Bats fill the sky outside of Nickajack Cave on Wednesday, June 19, 2019 in Marion County, Tenn. Gray bats are federally endangered and afforded protection under the Endangered Species Act.

Updated at 4:13 p.m. on Thursday, June 20, 2019, with photos.

SOUTH PITTSBURG, Tenn. — Nearly 30 people gathered Wednesday evening outside a cave situated on Nickajack Lake. Some sat in boats with their backs to the cave's opening; others crowded the cliff line with their legs hanging high above the water.

Among them were representatives from the Tennessee Valley Authority and Tennessee Wildlife Resources Agency — both of which operate the cave. They had gathered to count federally endangered bats that call the cave home.

Gray bats are found in 13 states, all in the Southeast or Midwest but mainly in Tennessee, Alabama, Kentucky, Arkansas and Missouri. They are endangered largely because they live in very large numbers in only a few caves, according to the U.S. Fish and Wildlife Service. As a result, they are vulnerable to disturbances by humans.

TWRA and TVA officials have been doing the annual count since the 1980s when they gated the cave to protect the endangered species. They are federally mandated to monitor and protect the bat population, as they are with all the endangered species on their properties.

"As far as counting, monitoring bats is important because they're important to our ecosystem," TWRA wildlife manager Daniel Istvanko said. "It's also a really easy way to assess the population, especially when you have a big population like this in one place."

Some years it's about 80,000, other years it reaches 120,000.

"Gray bats move [caves] from year to year, so it's hard to tell what [count] we're going to get," TVA zoologist Jesse Troxler said.

After the sun set over the lake, the bats began to chirp, preparing to dart through the cave's opening. The groups laid back, staring at the ceiling to see the bats as they flew above. They divided the opening into sections. A handful would count each for a minute.

A ding signified the start. The first minute saw fewer than a handful of bats. The next round, the total was closer to 10. By round three, they were counting by 10s. By round five, it was by 50s — about 2,000 total in one section during one minute as the bats poured out in thick groups.

Between rounds, the team paused for a minute, tallied the numbers and then counted for another minute. They did that over and over for about an hour. At the end, they averaged the numbers, counted the total and doubled it to account for the minutes they were jotting the numbers.

In all, there were 97,026 bats Wednesday night — a good number and close to what the agency usually sees, according to TVA zoologist Liz Hamrick. She had expected around 100,000 before the night started.

The group counting method is archaic but effective, agency scientists said. There are now thermal cameras that count, as well. The agencies use that, too, but the counting method has worked for decades. The counters tally the totals in their head. At the end of the minute, they relayed their count to Troxler. Often, the counts would be shockingly close from person to person, even during the thickets of minutes when counts exceeded 2,000. The infrared findings aren't yet available, but the scientists often find the total number to be within a hundred or so of the manual count.

"It's unbelievable," TVA spokesman Scott Fiedler said.

"It's old, but it still works," Hamrick added.

Contact Mark Pace at or 423-757-6659. Follow him on Twitter @themarkpace and on Facebook @ChattanoogaOutdoorsTFP.