Nocturnal singing in diurnal birds is poorly described in the literature and is not well understood. Most research has been limited to the documentation of the behavior, with little information regarding the patterns and function of nocturnal song. We used autonomous acoustic recording units and automated detection and classification algorithms to examine the seasonal and nocturnal patterns of song in the Field Sparrow, Spizella pusilla, a diurnal songbird. We used a generalized linear mixed model to investigate the patterns in the probability of recording each of the two song types described for the species (simple and complex) at night from 7938 10-min recordings from 14 different individuals singing in six different grassland patches. We fit different models investigating the influence of date within the season and time period during the night, and compared models using the Bayesian information criterion. Male Field Sparrows sang both simple and complex songs during the early stages of territory settlement and mate acquisition. However, as the breeding season progressed, we detected fewer instances of simple song, while the occurrence of complex song increased, reaching its peak during the height of reproductive activity. The seasonal pattern of simple and complex songs suggests that they may have a similar function at night and during the day, with simple songs used for inter-sexual interactions and complex songs used for intrasexual functions. Consequently, the role and function of nocturnal song may be more important for reproductive activities than previously assumed.
