DCist is providing special coverage to climate issues this week as part of Covering Climate Now, a global collaboration of 250 news outlets designed to strengthen coverage of the climate story. Many of these stories originated as questions from our readers. 

On a pleasant warm late-August early evening near Alexandria’s Dyke Marsh Wildlife Preserve, Deborah Hammer sits on a bench and watches the sky. She isn’t looking for any old species of the world’s only true flying mammal, but specifically the Brazilian free-tailed bat, sometimes referred to as the Mexican free-tailed Bat.

Quite simply, these bulldog-looking bats shouldn’t be this far north. They shouldn’t be fluttering about the Washington Monument and shouldn’t be slurping up insects over the Potomac. But, over the last five years or so, citizen scientists like Hammer have seen them in D.C.’s night sky on a constant basis.

“We hear them every night,” says Hammer as she clutches her iPhone with a plugged-in EchoMeter Bat Detector and the corresponding app open. “And there are more and more of them every year. The question is why.”

The “why,” of course, is complex and hasn’t been fully answered yet. But recent research and studies have pointed, at least partially, to climate change and the warming of our planet as reasons that the velvet-furred, human thumb-sized mammals (not including their foot-long wingspan) are growing in numbers in the area.

With a cackle and chirp, an ID pops up on Hammer’s iPhone. “Mexican free-tailed! Look! Look! Over there!” she says excitedly.

It’s believed that there are eight different species of bats in the D.C. area. However, in recent years, the bat population has undergone major changes. The epidemic of White-Nose Syndrome, a fungus that infects cave-dwelling bats while they hibernate, is estimated to have killed seven million North American bats since 2006. In this region, it has completely decimated the little brown bat population.

About 1600 miles to the southwest of D.C.—near San Antonio, Texas—-is Bracken Cave, summer home to the world’s largest bat colony, with more than 15 million Mexican free-tailed bats.

Mexican free-tailed bats, so-called due to the ranges they inhabit, is a regional name given to certain groups of Brazilian free-tailed bats. In other words, they are very similar save for slight behavioral differences—in this case, their migration habits.

As the most widely-distributed species of bats in North and South America, massive colonies of Brazilian free-tailed bats can be found in Brazil, Chile, Mexico, Texas, Florida, New Mexico, and Nevada. They eat bugs by the thousands, with moths, beetles, and flies being their favorites. Soaring up to 60 miles an hour, Brazilian free-tailed bats snack on the fly using echolocation to find their next meal. They’re such good eaters that they are an extremely valuable source of natural agricultural pest control.

In the late 1990s and early 2000s, Leslie Sturges was a keeper at a National Zoo when she fell in love with these fuzzy flying creatures. It was about this time that she attended a “bat boot camp” at Bat World Sanctuary in Texas and, soon, transitioned to rehabbing injured and ill bats out of her home in Annandale, Virginia (she later moved further southwest to Staunton). She says it was in 2014 when she got her first Brazilian free-tailed into rehab. “It was a tiny female orphaned pup [found] in Charlottesville,” says Sturges. “That blew it wide open because it was like ‘So, where’s the mother?’”

Sturges first thought it could have been a fluke with the poor little critter getting stuck on a Charlottesville-bound flatbed truck. But then she kept receiving the species into her care and people kept having sightings. Worse, Sturges says, most of her ailing bats come in the winter, often with frostbite and other cold-weather-related ailments—the species is not well-adapted to cold, snowy D.C. winters. In fact, in her care right now, is a frostbitten male named Nog that was found in Lynchburg, Virginia.

All of this has lead Sturges to believe that, over the last five years, the Brazilian free-tailed bat has found a new home in the D.C. region. She even suggests that, since there’s evidence that some insects are hatching sooner due to the earlier springs, the bats could be simply following their meals. “My feeling is that they are getting here under their own steam,” Sturges says. “But then they are getting stuck.”

In the April 2018, one of the few scientific studies looking into why Brazilian free-tailed bats have recently been found further north was published in the Journal of Mammalogy. It’s conclusion is familiar. “We hypothesize that… their rapid northward expansion is facilitated by climate change.”

It also says that these bats’ natural curiosity and ability to adapt to unfamiliar roost (a sleeping/hibernation location) sites are also factors. Later, the paper goes a bit further. “These largely tropical bats … with climate change … may be at the vanguard of faunal expansion into areas with formerly cooler climates.”

Veronica Brown, one of the co-authors of the study, is based out of the University of Tennessee in Knoxville. She’s a molecular ecologist by training and began by studying bats. She is credited with finding Knoxville’s first Brazilian free-tailed bat on a snowy January day in 2011. Despite her initial disbelief, she knew what she had.

“They look different enough from every other bat here, that you can tell,” says Brown. “You don’t need DNA.” That lead her on a years-long mission to figure out why these tropical bats were being found in northeast Tennessee and, as was later discovered, even further north.

Brown cautions that they were careful when it came to the topic of climate change. With all the authors of the paper being biologists and genome sequencers, none were climate scientists or experts. She’s adds that the topic is so polarizing and fraught that they were fearful that the paper wouldn’t be published if it ventured too far into that subject matter.

Nonetheless, Brown is convinced that the changing climate and the warming of our planet is playing a big part in why tropical Brazilian free-tailed bats are in Washington D.C. “It’s a combination that’s making it easy for free-taileds to take over,” Brown says. “Climate change is making the climate better suited for them and the other bats are disappearing [due to White Nose].”

Hammer, Sturges, and Brown all agree that the Brazilian free-tailed bat being here, in and of itself, isn’t necessarily a bad thing. They eat similar insects to the little brown bats, meaning they’re essentially filling the food chain gap left by the thousands of others that have succumbed to White Nose. While they are known to roost in warm buildings and attics, few, if any, colonies have been observed here as of yet. Brazilian free-tailed bats can carry rabies, but the percentage of them that have the virus compared to the population size is extremely low.

Plus, as Brown puts it, they are easy on the eyes. “They are super cute,” she says. “Free-tailed bats are so adorable.”

The real issue with them being here is what it indicates about the future of the planet. But, at this point, much of what we know are educated guesses from those who know these bats best.

Back in Alexandria at the Dyke Marsh Wildlife Preserve, it’s growing darker and harder to see the bats fluttering above. But the free-taileds are there, enjoying an all-bat-can-eat bug buffet.

As her phone crackles with yet another ID of a bat, Hammer nods. “They are here and well-established,” she says. “And that’s brand new … Things are changing.”

This story has been updated to correct the the spelling of Leslie Sturges’ last name.