More than noise: light, moon phase, and singing behavior in a passerine

Most studies on the effects of urbanization on animal communication have focused on the influence of acoustic pollution on vocal production. Comparatively few studies have addressed the potential impact of light pollution on animal acoustic signals, with no studies having looked at the structure of vocalizations. We know, however, that light is related to melatonin production, influencing activity patterns of organisms, which opens the possibility that artificial light at night may affect also the structure of animal vocalizations. Here we studied whether there are associations between urban noise and nocturnal light with the structure of songs produced by vermilion flycatchers. We also considered the effects of moonlight by recording a subsample of males during full moon, new moon, and full moon again. Contrary to previous findings, males did not sing longer songs with noise; they produced longer songs and at a higher rate in brighter territories. Males also increased song rate and sang songs with more elements during full moon nights when compared to new moon nights (vocal flexibility). This study highlights the importance of considering several pollutants along with moon phase to better understand how they influence animal signals, links light at night with birdsong structure, and provides additional evidence of a circa-lunar rhythm of song output. We discuss our results in the context of proximate and ultimate causes and provide a possible explanation of the seemingly contradictory result with previous research.