Moonlight predicts the timing and duration of parental behavior across latitudes in a diurnal songbird

Light cycles influence the timing of daily activities that can affect organismal fitness. Moonlight has a profound influence on nocturnal and crepuscular species. Moonlight and lunar rhythms can also influence diurnal vertebrates’ behavior, including the timing of dawn chorus in birds. However, we know little about the impacts of moonlight on parental care, a key element of reproductive performance, in diurnal species. We studied the relationship between moonlight availability and the timing and duration of offspring provisioning in tree swallows (Tachycineta bicolor) by tracking adult nest box attendance over several years in New York, California, and Alaska. In all locations offspring provisioning shifted in accordance with moonlight. At lower latitude sites both parents began provisioning later on days following brighter moons, resulting in a shorter total duration of provisioning throughout the day after brighter and cloudier nights. In contrast, Alaska birds began provisioning earlier after nights with brighter moons, resulting in longer provisioning durations. These findings suggest that moonlight intensity may be an overlooked contributor to variation in diurnal parenting behavior, and that the response to moonlight differs across populations. Investigating how moonlight influences parental behavior is a key step in understanding the impacts of light at night across latitudes.