Are wildlife overpasses of conservation value for birds? A study in Australian sub-tropical forest, with wider implications

- Wildlife overpass crossing rates documented for range of avian species and guilds.
- Rates and methods of crossing differ markedly across species and guilds.
- Comparison made with crossings over three high-traffic roads of different widths.
- Conservation value of wildlife overpasses lies with defined forest-dwelling species.
- Linear strips of low/medium vegetation strata advocated to benefit these species.

A major conservation objective of wildlife overpasses is to maintain levels of dispersal between fragmented areas of habitat, thereby allowing gene-flow and sustaining population viability of target species. We documented crossing rates of a range of avian species, of four defined species guilds, over a vegetated 15 m wide wildlife overpass in suburban Brisbane, south-eastern Queensland, Australia. The overpass spanned a 60 m wide major arterial road, and linked two areas of isolated sub-tropical eucalyptus forest. Crossing rates of these species guilds were also determined over the major road, and two other high-traffic roads, 20 and 90 m wide respectively.Species guilds differed markedly in their crossing rates of the overpass, and in their method of crossing. They also differed in their relative ability to cross high-traffic roads of different width. Our results indicate that, although honeyeater and large forest insectivore guilds made extensive use of the wildlife overpass as a foraging resource and as a conduit between forest areas, the conservation value of wildlife overpasses in relation to potential gene-flow lies mainly with the small insectivore species guild, predominantly with forest-dwelling species of lower bodyweight and with flight characteristics not well-suited to sustained direct flight. Differences between species within this guild may be linked to a range of inter-related factors, including flight capacity, habitat use, and evolutionary history.Overpasses specifically for birds have rarely, if ever, been constructed. It would, however, be relatively simple to incorporate narrow, linear strips of appropriate vegetation strata into multi-purpose overpasses to benefit these target species.