Hard Freezes Are Killing Texas Bats

On a recent day in November, under a mesh tent in their Austin backyard, Dianne Odegard and Lee Mackenzie took turns hand-feeding the dozens of bats in their care. Partners both in life and in the Austin Bat Refuge, a nonprofit they run from their home in the Cherrywood neighborhood, the septuagenarian couple has rescued thousands of injured bats over the past two decades. Their garden is full of night-blooming greenery: fragrant jasmine, Mexican honeysuckle, and burnt red shrimp plants, all in service of their nocturnal charges, who take practice flights under the tent come nightfall. Mackenzie, his shoulder-length gray hair tied into a ponytail, wore a black latex glove as he fed live worms to Sylvie, a hoary bat (so named for its white-tipped fur, resembling hoarfrost). 

“I won’t let anything happen to this wing,” he whispered as he unfolded her injured wing to see how it was healing. “You are really something, Sylvie. Stunning,” Mackenzie said as he gingerly placed the bat back in her temporary home, an upturned wicker basket on the edge of the tent. Nearby, an antique pie safe, about the size of a small dresser, contained several more adult bats and pup tents. Sylvie remained in the couple’s care, being fed multiple times daily, for ten more days, until—after 176 days total—she was released (along with a northern yellow bat named Vermicelli) near Fiesta Gardens. 

Odegard, who spent twelve years as the education and public outreach manager for Bat Conservation International, drew Mackenzie into her passion for bats long ago. Now they receive calls at all hours of the night from Austin residents who find the creatures fallen from their perches in trees or stuck immobilized with a broken wing from a car accident. After a hard freeze in January, the couple and their team of volunteers collected 1,200 cold-stunned bats that had dropped from their perches under bridges throughout the city, attempting to revive them on heating pads and under heating lamps. Fewer than three hundred survived. In Houston, volunteers with the Humane Society rescued more than 1,100 hypothermic bats that same week.

Harsher winter weather is just one of the many effects of climate change—including severe heat, drought, and flooding—that are beginning to threaten the 32 bat species that live in Texas. The state has more bats than anywhere in the nation, in part since it’s home to the largest bat colony on the planet: 15 million Mexican free-tailed bats live in Bracken Cave, outside San Antonio, during the summer months. Climate change is expected to threaten the survival of a whopping 82 percent of North American bat species in the next fifteen years, according to Bat Conservation International. 

Last month’s cold snap was just one of several recent freezes that have put Texas bats at risk. Similar events in 2023 and 2022 also killed thousands of the animals. Exact numbers are hard to come by, but the statewide death toll from the worst freeze, in 2021, is likely in the tens of thousands, according to Jonah Evans, nongame and rare species program leader at Texas Parks and Wildlife. Mass die-offs of bats could devastate Texas biodiversity. Despite centuries of bad press, these nocturnal creatures are great for their environment. Nearly all of the millions of bats that call Texas home are insectivores, each capable of eating thousands of mosquitoes, moths, and other insects per night. This combined bat power saves Texas farmers an estimated $1.4 billion annually in pesticides, according to an estimate from Texas Parks and Wildlife. That reduction in pesticides in turn limits the amount of polluting chemicals damaging habitats, destroying plant species, and leaching into the water.

“All organisms are influenced by climate. With bats, they seem to be particularly susceptible,” said Samantha Leivers, bat specialist at TPWD. The scale of the problem is difficult to quantify, she said, but it is severe. “The main problem that we run into is that to actually see the effects of climate change, we need long-term data. By the time we have that long-term data, it’s going to be too late to do anything,” she said.

This month marks the two-year anniversary of the devastating winter storm of February 2021. So-called Snowmageddon caused at least $80 billion in damage statewide, left 11 million Texans in the dark, and set a record as the coldest winter storm in Texas since the 1980s, with a final death toll numbering nearly 250 people. The bat population was also decimated. Many Texas bats roost in the warm concrete under bridges in cities such as Austin and Houston, and after the storms, the underpasses were a “carpet” of dead bats, according to Texas Parks and Wildlife. It could take decades for the population to recover, because each female bat typically gives birth to only one pup per year—a much slower rate than many other mammals (a rodent, for instance, can produce as many as one hundred offspring in that time). 

On the flip side, changes in climate mean a winter season that can fluctuate between freezing temperatures and mild days, and those major shifts in temperature also challenge bats’ survival. Tree-roosting bats need consistent temperatures to enter the energy-saving mode of torpor during winter. Constant changes in temperature can arouse bats and cause them to burn through their precious winter fat reserves too quickly. That’s true for the threatened Rafinesque’s big-eared bat, which has long, bunnylike ears and roosts in leaf-littered tree hollows along the Gulf Coast, as well as for the eastern red bat, which camouflages itself as a dead leaf, dangling from one leg in the branches of deciduous trees across Texas.

Both tree roosters and cave dwellers face another climate problem: drought. Drought is a multipronged issue, both because it reduces the fresh drinking water bats need to survive and because less water means fewer hatching insects for the bats to eat. Texas hasn’t yet seen the massive wildfires that affect California, but if the drought periods continue to extend, the ancient oak, cedar, and elm trees favored by bats will be easy targets for either fire or hurricanes.

The problems faced by the Texas bat population can all be seen on a micro level at Bracken Cave, outside San Antonio. Fran Hutchins has been the warden of Texas’s largest bat colony for nearly twenty years, and in that time, he’s already started noticing important changes in the bats’ habitat and behavior. Bracken bats tend to be less affected by freezes because temperatures remain stable inside the humidity of the cave, but the increasing duration and intensity of droughts means that they are having to travel farther and longer to find enough fresh water to drink and insects to eat. A 2018 study found that warming temperatures have prompted bats to arrive earlier to Bracken, showing up to their summer residence as early as February.

Driving through South Texas and seeing sorghum or corn that’s only a foot high, or burnt to a crisp, is usually a good indicator that the local bats will struggle to find food. The bats of Bracken Cave alone eat 140 tons of bugs—mostly crop pests—per night. As Bracken is a maternity colony, populated almost entirely by pregnant females, the bats can’t forego feeding, so if the pickings are slim, they will simply spend more time hunting. Many of the millions of Mexican free-tailed bats now take flight when it’s still light out, exposing them to owls and hawks. Flying (during which time a bat’s heart can pump at least eight hundred beats per minute) is energetically costly, too, meaning the bats could soon be expending more and more energy for smaller returns. 

Between white-nose syndrome—caused by a deadly fungus that kills bats by eating their skin and disrupting their hibernation until they exhaust their fat reserves—and the effects of climate change, bats are now facing deadly threats during all seasons. “A lot of this starts becoming cumulative when you have a disease that’s stressing you during the winter months, and then during the spring and summer, droughts are affecting the food supply. Development is affecting where you can live and feed,” Hutchins said. “They’re not getting a break.”

The gaping mouth of Bracken Cave measures sixty feet across, yawning open at the bottom of a steep hill covered in vegetation. Each night, millions of bats emerge from that abyss, the sounds of their high-pitched echolocation calls and flapping wings marking the beginning of their hunt. Their wingspan gives the illusion of bulk, but a Mexican free-tailed bat weighs no more than half an ounce, a little more than a bullet. For now, the bats are adjusting their hunting tactics to survive, flying a little farther, and staying out a little longer. Only time can tell if that will be enough.