The impact of predation pressure, natural light, and species-specific factors on the prevalence and intensity of nocturnal singing by diurnal birds.

Bird vocalisation has primarily been studied during daylight hours, but research indicates that diurnal species also sing at night. The aim of this study was to determine the prevalence and intensity of nocturnal singing by diurnal bird species across four distinct locations in Poland. We examined the impact of moon illumination, habitat type, predation pressure, and species-specific characteristics on nocturnal singing by diurnal birds. Our findings demonstrate that nocturnal singing by diurnal birds is a relatively common behaviour in Central Europe. We recorded 35 diurnal species at night, comprising 76% of all detected species during the nocturnal observations. The intensity of nocturnal singing by diurnal birds varies across orders, families, and even individual species. Birds tended to sing less at the beginning of the night, potentially due to physiological recovery during sleep, but then increase their vocal activity, peaking just before sunrise when the environment begins to be illuminated. We found that more diurnal birds sang in open areas compared to forested habitats, probably due to the greater availability of light in open spaces and reduced predation pressure from owls. We also observed a positive relationship between moon illumination and both the probability of singing and the number of songs produced by both diurnal and nocturnal species, underscoring the significant role of natural light in stimulating night singing. Furthermore, we noticed that the number of species singing at night was higher when diurnal predators were present around the recording locations, suggesting that birds may intensify their nocturnal vocalisations to avoid the threat of daytime predators. In conclusion, nocturnal singing by diurnal birds is a complex phenomenon influenced by various environmental factors that impact its occurrence and intensity.