Nocturnal vocalizations in diurnal birds: a systematic review of current knowledge, research gaps and future directions

Nocturnal vocalisations by diurnal birds have been increasingly documented in recent decades, yet the functions and mechanisms underlying this behaviour remain poorly understood. Here, we conducted a systematic review to synthesize the current knowledge regarding nocturnal vocalisations in diurnal birds. The study is structured around 12 non-exclusive hypotheses proposed to explain why diurnal birds vocalise at night. We identified 39 peer-reviewed studies, comprising 92 case studies. The studies were geographically biased towards Europe and North America, with no studies conducted in Africa, Central America, or Antarctica. Most studies focused on passerines (67% of the case studies), males (73% among those reporting sex), songs (69%), and examined a single species (85%). We also found substantial variation among studies regarding what was considered nighttime, which may hinder future comparisons. We found uneven research and support among the 12 considered hypotheses. The hypotheses related to Elevated Light Levels, Mate Attraction, and Predator Avoidance received the strongest empirical support, with more than 80% of assessments supporting these mechanisms. In contrast, hypotheses such as Enhanced Sound Transmission, Reproductive Stimulation, Mate Guarding, and Territory Defence received mixed support. We did not find studies testing two of the proposed hypotheses (Nocturnal Roosting Groups and Pair Bonding). We found evidence that nocturnal vocalisations by diurnal birds are driven by multiple, non-exclusive mechanisms operating under different ecological and social contexts. Our review found that current knowledge regarding nocturnal vocalisations by diurnal birds presents major geographic, taxonomic, and methodological gaps, as well as a lack of standardized definitions. Overall, our review emphasizes a need for further research in this topic, which should prioritize comparative, experimental, and multispecies approaches for improving our understanding regarding the evolutionary and ecological drivers of nocturnal vocal behaviour in diurnal birds.