Bat Conservation Texas Style
Article by Lorie A. Woodward
Photos courtesy of Jonathan Alonzo/Bat Conservation International
Thanks to its vastness and diversity of habitats, Texas is home to more bat species than any other state in the nation.
“Thirty-three bat species have been found in Texas, giving us the distinction of not only having the most bat species of any state, but having the highest diversity of bat species in the nation,” said Mylea Bayless, Senior Director of Networking and Partnerships for Bat Conservation International (BCI). “While diversity in nature is always a very good thing, it lends itself to a complex approach for bat conservation in Texas.”
Globally, there are 1,411 species identified in the order Chiroptera. Of the 44 bat species in the United States, 41 are insectivores and three are pollinators.
“In Texas, all of our bat species feed on insects, with the exception of the Mexican long-nosed bat, found in far West Texas, which feeds on nectar from agave plants and pollinates them in the process,” said Dr. Amanda Adams, Conservation Research Program Manager for BCI. “And, just because people always ask me, vampire bats do exist. There are three species in the world, but they only live in tropical Central and South America.”
Because bats eat insects, including mosquitos and agricultural pests, they benefit humans. For instance, Mexican free-tailed bats feed on high-flying swarms of corn worm silk moths, a major agricultural pest. A single Mexican free-tailed bat can eat up to two-thirds of its body weight in insects each night. According to the Encyclopedia Britannica, the multiplication of that singular appetite across millions of individuals totals up to 10,000 tons of insects eaten in Texas annually.
“It’s been estimated that Mexican free-tailed bats save Texas corn farmers $1.4 billion a year by reducing crop damage and pesticide use,” said Jonah Evans, Mammologist for the Texas Parks and Wildlife Department’s Rare and Nongame Species Program. “On a national level, it’s estimated that bats, as a group, save farmers $20 billion - $50 billion annually.”
Bats occur in every county in Texas. In 1995, the Texas State Legislature named the Mexican free-tailed bat as the "state flying mammal."
“There are no ‘bat-free zones’ in Texas,” Evans said. “Many ranges overlap, but each species has a slightly different strategy for survival that is unique to its environment.”
Roost sites, where the nocturnal feeders rest during the daylight hours and raise their young, provide a good example of adapting to local conditions.
Mexican free-tailed bats, which can be found in Central Texas and throughout the Edwards Plateau, roost in caves and under rock outcrops as well as in man-made structures such as bridges. Red bats, which live in the Pineywoods, roost in the foliage of deciduous trees and on cold, damp days have been known to burrow into fallen leaves and remain in a state of semi-hibernation.
In the Cypress-Tupelo swamps of southeast Texas, Rafinesque’s big-eared bats and Southeastern myotis roost in the hollow trunks of old growth black gum and water tupelo trees. Meanwhile down in deep South Texas and along the Gulf Coast, the Southern yellow bats roost and raise their young hidden in palm fronds, so landowners are encouraged to refrain from trimming palms or at least wait until the end of fall when all pups have matured.
While Texas enjoys an unprecedented diversity in bat species and habitats, the flying mammals are not immune to the threats that are causing bat populations to decrease across the nation.
According to Bayless, the challenges include: land conversion and fragmentation, White-Nose Syndrome, wind energy development and water scarcity.
“As bat conservationists, we’re facing high-level challenges, but we have the opportunity to mitigate the effects,” Bayless said. “We need passionate people to implement the solutions we know work to benefit bats and people.”
Bats are unique. In addition to being the world’s only flying mammal, they live for decades despite weighing just a portion of an ounce. Bats can live up to 40 years.
“There is a lot of scientific interest in understanding how they, as relatively small creatures, live so long because generally that’s not normal in nature,” Bayless said. “Somehow they’ve unlocked the fountain of youth.”
Bats are also unique when it comes to reproduction.
“Unlike most small mammals such as mice and rabbits that have large litters of offspring, bats, depending on the species, have one or two pups each year,” Evans said. “Reproductively, bats are much more like brown bears than other mammals in their size range.”
The combination of longevity and a low reproductive rate make each bat important to the population.
“One dead bat has a much larger impact on the population than one dead mouse, because it isn’t quickly or easily replaced,” Bayless said.
While each species’ reproductive strategies and life cycles differ slightly, the migratory Mexican free-tailed bat, Texas’ most abundant and iconic bat species, provides a good example.
Most Mexican free-tailed bats leave Texas in October, migrating to Mexico where they overwinter and mate. (A small percentage of the population remains in Texas.) While some bat species are monogamous and others form harems, female Mexican free-tailed bats come into estrus and may mate with a different male each breeding season.
The bats return to Texas beginning in late February. Males form small bachelor colonies, while the females congregate to create huge maternity colonies such as the well-known ones at Bracken Cave, the world’s largest known maternity colony, and under the Congress Avenue Bridge in Austin. In June, each female gives birth to a single pup that arrives weighing 25 percent of its mother’s weight.
The female bat leaves her pup in nursery section of the cave, which is separate from the adult roost, until the pups are able to fly.
The cave walls are densely packed with up to 500 naked pups per square foot, so before the mother leaves to forage she spends up to an hour getting acquainted with her pup’s scent and vocalizations. She, then returns, at least twice a night to nurse the pup.
“Think about the feat the mother bat accomplishes by finding her pup among millions of others,” Bayless said. “The ability to use smell, vocalizations and spatial memory is incredible especially in the context of a huge social colony.”
In about 38 days, the young bats begin to learn to fly within the cave. It is difficult at best, as this excerpt on Bracken Cave taken from the BCI website describes: “If all goes well on its first flight, a young bat drops into complete darkness, flies at a speed of at least 20 feet per second, and turns an almost complete somersault with millimeter precision to land on the cave wall just seconds after taking off. It must also avoid several collisions a second with thousands of other young fliers testing their skills, while at the same time relying on an echolocation system that is itself being tested for the first time.
Collisions with other bats or cave walls can be fatal. An emergency landing is certain death; the floor is teeming with millions of carnivorous dermestid beetles that can reduce a young bat to a cleaned skeleton within a few minutes. Fatalities are high, and at least half won't survive their first year.”
By late July, young bats are ready to fly outside the cave and join their mothers to feed on insects. Although they are weaned soon after, the pup’s initial energy demands are high, forcing nursing mothers to consume more than their body weight in insects each night to produce enough milk. To find sufficient food, the bats often emerge up to three hours before sundown. Pups reach adult size at about 60 days.
Because the number of bats leaving a maternity colony doubles when the sub-adults join the throng, the best time to view a Mexican free-tailed bat emergence is late July until the bats leave for Mexico in early October. Most young Mexican free-tailed females reach reproductive maturity at about 9 months old, while young male bats mature at about two years.
White-nose Syndrome is a devastating disease caused by Pseudogymnoascus destructans, a fungus that thrives in the wet, cold environments of caves and abandoned mines.
“Scientists suspect the fungus was transferred from Europe,” Evans said. “It occurs there, but the native Old World bat species seem to be largely resistant to its effects.”
It takes several years of the proper conditions for the fungus to manifest itself as the disease, which affects bats’ skin and ravages bat species that hibernate.
“When bats hibernate, they suppress their immune system to reduce their metabolic rate so they can survive without eating,” Bayless said. “The disease strikes when they are most vulnerable.”
The fungus disrupts the skin’s functions which can affect gas exchange, cause dehydration and irritation, which can rouse them from torpor. Each time the bats wake, their body systems “wake up” as well, using valuable fat stores. Eventually, the bats either starve to death or are prompted to leave the roost to forage during the winter and succumb to the lack of food and cold temperatures.
The fungal disease was first identified in the United States in 2006 in New York. Since that time, the disease has spread to 33 states and seven Canadian provinces with an estimated death loss of at least 6 million bats.
“Our North American bats species are not adapted to it and succumb really quickly,” Evans said. “The disease has spread in all four directions moving an average of 200 miles each year toward Texas.”
In 2017, the fungus was detected in the Texas Panhandle.
“TPWD and BCI predicted it would arrive in the Panhandle first and have been surveying for years,” said Adams, noting that eight bat species including Tri-colored bats, which have suffered 96 percent loss at various sites four to seven years after first detection of the fungus that prompted White-nose Syndrome. The Tri-colored bats are predicted to be at highest risk in Texas.
By the end of the 2019 survey season, the fungus had been detected in 22 sites in 16 counties stretching from the Panhandle through Central Texas; 11 of those sites including Bracken Cave were newly identified in 2019.
“So far, we have the fungus, but it hasn’t yet prompted the disease,” Adams said.
Species such as the Mexican free-tailed bat and Eastern Red bat that don’t hibernate seem to have an advantage.
“While Texas is on the leading edge of the disease, it appears that we may have some advantage,” Bayless said. “When bats are active and the weather is warm, their immune systems are active as well, so much of the state is unlikely to see the disease.”
With that said, there have been significant declines in states with warm climates such as Alabama. What happens in Texas remains to be seen. Some bat species unaffected by the disease may still serve as vectors spreading the fungus.
“There is no silver bullet, but there has been good work done on potential treatments,” Evans said.
At the moment, TPWD is conducting a research study in East Texas involving a two-part treatment using culverts where Tri-colored bats roost. In the summer, when few bats are using the sites, researchers spray the culvert walls and ceilings with polyethylene glycol, the active ingredient in Miralax®, which is slippery. It is postulated the compound will prevent the fungus from attaching to the surface and growing.
During the winter, once the bats have begun to hibernate, the researchers plug both ends of the culvert for about an hour and fumigate the space and sleeping bats with an antifungal VOC known as Decanal in hopes of retarding fungal growth and spread.
As an adjunct to this work, TPWD researchers are also conducting “disturbance studies” in culverts where the fungus isn’t found to ensure that the treatments themselves aren’t inadvertently disturbing the bats.
“They’ve done similar work in other states and have reported no ill effects,” Evans said. “Here in Texas, we’re being proactive—but cautious—when it comes to White-nose Syndrome.”
For reasons not clearly understood, bats are on a collision course with wind turbines.
“In the U.S. and Canada, at least 24 species of bats have suffered recorded mortalities due to wind turbines,” Bayless said. “There doesn’t seem to be a ‘good’ relationship between the pre-construction bat counts and post-construction mortality—the numbers suggest that bats are attracted to the turbines.”
She continued, “Because there are so many diverse bat species, it’s a definite challenge for wind developers and conservationists.”
It is a challenge that hits close to home. According to the American Wind Energy Association, Texas tops the nation when it comes to wind energy with an installed wind capacity of 25,629 MW produced by 13,672 turbines—and that foot print is growing. In areas where Mexican free-tailed bats exist, they tend to make up the highest proportion of the kills.
“All of our renewable energy goals point to expanded wind development which in turn points to more bat mortality unless we implement conservation measures that are proven to save bats,” Bayless said.
One of the most promising strategies is operational minimization. It is based on two facts: Bat activity increases on nights when wind speed is low. Electricity production decreases on nights when wind speed is low.
“In a nutshell, the wind operator delays the cut in point on the turbines,” Bayless said. “The turbines are feathered until the wind speed reaches a threshold where bat activity dies down.”
Currently, the turbines “cut in,” start turning to produce energy, when the wind is blowing around 4 meters/second. Bats are still active. Studies show that if the cut in point is delayed until the wind speed are higher when most bats become less active, mortality loss can be reduced by about 50 percent.
“The challenge is finding a sweet spot where the wind operators can operate profitably and reduce bat deaths,” Bayless said. “If you’re a landowner who has or is considering wind energy development, I’d encourage you to ask the operator what their company is doing to conserve bats—widespread demand for conservation is going to make it easier for companies to voluntarily change their practices.”
Another promising option is ultrasonic acoustic deterrents (UAD). The devices, developed by NRG Systems, emit a continuous high frequency sound (between 20-50 kHz) designed to “jam” the bats’ echolocation by creating a disorienting airspace around the wind turbines. In both 2017 and 2018, Sara Weaver, who was conducting doctoral research at Texas State University, tested the deterrents’ efficacy at a Duke Energy wind farm in South Texas. The sampling lasted from July 31 to October 30.
Each year, Weaver outfitted eight turbines with deterrents and used eight turbines as the control. Every day, the team conducted a search in a 100-meter radius of each turbine to recover and identify any bats killed the night before. They identified Mexican free-tailed bats, Northern yellow bats, Hoary bats, Southern yellow bats, Evening bats, Eastern red bats and Cave myotis. More than 77 percent of the bats killed were Mexican free-tailed bats; Hoary bats suffered the second highest death loss.
According to Weaver’s presentation, “Testing Ultrasonic Acoustic Deterrents for Reducing Bat Fatalities at Wind Turbines in South Texas,” fatalities for Mexican free-tailed bats were reduced by 54 percent and by 73 percent for Hoary bats. The deterrents did not have a measurable impact on the other species.
“At the conclusion of the study, overall bat fatalities due to collision were reduced by more than 50 percent at the turbines outfitted with deterrents,” Evans said. “It is exciting work with tangible real-world results.”
Weaver, who is now on staff at Texas A&M University-San Antonio, is continuing to refine the work. Duke Energy, according to press issued on June 26, 2019, will be outfitting 255 wind turbines in the Rio Grande Valley with the deterrent system over the next five years.
Best Management Practices
Landowners can make a difference for bats just as they make a difference for other native species.
“Bats like healthy native habitat just like every other species,” Evans said. “Generally, if landowners are managing for white-tailed deer, quail or other iconic species that require a diverse mix of native vegetation they’re doing good things for bats.”
Of course, each bat species has different habitat requirements. (Bats of Texas by Loren K. Ammerman, et al, which contains range maps and detailed information on each Texas species, is considered the “go to” reference for Texas bat biology). Overall bats, though, need areas to forage, insects to eat, accessible water to drink and places to roost.
Native habitat produces native insects, the food source for most Texas bats.
“If you’ve got native range maintain it,” said Bayless, noting BCI includes native re-seeding and prescribed fire as part of its management of the 1,500-acre Bracken Cave preserve. “If you’ve got invasive plant species, try to get a handle on them.”
Evans cited a recent study that showed a well-established stand of KR bluestem supported very few insects, making it a “food desert” for insect eaters.
Because insects are such an integral part of the food web, land managers are also encouraged to minimize or eliminate pesticide use.
“Recently, a report was released indicating that we have lost 3 billion birds since the 1970s,” Evans said. “The cause? Invasive species such as feral cats and collapsing insect populations due to large-scale insecticide use.
He continued, “Diminishing populations of insects spell trouble for bats as well. We’ve got to strive for balance in the ecosystem.”
The same principles apply to urban landscapes.
“Urban and suburban lawns now cover a lot Texas,” Adams said. “In a perfect world, homeowners would let their lawns go, but reducing chemical use and dedicating a portion of the yard for native plants is a step in the right direction.”
All animals need water, but bats are particularly susceptible to physiological water stress due to their large body surface to volume ratio and water loss through their wing membranes.
“We’re paying attention to changing weather because we have so many habitats in Texas that could be affected as temperatures rise and rainfall becomes more erratic,” Bayless said.
Research has documented bats losing up to 30 percent of their body weight in a 12-hour period due to evaporative water loss. Insectivorous bats are especially susceptible as they receive less metabolic water from their food than bats that feed on fruit or nectar.
Bats drink “on the wing,” using a touch-and-go method that mimics airplanes and requires access to flat, slow moving open water. (Riparian areas also provide excellent foraging opportunities.) If natural water sources aren’t available, bats can use man-made water sources such as stock ponds and stock troughs. Landowners can help make stock troughs bat friendly.
“If you have stock troughs, keep them full, so the bats can easily skim the surface without running the risk of drowning,” Bayless said. “It’s also important to add escape ramps, which benefit both bats and birds—and we have plans on BCI’s website that show you how.”
As skilled flyers, bats can reach suitable food and water, but they can’t build suitable roosts.
“The number one thing a landowner can do is protect a roost site, whether it’s a cave, a rock outcropping, snag or hollow tree,” Evans said. “Find out what bats live in your area, identify their potential roosts and conserve them.”
Avoid disrupting the colonies, especially during maternity season.
“Some Mexican free-tailed bats and the big brown bats adapt to human development, most bat species are sensitive to disruption of their habitat,” Bayless said. “They are all sensitive to disruption in May through late July while they are raising babies.”
Caves require particular care.
“All caves are delicate ecosystems,” Bayless said. “Access should be limited.”
If a landowner has trouble keeping the public out, there are contractors who can build specially designed bat gates adapted to particular species. BCI can provide a list of qualified contractors.
“Bat gates are equal parts art and science, so it is important to work with people who know what they’re doing,” Bayless said. “A badly built or installed gate can do more harm than good.”
Landowners who provide access to caves should require guests to follow decontamination protocol to stop the spread of White-nose Syndrome, Evans said. The protocol is available on the whitenosesyndrome.org website.
Some bat species, such as Mexican free-tailed bats and big brown bats, take up residence in abandoned buildings, barns and attics.
“If they aren’t disturbing you, let them be,” Adams said. “If they are causing a problem, then seek out an excluder, who will keep the bats from returning to the space, instead of an exterminator, who will kill them.”