Male-specific nocturnal song functions similarly to day song in a diurnal bird species

Historically, birdsong research has been biased towards song of male birds at dawn and during the day, even though some diurnal birds sing at night. To address this gap, we studied how song in the willie wagtail, Rhipidura leucophrys-a diurnal species with prolific male-specific nocturnal song during the breeding season-varies with time of day, breeding status and simulated intrusions. We recorded male nocturnal and dawn song over three breeding seasons and examined how this related to fertile and non-fertile breeding stages of females. To test whether song functions for territory defence, we simulated territorial intrusion experimentally in both sexes with daytime and nighttime playback of male and female song. To test whether nocturnal song could function for mate guarding or post-pairing mate attraction, we describe the mating system of willie wagtails using molecular genetic methods. We showed that both nocturnal and day song of male willie wagtails has roles in mate attraction and territory defence, while day song by females functioned primarily for territorial defence. Males increased song behaviour during fertile periods of resident females, suggesting possible roles in mate stimulation and mate guarding. Males and females responded similarly to simulated daytime intrusions and no differences were seen in male responses dependent on the time of day. We found 10%-14% of offspring were fathered by extra-pair males, suggesting song may also function for mate guarding and post-pairing mate attraction. In a species with small repertoires and simple songs like the willie wagtail, differences between males in overall song output achieved through nocturnal singing may be important in mate attraction and territory defence.