Animals Using Culverts to Cross Roads

Stone culvert / Photo by Wikipedia user Valerius Tygart

Researchers in Maryland studied whether and how animals (including birds) were using culverts to cross under roads.

Raccoons will use any kind but deer avoid culverts with cobbled floors and eastern gray squirrels don’t seem to like arch-shaped passages, the study’s lead author, ecologist J. Edward Gates, said Friday. Great blue herons prefer box-shaped culverts with sandy bottoms, the study found....

Culverts are tunnels, usually made of concrete or metal, that allow water to flow beneath roadways. Animals moving along stream banks may naturally follow the water through culverts, Gates said.

Researchers documented 57 species using the conduits, including nesting barn swallows, feral cats and white-tailed deer — a species of particular concern because they cause dozens of deaths in collisions each year around the country....

The study found deer using tighter passageways than previously documented — as small as 3.2 feet high and 4.7 feet wide....

He said preferences for certain culvert shapes may have more to do with what’s on the tunnel floor. Many box-shaped culverts studied were made of concrete whereas the rounded conduits were all made of corrugated metal. The boxy shape preferred by herons also provides room to spread their wings and quickly escape, the study says.

The study was funded by the Maryland State Highway Administration to see if culverts could be used to provide wildlife with safer ways to cross roadways. Highways present a real barrier to large wildlife. If they choose to cross, they risk death or injury. If they choose not to cross, they lose access to the foraging habitat on the other side of the road as well as the opportunity to mate with members of their species that live on the other side. (The latter is especially a problem when endangered or declining species are involved; in the mountain west, wildlife bridges are being built over interstates to avoid genetic isolation.) For drivers, there is a risk of vehicle damage or injury when larger animals try to cross the road. Perhaps the solution to the problem is more or better culverts where roads can accommodate them.