Increasing canopy cover elevates vehicle collision risk for barbastelle bats (Barbastella barbastellus) at roads

Roads can form barriers to movement and act as mortality sinks for bats. However, understanding flight behaviour over roads has been limited by reliance on human observers to characterise flight patterns. Moreover, both species-specific behaviour and the local environment may alter bat-road interactions. Using paired near-infrared cameras with 3-D tracking analysis, we reconstructed the flight trajectories of barbastelle bats (Barbastella barbastellus) at nine road sites in West Sussex, England. Of the 400 flight paths, 56% occurred at heights that placed bats at risk of vehicle collision, and this risk increased as bats flew closer to canopy cover. Increasing canopy cover was associated with bats flying along rather than across roads, and with faster flight speeds (by 0.14 ms^{- 1} per 10% increase in canopy cover). Flight behaviour was not influenced by traffic density. These findings suggest that hop-overs, in which trees are planted to provide continuous canopy cover, may create an ecological trap by encouraging B. barbastellus to cross roads at unsafe heights. Our results also highlight the value of 3-D tracking to understand bat behaviour, an approach that could be applied to predict better the impact of road developments on bats and evaluate the effectiveness of mitigation in reducing collision risk.