A vocalization in male Japanese bush warblers in response to both predators and conspecific females

Male Japanese bush warblers (Cettia diphone) produce not only typical songs but also long, conspicuous vocalizations (continuous songs) during the breeding season. In my preliminary observations, male warblers produced continuous songs in response to both the appearance of a predator and the calls of conspecific females. Although predator-elicited vocalizations by males have been studied in some species, there are no known cases in which an acoustically identical vocalization is also triggered in response to conspecific females. Here, I examined whether these continuous songs triggered in response to predators and females were acoustically similar. In field experiments, I presented dummies of a Eurasian sparrowhawk and a female warbler before (April) and after (June) female arrival at the breeding ground. Before female arrival, 11.1% of males responded with continuous songs to each of the hawk and female presentations; after female arrival, the respective values were 27.6% and 48.3%. Thus, more males responded to the model presentations with continuous songs after female arrival. There was no difference in the acoustic characteristics of the continuous songs between the two models. The males approached the female model more intensively than the hawk model. These findings do not support the hypothesis that the song functions as an alarm or predator deterrent. The function of these continuous songs remains unclear, but this study is the first to show an example where males produce the same vocalization in response to both predators and conspecific females. The continuous song contains a trill-like structure, which can be considered a performance-related song trait. Therefore, the continuous song may possibly be an honest signal of male quality for females. Further studies are required to understand the function of this novel vocalization, particularly the effects of female breeding stage on male singing activity and the female response to the songs.