Motorways and bird traffic casualties: Carcasses surveys and scavenging bias

► We analysed biases affecting carcasses surveys: detectability and persistence time of carcasses, scavengers activities. ► Passerines and the Barn Owl were the most numerous killed birds. ► Encounter probabilities were constant and high. ► Persistence probabilities were higher for large and old carcasses, during summer, differed between motorways. ► Scavenging activities varied between years, being higher during diurnal periods in spring.