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When millions of migrating bats fly toward their breeding cave near San Antonio, Texas, each spring, the shadowy, swirling swarm is so dense, it shows up on weather radar. Scientists reviewed years of that radar data and found that the flying mammals are arriving about two weeks earlier than they did just two decades ago.

The scientists suspect the changes in the bats' timing and seasonal cycles are linked with the way global warming is altering the food chain and weather patterns.

Spring is warming faster than other seasons in many parts of the world, including Texas. An earlier bat migration fits with many pieces of the climate change puzzle, including earlier migrations of some bird species and earlier blossoming of many plants, said Phillip Stepanian, a meteorologist with Rothamsted Research who co-authored a new study in the scientific journal Global Change Biology on bat migration using the radar data.

Successive images from the San Antonio/Austin weather surveillance radar show a swarm of bats leaving Bracken Cave. The cave entrance is marked by the red box. Credit: Stepanian and Wainwright, 2018

"We're able to see there is this advancing pattern. Spring is happening earlier, winter is ending sooner," Stepanian said. "In the spring, the bugs advance up from Mexico, hitting Texas, hitting Kansas and, suddenly, the bats have something to eat."

That may also explain why the number of Brazilian free-tailed bats now staying through the winter in Texas' Bracken Cave has swelled in recent years. Surveys in the 1950s found no bats spending the entire winter there, but in 2017, as much as 3.5 percent of the entire population—100,000 bats—remained during the winter months, Stepanian said.

"To us, that sort of says winter conditions are becoming more tolerable and, rather than just going farther south, the bats are saying: We're going to just hang out in Texas."

The data show that the world's largest colony of migrating bats have arrived at Bracken Cave earlier steadily since the start of the study period in 1992, eventually moving the average arrival time up from late March to the middle of the month. Before that, migration date records are sketchy, the scientists said.

The Bat's Critical Role in Texas: Pest Control

Bats and the insects they eat are linked in a complex seasonal and evolutionary cycle that the state's agriculture areas rely on. If climate change disrupts the cycle, this natural pest control might fail, causing costly crop damage and an increased need for pesticides.

In a worst-case scenario, the timing could be so disrupted that it might threaten the bats' ability to reproduce. Females produce just one pup each, and during the feeding phase, the bats are heavily reliant on one main species of insect—the corn-earworm moth.

Texas farmers rely on Brazilian free-tailed bats to assist with insect control. Credit: Kathleen Smith/Florida Fish and Wildlife Conservation Commission

"The cycle of the moths showing up right when the bats need them is likely to get disrupted because of climate change," said University of Tennessee ecologist Jennifer Krauel.

Rising Temperatures Could Take a Toll

Krauel, who studies bat and moth migration, has done research during the fall, which is also getting warmer and drier in South Texas, and she is concerned about the impact on the bats during that season.

"In this part of Texas, it's really hot and dry, and climate change is just going to make it worse," she said. "There is not much for the bats to eat, they are at really low fat levels. They need to boost that before they migrate."

Global warming is also projected to worsen drought in the region, which would suppress insect numbers and diversity, she said.

"We don't know exactly what the change is going to be," she said, "but it's going to be bad."