Part-night lighting fails to restore natural song rhythms in urban European Robins

Artificial Light at Night (ALAN) disrupts the natural circadian rhythms of animals, often extending the activity of diurnal species into nocturnal periods. To mitigate ALAN's ecological impacts and reduce energy consumption, many municipalities have implemented Part-Night Lighting (PNL), which involves switching off streetlights during core nighttime hours. However, the effects of such temporal lighting reductions on animal activity patterns remain poorly understood. We investigate how PNL influences daily singing behavior in European Robins Erithacus rubecula during spring in an urban area of France, where streetlights are turned off between 11 p.m. and 6 a.m., creating brief illumination windows at dawn and dusk. We hypothesized that PNL would reduce the effects of ALAN on Robin vocal activity compared to Full-Night Lighting (FNL). Using passive acoustic monitoring, we recorded Robin song activity across three types of sites (unlit, PNL, and FNL) and four time periods (dawn, morning, afternoon, dusk). Robins sang significantly earlier at FNL and PNL sites than at unlit sites during dawn and morning, and later during dusk, with no significant differences between FNL and PNL. In the afternoon, song activity tended to be delayed at PNL sites compared to both FNL and unlit sites. Our findings indicate that even short illumination periods at dawn and dusk under PNL do not replicate unlit conditions. In an urban context, the similarity in vocal activity between PNL and FNL sites suggests that residual light from nearby lit areas and/or lingering effects of ALAN may continue to influence animal behavior under PNL regimes.