In 2005, we surveyed the birds inhabiting the Tokyo University Forest in Chichibu, in western Saitama Prefecture, Japan. Special attention was given to the Bush Warbler Cettia diphone, which appeared at the study site after 22nd March. We captured seven male Bush Warblers and banded them with individually unique combinations of color rings. Six males of them maintained the same home ranges (territory) throughout the study period, and no other males were observed. The population density of the males was 19.2 individuals per km2. Although we were unable to locate an active nest to study the breeding and rearing process, juveniles were captured in August. The tarsus length of juveniles reached the size of adults earlier than the natural wing length. Thus, in the juveniles, sexual size dimorphism was evident in the tarsus length to the same extent as seen in adults. We found nests of the Japanese White-eye Zosterops japonicus, Long-tailed Tit Aegithalos caudatus, Short-tailed Bush Warbler Urosphena squameiceps, Blue-and-white Flycatcher Cyanoptila cyanomelana and Red-billed Leiothrix Leiothrix lutea. The clutch of the Short-tailed Bush Warbler was believed to have been parasitized by the Oriental Cuckoo Cuculus saturatus. We confirmed that 40 species of birds occurred in the study area during the study period.