Wildlife crossings (or ecoduct, or ecopassage) are structures that allow animals to cross human-made barriers safely. Wildlife crossings may include: underpass tunnels, viaducts, and overpasses (mainly for large or herd-type animals); amphibian tunnels; fish ladders; tunnels and culverts (for small mammals such as otters, hedgehogs, and badgers); green roofs (for butterflies and birds)