Patterns of wildlife-vehicle collisions in Poland: a cross-taxonomic analysis of a nationwide citizen science dataset.

Wildlife-vehicle collisions (WVCs) are a global biodiversity threat impacting most terrestrial vertebrates and posing scientific, economic and conservation challenges. Research on WVC is nevertheless geographically uneven, and large-scale multi-taxon studies are scarce. We analysed data from the Polish Roadkill Observation System, a nationwide volunteer-based programme recording WVC carcasses across all terrestrial vertebrates. Based on 28,709 casualties recorded during 2000-2022, we examined the mortality patterns in vertebrate classes and some correlates of WVCs distribution. In total, 205 species were identified, representing 55% of Poland's terrestrial vertebrate fauna, including 16% of threatened species. The approximate proportion of roadkilled mammals, amphibians, birds and reptiles was 8:8:5:1; mammals and amphibians are thus the most frequently reported vertebrates on Polish roads by citizen scientists. Reports of multiple casualties concerned mostly amphibians and reptiles, and were often associated with protected areas. Regression models revealed that observed roadkill was positively correlated with body mass and carcass persistence but not with species distribution range in Poland. Prominent seasonal patterns were also evident, with peak mortality occurring in warm seasons and coinciding with taxon-specific phenological events. Our study demonstrates the value of citizen-science data for identifying nationwide patterns of WVCs, while also highlighting the need to move beyond opportunistic observations toward a nationally coordinated, systematic survey to generate more accurate and policy-relevant estimates.