The effect of variation in moonlight on nocturnal song of a diurnal bird species

Abstract: The lunar cycle is known to affect the behaviour of strictly nocturnal species, but for diurnal species that are periodically active during the night, this has been less investigated. Nocturnal bird song is relatively common in diurnal species, yet research on this behaviour accounts for little of the research on avian vocalisations. This is surprising given that diurnal species are adapted for bright environments and therefore may be particularly sensitive to change in the lunar cycles. We used automated bioacoustic recorders and automatic song detection software to measure nocturnal song rate in a diurnal bird where both sexes sing, the willie wagtail (Rhipidura leucophrys). We deployed recorders at eight locations across four naturally dark sites resulting in 457 h of nocturnal audio. We confirmed anecdotal evidence suggesting that willie wagtails are prolific nocturnal singers during the breeding season and demonstrate that while both male and females sing during the day, nocturnal song is largely sung by males. Moreover, we show that nocturnal song increased with lunar illumination, contrasting with previous research on other diurnal species that sing at night. Our data allow us to hypothesise possible functions for nocturnal song in this species, such as territory defence or mate attraction. Significance statement: Despite being taxonomically widespread, nocturnal birdsong is largely overlooked in the literature. Anecdotal evidence suggests nocturnal song is positively influenced by the intensity of moonlight but evidence corroborating this is minimal and conflicting. More problematic is that all previous studies are biased towards male song from Northern Hemisphere species. We addressed these gaps by measuring nocturnal song, over the entire lunar cycle, in the willie wagtail, an Australian species. We demonstrate that nocturnal songs are from chorusing males during the breeding season exclusively and that nocturnal song rate increases with lunar illumination. Our work provides a foundation for hypothesizing the function of nocturnal song and contributes to understanding these patterns on a global level.