alamagan inhabits areas with open brushy overstory and understory, and wooded edges adjacent to open grassland.