Assessing the Effects of Artificial Light at Night (ALAN) on Barn Owls’ (Tyto alba) Spatial Behavior in Western Switzerland

The main message of our research revolves around understanding how artificial light at night (ALAN) impacts the movement behavior of barn owls (Tyto alba) – a near threatened nocturnal raptor in Switzerland – specifically focusing on around 53 breeding males in Western Switzerland.

Our study aims to fill the knowledge gap surrounding the effects of artificial light on space use of nocturnal birds of prey, specifically focusing on their habitat selection and movement pattern during their night foraging activities.
Since barn owls live in close contact with human infrastructure, and thus artificial light, they serve as an ideal model to investigate how ALAN influences their movement trajectory and space use during their foraging and commuting activities and, as well, their perching site selection.
We used movement data collected during the 2019 and 2020 breeding season deploying GPS devices across monitored nest boxes on both parents. The present study focused on males because male barn owls carry out the majority of prey provisioning during chick rearing (Marti, 1994, Taylor, 1994; Roulin, 2004). Male spatial behaviour therefore provides the most direct representation of active foraging patterns during the breeding period.

The individuals were equipped with GPS multi-sensor loggers to track their movement in space and time and implement a monitoring protocol based on the species’ ecology, running from April to August to capture key behavioural and environmental interactions during their reproductive season.

Our results show that barn owls generally avoided artificially illuminated areas across hunting, movement, and perching behaviour, indicating that ALAN acts as a strong ecological constraint. Illumination also modified habitat selection, reducing the use of grasslands and biodiversity structures, habitats that were otherwise selected under dark conditions, while slightly increasing reliance on forest edges. This latter effect may reflect the availability of shaded or darker microhabitats within forest edge, which remain comparatively less exposed even when illuminates. Behavioural responses were further shaped by individual phenotype, with darker morphs showing stronger avoidance of lit areas during movement, and by natural illumination, as moonlight reduced avoidance during hunting. These results go along what San-Jose found in the 2019 paper that reddish barn owls are less successful in moon lit nights indicating that ALAN might replicate the perceptual challenges associated with moonlit nights, thereby disproportionately affecting darker individuals. Together, these results reveal that the nocturnal lightscape strongly influence how barn owls use space and select habitats.
These findings could provide critical insight for mitigating light pollution to support night active birds’ populations in human-oriented landscapes.