Diurnal and Nocturnal Birds Vocalize at Night: A Review

In contrast to diurnal songs and calls, avian nocturnal vocalizations are understudied, and their occurrence and function are poorly understood. Here, using primarily the Birds of North America species accounts, I systematically examine the occurrence and potential factor(s) that contribute to nocturnal vocalizing across 749 species of breeding birds in North America and review proposed hypotheses on its function(s). Nocturnal vocalizations are reported from at least 30% of North American birds, across 18 of 22 orders, of which over 70% are considered diurnal. This indicates that nocturnal vocalizations are a taxonomically widespread behavior and are not restricted to nocturnal species as traditionally believed. Furthermore, it indicates that diurnal birds are extending activities into night-time hours, which may influence energy dynamics and reproductive success. Lack of significant phylogenetic signal suggests that nocturnal vocalizations are not merely a result of phylogeny, which leaves ecological factors such as elevated light levels and mated status as potential promoters of its occurrence. With less than 4% of avian vocalization literature concentrating on nocturnal vocalizations, the goal of this paper is to synthesize empirical knowledge and to stimulate more research in this field.