Seasonal variation in vertebrate traffic casualties and its implications for mitigation measures

Wildlife mortality on roads is a global conservation problem. To implement a cost-effective programme of mitigation measures, it is essential to determine spatial and temporal patterns of traffic collisions. Identifying seasonal road-collision patterns could make it possible to schedule and optimize monitoring programmes. We evaluated seasonal variation in the roadkills of four vertebrate taxonomic groups (amphibians, reptiles, birds and mammals) across an environmental gradient in northeastern Iberia. Seven roads were monitored twice a month for one year and seasonal roadkill aggregations were examined using Poisson tests. Roadkill patterns were correlated with environmental variables using Generalized Linear Models. Amphibians were the group most commonly killed. Overall, the roadkill pattern was seasonal, and most roads showed seasonal peaks of casualties in autumn and spring. Roadkill peaks showed slightly seasonal differences between taxonomic groups. The total number of roadkill incidents was positively associated with temperature and negatively associated with solar irradiance. The roadkill numbers by group were related to different environmental factors: amphibian roadkills increased with relative humidity, while this relationship was negative for birds; mammal roadkills were associated with temperature, and reptile roadkills correlated with precipitation, solar irradiance, and temperature. Our results suggest that roadkill rates in Catalonia are seasonal and mostly associated with several climatic factors, although they can vary depending on taxonomic group and environmental factors. Our results highlight the fact that an understanding of seasonal variation in roadkills is critical for optimizing monitoring programmes and temporary mitigation measures aimed at particular species or taxonomic groups.