The song of the Japanese Night Heron Gorsachius goisagi
has long been known from its distinctive low, carrying call which sounds like ‘ivoh—’. The actual status of the song, however, has little been studied. The author conducted a comparative survey of the song at song posts and during the breeding period at breeding sites in Tokyo in 2008, using a digital voice recorder and a digital video camera to record the song. The study documented the song at song posts during night for 10 consecutive nights. The song usually continued from sunset until next morning and in an extreme case, lasted for 10 hours. However, once a breeding pair was established, they immediately stopped singing and thereafter, the song was not observed during the breeding period regardless of day or night. Results of this study suggest that the singing activity of the Japanese Night Herons is done intensively at night for a short period from immediately after their arrival in Japan up until pair formation occurs, and that thereafter, no singing occurs either during the daytime or at night. The Japanese Night Heron derived its common name from the belief that it was a nocturnal species. This might be because male birds sing intensively only at night. However, the results of the present and previous studies revealed that this species to forage exclusively during the daytime both in the breeding and non-breeding seasons. This species can therefore be considered diurnal, except for short periods.