Accounting for cloud cover and circannual variation puts the effect of lunar phase on deer–vehicle collisions into perspective

Although several studies have focused on the influence of moonlight over deer-vehicle collisions, findings have been inconsistent. This may be due to neglecting the effects of cloud cover, a major impediment to moon illumination, and circannual variation in both deer and human activity.We modeled how median cloud cover interacted with the illuminated fraction of the moon in affecting daily roe deer (Capreolus capreolus) roadkill in Slovenia (Central Europe). Data included nationwide roadkill (n = 49,259), collected between 2010 and 2019 on a mandatory basis by hunters as well-trained citizen scientists.Roadkill peaked for medium-to-high cloud cover, while decreased at nights with low or extremely high cloudiness. This pattern was more pronounced at full moon nights. However, the effects of moon illumination and cloud cover had a lower predictive potential than circannual variation.Our results suggest that moonlight could influence deer movements through compensatory foraging. However, at full moon nights collisions could then be modulated by weather conditions, affecting human movement and vehicle traffic, and likely also road crossing by roe deer.Moon illumination may indeed affect wildlife-vehicle collisions and roadkill, but its effects should be quantified as a function of cloud cover. Because collisions with roe deer, the most common road-killed large mammals in Europe, peak at nights with full moon and casted skies, and at some precise periods of the year, interactive warning signs that detect ground illumination at these periods may improve drivers’ awareness and increase their safety.