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Keywords: acoustic recording system automated radiotelemetry system birdsong extrapair mating field sparrow individual acoustic identification mate attraction nocturnal behaviour nocturnal songWe addressed the role of nocturnal singing in the diurnal field sparrow, Spizella pusilla. We were particularly interested in whether nocturnal song is used to repel intruders (intrasexual function) and/or to attract females seeking extrapair copulations (intersexual function). First, we used autonomous acoustic recording units (ARUs) and an automated detection and classification system to examine the associations of singing behaviour of mated field sparrows with social factors (fertility stage, presence of neighbour song and presence of intruder song). Second, using an automated radiotelemetry system (ARTS), autonomous acoustic recording units (ARUs) and automated playback systems, we conducted a nocturnal playback experiment to explore how mated male and female field sparrows respond to nocturnal songs at night and across prefertile, fertile and postfertile stages. We found that neighbours and intruder males tended to sing complex songs on the same nights when focal males sang. While not all focal males sang at night, all male birds (resident males, neighbours and intruders) that sang tended to sing according to the fertility periods of the resident male's female. Our playback experiment demonstrated that, although field sparrows usually slept and were inactive at night, they moved in response to nocturnal field sparrow songs more than to control stimuli (other species vocalizations). Interestingly, we found that males did not respond by singing or countersinging to field sparrow nocturnal songs (simulated intrusions). We also demonstrated that male activity responses were similar during the prefertile and postfertile stages, while females responded more during the prefertile and fertile stages than during the postfertile stage. Consequently, the nocturnal song in the field sparrow appears to play a role in extrapair mate attraction (intersexual function) more than in repelling intruders (intrasexual function). While nocturnal singing is an infrequent behaviour of most diurnal birds, it may be important in the mating system of these birds.