Island and mainland populations of birds often differ in their breeding biology because of differences in ecological conditions. We studied the breeding biology of Japanese Bush Warblers Cettia diphone on Hahajima in the Ogasawara (Bonin) Islands group. Based on observations of four nesting attempts, clutches consisted of three or four eggs, which is smaller than on the mainland. Males fed the chicks, although the frequency was fairly low. Brood sizes declined during the nestling period due to starvation. Neither such male behavior nor starvation, have been reported on the main islands of Japan or on their small peripheral islands. These features of their breeding biology may have resulted from food scarcity and strong intraspecific competition on the island.