A Predator-Elicited Vocalization in Male Japanese Bush Warblers: Temporal and Spatial Singing Patterns in Relation to Presence of Conspecific Females

Recent studies have suggested that, in some bird species, predator-elicited vocalizations of males may function as an advertisement to conspecific females, but the evidence remains scarce. Male Japanese bush warblers, <i>Horornis diphone</i>, are known for producing long, conspicuous songs (continuous songs), in response to both predators and conspecific females. In this work, I investigated the temporal and spatial associations of continuous songs with the presence of females. In seven surveys throughout a breeding season, male singing activity of continuous songs was low before females arrived at the breeding ground but increased thereafter. Singing activity was higher in territories where the presence of females was confirmed than in those where it was not. Over multiple days, I observed a single male from sunrise to sunset and observed that the locations where the male sang continuous songs were concentrated at the locations where the females were observed. These temporal and spatial associations of the continuous song with the presence of conspecific females support the hypothesis that the continuous song is an advertisement to females. Additional observations showed that females did not fly away or enter the bushes in response to the continuous songs as they did when they encountered danger, and that the male sang continuous songs even towards females that were not considered their mating partner. These findings are inconsistent with the hypothesis that the continuous song is an alarm to alert their mates of danger. This study shows the possibility that male Japanese bush warblers advertise their quality to potential mates by singing continuous songs under dangerous situations.