Some animals choosy about their culverts

HAGERSTOWN — A wide variety of wild animals use drainage culverts to cross beneath roads but some species are choosy about tunnel types, a University of Maryland study has found.

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.

The $312,000 study was commissioned by the Maryland State Highway Administration to help engineers design roads that minimize animal-vehicle collisions and mitigate destruction of wildlife habitat.

The two-year, statewide study by the university’s Center for Environmental Science is among the largest investigations to date of how wildlife use existing culverts.

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.

“If you’ve worked in this area, you know there’s a tremendous amount of money that’s spent every year on repairing vehicles that have hit deer or large animals,” said Gates, a professor at the center’s Appalachian Laboratory in Frostburg.

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

It also documented 72 cases of does leading fawns through culverts.

“I’m sure it’s a learning process,” Gates said. “We have some indication that perhaps the does that are using the culverts actually show their fawns the passageways so when they grow into adults they continue using it.”

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.

Civil engineers have long known that wild animals use culverts as passages, including grizzly bears and moose in Canada and panthers in Florida. In recent decades, wildlife advocates have persuaded road builders to design culverts to better accommodate animals and erect roadside barriers that funnel them to the tunnels.

Maryland highway agency spokesman Charlie Gischlar said keeping animals off the roads is good for wildlife and people.

“It’s a safety issue but also we want to prevent as many animal-vehicle collisions as we possibly can,” he said.

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