Do seabirds dream of artificial lights? Understanding light preferences of Procellariiformes.

Seabirds, and particularly fledglings of burrow-nesting species, are greatly impacted by light pollution. During their inaugural flights from colony to sea, fledglings become grounded after encountering artificial light. Such groundings, or fallout events, affect many fledglings each year, causing mass mortality events. To mitigate this light-induced mortality, rescue programmes have been implemented for decades at many locations worldwide. Despite the notoriety of fallouts and their conservation implications, the contributing behavioural and biological factors remain mostly unknown. How the mechanisms of light attraction and light avoidance interact and how they manifest in different groups (e.g. age, personality, populations) or light pollution levels remain open questions. We tested behavioural choices of Cory's shearwater Calonectris borealis fledglings, rescued after being grounded in urban areas, and choices of breeding adults for contrasting light sources. Fledglings and adults were exposed to one of three treatments in an experimental Y-maze set-up: white light versus no light, blue versus red light, and a control with no light on each arm of the Y-maze. Both age groups clearly chose the no-light arms and the red light arm. This choice for longer wavelengths and darker environments, along with slower responses by fledglings, suggests that close range artificial light causes disorientation in seabirds. Our study helps to clarify the behavioural components of fallouts and provides further evidence on the disruptive effects of nocturnal artificial light on sensitive species like Procellariiformes.