Thousands of Austin’s bats will be displaced by I-35 expansion

The expansion of Interstate 35 through Austin is set to displace more than 100 homes and businesses. But an even larger community is being forced to relocate: thousands of bats that roost in the highway’s bridges.

While the Texas Department of Transportation (TxDOT) has taken steps to protect bats, some conservationists argue more could be done to avoid harming the nocturnal creatures that have become an integral part of Austin’s identity.

TxDOT is already demolishing bridges where bats live as part of the I-35 Capital Express project, a multibillion-dollar highway expansion through Travis County.

At least four bat colonies live under I-35 in Austin: at Howard Lane, Wells Branch Parkway, the upper decks and Onion Creek.

Howard Lane and Wells Branch together could host more than 100,000 bats, according to the Austin Bat Refuge, a rehabilitation center in North Austin that uses radar data to estimate bat populations.

The northbound I-35 bridge over Wells Branch Parkway was demolished last month. The northbound I-35 bridge over Howard Lane Bridge was torn down this week. Southbound bridges will remain open until the demolished crossings are rebuilt.

The larger bridges replacing the old ones will be built so bats can roost there again, TxDOT says.

Michael Minasi/KUT News. The northbound I-35 bridge over Howard Lane was demolished earlier this week. Bats lived under the bridge, but TxDOT tried to “exclude” bats – allow them to leave but not come back – before the structure was torn down. The larger bridge that replaces this one will be constructed so that bats can roost underneath.

Before demolishing the bridges, TxDOT evicts bats through a process known as “exclusion.”

Expanding foam is used to seal the crevices under the bridges where bats roost – ideally, after bats leave for the night but before they return. Conservationists aren’t fond of this method because of the risk that bats could be entombed alive. TxDOT says it needs a permanent solution.

“We don’t want things to fall out on cars, in case something got knocked loose, in case any pedestrian’s around and just pulls things out,” said Tracy White, a biologist with TxDOT’s Austin District.

The construction timeline also plays a role: “We want to allow the contractor to have some flexibility about when they can demo these bridges and keep their schedule going. So we want some kind of material that’s going to last a long time,” White said.

A preferred way to exclude a bat colony is by installing tubes that the animals can exit but not enter, according to Bat Conservation International (BCI) and Austin Bat Refuge.

Nathan Bernier/KUT News. Lee Mackenzie and Dianne Odegard run the Austin Bat Refuge in a residential neighborhood in North Austin.

“We don’t like that they’re using spray foam, but apparently that’s the method of choice,” said Lee Mackenzie, who runs Austin Bat Refuge with his wife, Dianne Odegard. “As long as they don’t use it in close proximity to bats, it’ll be fine.”

“And as long as they’re certain the crevices that they’re working with are vacant of bats,” Odegard added. “We feel certain that they’re doing their very best to get this done in a way that’s going to protect the bats as much as possible.”

Trying to replace bat roosts

To compensate for the lost habitat, TxDOT installed 36 artificial roosts known as “bat boxes” under the I-35 bridge over Walnut Creek. The boxes, designed in 2019 with help from Bat Conservation International, cost up to $4,000 each. They measure about 4 feet by 4 feet by 2 feet – about the size of a small chest freezer.

TxDOT. TxDOT biologist Tracy White examines some of the 36 bat boxes that were installed under I-35 at Walnut Creek. One bat expert says the boxes don’t provide enough habitat to replace the roosts lost when I-35 northbound bridges at Howard Lane and Wells Branch Parkway were demolished.

“It doesn’t seem like the most ideal habitat. It’s down below the level of the surrounding landscape. But it’s a nice gesture,” said Mackenzie, who has participated with Odegard in TxDOT Zoom calls seeking input from bat experts. “They can’t build enough for the number of bats in … both (Howard Lane and Wells Branch Parkway) bridges.”

White acknowledged it usually takes about a year for bats to find the artificial habitats. But TxDOT was on a “compressed schedule,” so many of the bat boxes weren’t installed until this summer, just a few months before the northbound bridges over Howard Lane and Wells Branch Parkway were torn down.

Another 24 bat boxes will be installed under an I-35 pedestrian bridge planned over Lady Bird Lake. The highway’s northbound frontage road at Onion Creek will get six bat boxes.

TxDOT. Twenty-four bat boxes will be installed in a stand-alone pedestrian bridge planned to be constructed over Lady Bird Lake on the east side of the I-35 bridge.

Austin used to fear bats

Bats weren’t always so beloved in Austin. A rabies panic hit in the early 1980s after TxDOT renovated the Congress Avenue Bridge. The crucial crossing into downtown had been closed almost two years when it fully reopened in April 1980.

In their placement of bridge beams, TxDOT engineers unintentionally created the perfect bat habitat. Hundreds of thousands started roosting under Congress Avenue Bridge, eventually establishing the largest urban colony of Mexican free-tailed bats in the world. During peak migration season, the colony is estimated to have more than 1.5 million bats.

People were terrified.

Austin History Center. The Congress Avenue Bridge under construction in 1979. Austin police had ordered the bridge closed in August 1978 after heavy rains washed out a road leading up to the crossing. TxDOT spent more than $5 million to widen the structure and replace the support beams. Bridge engineers unknowingly created the perfect habitat for the Mexican free-tailed bats.

“There was an association in people’s minds of bats with vampires,” said Frank C. Cooksey, who was Austin’s mayor from 1985 to 1988. “That was kind of a negative attitude or a fearful attitude.”

Local news reports in the mid-’80s, citing city health officials, warned thousands of rabid bats were living under the bridge and attacking residents. Angry calls grew to eliminate the dark-winged mammals.

Austin American-Statesman. One news report from September 1984 warned of bats sinking their teeth into the city “under a cloak of darkness,” a typical tone at the time when paranoia over bats ran high.

“I saw that this was a goldmine of opportunity,” said Merlin Tuttle, a prominent bat scientist who had shifted his efforts to bat advocacy just as the horror hit Austin.

Tuttle moved to Austin with his new organization, Bat Conservation International, and launched a charm offensive on behalf of the furry fliers, carrying a tiny Mexican free-tailed bat in his pocket that would melt people’s hearts with its cuteness.

Michael Minasi/KUT News. Merlin Tuttle, a bat ecologist and conservationist, is now in his 80s and still working to promote and protect bats.

The public education campaign worked. Tuttle effectively neutralized the city’s worst fears about bats. He educated local health officials. He spoke to the media in catchy soundbites, explaining how bats devour insects and pose less of a rabies risk to humans than dogs.

The Congress Avenue bats quickly “became a very positive tourist attraction,” Cooksey said. “It became an economic benefit.”

Why do bats like bridges so much?

Concerns lingered about whether bats were damaging bridges. Tuttle would go on to investigate – with help from TxDOT engineer Mark Bloschock, who designed the Congress Avenue Bridge – why bridges and culverts are so appealing to cave bats.

Tuttle’s team found no evidence bat colonies harmed the concrete structures. But they discovered the bridge’s beams, like many along I-35, were in the sweet spot for bats: between 0.75 inches and 1.5 inches apart, offering a synthetic cave whose roof – the road above – is warmed by the sun.

“It works really well,” explained Tuttle. “This gives them an almost perfect incubator-like situation for rearing young. Probably better than they find in most of the caves, actually.”

Michael Minasi/KUT News. A red light illuminates bats as they emerge late in the evening from under the Congress Avenue Bridge in August.

Tuttle’s efforts helped shift attitudes at TxDOT. The state agency now employs biologists who help engineers design bridges and culverts so bats can roost in them.

“We have to give TxDOT a lot of kudos here,” said Sarah Fritts, an associate professor of wildlife ecology at Texas State University who researches bats. Fritts reviewed TxDOT’s bat protocols for KUT News and said they seem reasonable.

“But also in the past, people didn’t care. People literally would just blow up bridges and do whatever construction and just kill bats and not care,” she said. “So I think we’ve come a long, long way from that.”

Similar praise came from the director of the world’s largest known bat colony, about 60 miles south of Austin in Comal County.

“They’re doing a really good job of mitigating any bad effects of road construction and bridge demolition and whatever they’re doing that might impact the bats,” said Fran Hutchins, director of BCI’s Bracken Cave, home to more than 15 million bats.

Patricia Lim/KUT News. The Bracken Cave in Comal County is home to as many as 20 million Mexican free-tailed bats, the largest known colony in the world.

Bigger bat exclusions are on the horizon. The most notable is the planned demolition of the I-35 bridge over McNeil Road, which is the largest highway bat colony in the Austin area. The site is a popular bat viewing alternative to the Congress Avenue Bridge.

“We were thinking about going to the Austin bat bridge, but you know with three young kids, it’s like – how long are they going to last anyways?” said Kyle Hampton, visiting the McNeil Road Bridge with his family on a hot night in September. “So what’s the point of going all the way down there?”

A project to widen I-35 from State Highway 45 North to RM 1431 could start as soon as 2028, according to the latest version of TxDOT’s long-range transportation plan. The expansion includes reconstructing the McNeil Road bridge.

“I’ve lost a lot of sleep over that,” said TxDOT’s Tracy White. “But I think everything’s going to be OK.”

This story was produced as part of the Austin Monitor’s reporting partnership with KUT.

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