When flyways meet highways – The relative permeability of different motorway crossing sites to functionally diverse bat species

Bat populations are potentially vulnerable to the barrier effects of roads. However, empirical data on how road features facilitate or impede bat movements are scarce and essential for planning mitigation measures. We investigated bat movements across a motorway where it intersects with potential bat commuting routes along prominent linear landscape elements (treelines along hedgerows, minor roads, and rivers). Bat activity was recorded acoustically at four types of potential crossing routes; two under-motorway routes (river bridges and underpasses) and two over-motorway routes (overbridges and severed treelines), while simultaneously recording activity in the adjacent habitat. Bat activity above and below motorway structures and the timing of crossing flights with respect to motorway vehicles were also recorded. All bat species recorded in the adjacent landscape crossed the motorway. Bats used under-motorway routes, particularly river bridges, more than over-motorway routes. Contrary to expectations, many species flew across motorway gaps in treelines despite the lack of a guiding structure, including clutter-adapted species (Myotis spp., Plecotus auritus). Furthermore, bat passes were also recorded for the more collision-prone flight route at crossing structures (2% and 6.4% recorded above motorway level at river bridges and underpasses respectively, and 50% recorded on the underside of overbridges). In a subset of 10 nights of motorway-edge recordings, 18.3% of bat passes coincided with, and 17.1% were within 1–5 s of, passing vehicles. Our results highlight the prevalence of bat flyways along motorway-disrupted landscape features, and also the challenge in locating and designing crossing structures to minimize the risk of collision mortality.

► Motorway was not an impermeable barrier to the movements of bats along formerly intact linear landscape elements.
► Under-motorway crossing routes were used by bats more than over-motorway crossing routes.
► Clutter-, edge- and open-adapted bat species flew over motorway between severed treelines.
► Bats did not limit motorway crossings to traffic-free periods, indicating risk for collision mortality.
► Our findings can inform environmental impact assessment and mitigation practice to facilitate road crossing by bats.