As the sun sets and shadows lengthen over Bracken Cave, a stirring of leathery wings becomes a rustle and then a whoosh as thousands of bats emerge from the cave for their nightly feast.
These Mexican free-tailed bats are early arrivers from their wintering grounds in Mexico, and their coming heralds spring in this part of Central Texas.
But according to a new study using 22 years of daily radar images taken from a weather station in New Braunfels, these bats now arrive an average of two weeks earlier than they did in the mid-1990s, a likely sign of warming temperatures as a result of climate change.
“It’s been pretty interesting – it kind of validates the on-the-ground observations we’ve seen over the years,” said Fran Hutchins with Bat Conservation International, which stewards this cave on the northeastern fringes of San Antonio.
The swirling cloud of bats now emerging nightly from Bracken is a smaller version of the full-on bat tornado to come.
Hutchins estimated that roughly 80,000 to 90,000 bats are using Bracken Cave now. By May, those numbers will swell to 15 million mother bats and their young, known as pups. Scientists think that makes the cave the most populous maternal bat roost in the world.
Every evening, the mother bats, and eventually, their pups, leave the cave in search of flying insects, a spectacular display that draws hundreds of onlookers each year.
The colony is so large that it shows up on weather radar as a giant, sometimes donut-shaped clump.
Phillip Stepanian, a meteorologist by training, works at the University of Oklahoma with a group of researchers who specialize in using radar to study bats, birds, and insects.
“It’s a really exciting method, especially to get something that systematic and precise,” he said.
At first, Stepanian simply wanted a way to monitor the size of bat populations in Bracken Cave that didn’t involve visiting the cavern every night for 20 years and trying to count them.
“When you’re talking about millions of bats in a dark cave, it’s like guessing the number of M&Ms in a jar,” he said. “[Using radar,] you just don’t have that squinting at the ceiling, trying to guess how many bats are there.”
But when he and a colleague analyzed weather radar from March 1995 to November 2017 from the National Weather Service station in New Braunfels, they realized something interesting: The bats that used to arrive around mid-March have started showing up in early March, a likely sign of warmer temperatures arriving sooner in the spring.
In their journey north from their wintering grounds in Mexico, the bats follow migratory insects and are known for their tendency to chow down on agricultural pests, such as the corn earworm moth and cotton bollworm moth.
“As their food sources are moving into the region, the bats are going to follow,” Stepanian said.
The shifts could have an effect on regional agriculture. Scientists have estimated that Mexican free-tailed bats provide an average annual benefit of $741,000 per year in free pest control services in eight cotton-growing counties in South Central Texas alone.
The bats offer another sign of the shifting climate, as well. In his paper, Stepanian quoted scientists from the 1950s who visited several caves in the San Antonio area over three years and found zero bats hunkering down for the winter in any of them.
Nowadays, a small population of bats does spend the winter in Bracken Cave. That’s a sign of warmer temperatures and more available food, Stepanian said.
The change in migration timing is simply a sign of a trend that climate scientists have already observed in temperature data.
According to a 2014 report by Texas Tech University climate scientist Katharine Hayhoe, average temperatures have steadily been rising in San Antonio over the decades.
Winter has seen more rapid warming than summer. From 1961 to 1990, average winter temperatures rose 0.7 degrees Fahrenheit. Summer average temperatures rose 0.5 degrees.
How these changes are affecting Mexican free-tailed bats as a group isn’t clear. Stepanian wrote that the average number of bats spending their summer in Bracken Cave was stable over 22 years of radar data, though the peak populations during the height of breeding season “appear to be showing a significant decline.”
“It’s hard to tell in terms of the full population by only looking at one cave,” he said. “It could be that the bats are moving around between Austin, San Antonio, Corpus Christi, and Houston.”
In the future, Stepanian said he and his colleagues hope to broaden their radar studies to a broader area – the entire South Central United States. That could help them understand what’s happening to bats on a much broader scale, he said.