It was the guano that gave them away.

A bridge inspector found it in 2013 on an island under the I-90 Interstate Bridge near La Crosse, Wis.

Piles of it.

High above, tucked into a structural gap just a few inches wide and 50 feet long, were 3,000 little brown bats that for decades have used the bridge as a safe place to rear their young.

But now the bridge is coming down, and, courtesy of the Minnesota Department of Transportation, the bats have $20,000 worth of newly constructed condominiums — should they choose to use them next year.

The little brown bat needs all the help it can get. One of the most common species in Minnesota and Wisconsin, it is among the cave-dwelling bats that are threatened by white-nose syndrome, a fungal disease sweeping the country.

“It’s one of largest colonies we know of in the state,” said Heather Kaarakka, a conservation biologist with the Wisconsin Department of Natural Resources.

“We’ve been working the [transportation departments] in both states to figure out what has to happen to save the colony.”

The disease strikes bats in winter when they hibernate and has devastated colonies in many eastern states.

Evidence of it has been found in Minnesota, but it has not yet been shown to harm any colonies.

Tiny creatures, big appetites

“It could expand to impact our bats at any time,” said Carrol Henderson, nongame wildlife manager for the Minnesota Department of Natural Resources.

White nose syndrome is becoming widespread in southwest Wisconsin, which has allowed the state to list the bats as threatened and to take pre-emptive steps to protect them, including ensuring summer roosting places.

Like building bat condos.

Although, from a bat’s perspective, it might be hard to beat the space under the bridge.

The bats are tiny, the size of a house mouse, but with an eight- to 12-inch wing span. They like tight, warm spaces for their young, so the expansion joint between the freeway lanes was ideal.

It’s also over a river where they could hunt insects at night — primarily mosquitoes.

“They can eat up to 1,000 mosquito-sized insects in one hour,” Kaarakka said. “A female consumes her body weight in insects in one night when pregnant.”

Bats live for as long as 30 years, and return to the same roosting spots year after year, and so do their young.

Accommodations common

Solving wildlife problems is nothing new for contractors on major construction projects.

Mark Anderson, director of the $187 million I-90 bridge project, said swallows and other migratory birds often inhabit the undersides of bridges.

Henderson said utility companies will build nesting platforms for ospreys to get them off power line poles.

The bat condo is a well-known strategy for homeowners who want bats to stay out of the attic: Build them their own place nearby.

Still, taxpayer-funded homes for critters are rare. And these are a lot bigger than the average bat house.

The main one is wooden, 4 feet square, filled with tiny baffles to give the bats the tight spaces they like.

It’s dark, to attract the sun’s warmth during the day. Nearby, several smaller plastic and metal bat bunkers will hold several hundred additional bats each.

Now comes the test.

The bats have left for the winter — bats hibernate in caves or underground spaces — so construction crews will remove the section of the bridge where they roosted.

Next spring when they return from hibernation, with just a little luck, the bats will find their way to their new homes.

“There is a lot of research that needs to be done on the type of bat houses they prefer,” Kaarakka said.

“We don’t know what a bridge colony will want after roosting in a little space along noisy bridge.”

How will she know if it works?

Bat guano underneath.