A Deeper Look at Colorado’s Colony of 250,000 Bats

This photo shows how bats tend to group together while exiting their living space in a nightly, unified event. Photo Credit: Umkehrer (iStock)

You can smell the Orient Mine a few steps before you turn the last corner on the rocky trail and see the 400-foot deep pit.

A slight wind drifts from the cavern in the side of the Sangre de Cristo Mountains like cold, damp breath.

As you get closer, the air grows as thick, sharp and musty as an old shed long used by stray cats. It’s the smell of tons and tons of guano dropped by the 250,000 Mexican free-tailed bats that swirl from the cave each dusk like a cyclone and disappear into the night.

A Deeper Look at Colorado’s Colony of 250,000 Bats

Mexican Free-Tailed Bat. Photo Credit: USFWS Headquarters (Flickr).

The bats, which are, for unknown reasons, almost all male, spend the summer in this rural roost, feasting on agricultural pests in the patchwork of grain fields that make up the San Luis Valley. This is where much of Colorado’s barley for beer is grown, and the hungry bats probably help keep the grain healthy.

In mid-September, the bats return to unknown haunts in Mexico. But for the next few weeks, anyone walking the trail can witness one of nature’s most spectacular displays.

“It’s far and away the largest colony in the state. In any other case, if we find a thousand bats, we’re thrilled,” Kirk Navo, the Colorado Division of Wildlife’s bat expert, said in a whisper on a recent evening as he waited for the bats to emerge.

It was 8:15 p.m., and he was standing at the mouth of the mine, keeping his voice low, so as not to disturb the resting residents. Colorado has 19 species of bats, Navo said, and they’ll roost anywhere from bell towers and bushes to specially made bat boxes that mount on the sides of buildings.

But the estimated 23,000 abandoned mine shafts that riddle the Rockies make particularly good hideaways. Open mines, with their crumbling ceilings and vertical shafts, can be deadly to people; 18 people are known to have died in abandoned-mine incidents in Colorado since 1955. So for years, the Bureau of Reclamation and Mining Safety actively closed abandoned mines, usually by caving in the entrances with dynamite.

“That was really bad for the bats because they either got trapped inside or lost their habitat,” said Navo.

So, in 1990, Navo and others started a program to close the mines to people but keep them open to bats. In an effort that continues today, they surveyed thousands of mines. If they found evidence of bats, or even a suitable habitat, the mine shafts were kept open, but a special grate was put over the mouth to let bats come and go while keeping out people. To date, 800 grates have been installed to cover 500 mines.

Mysterious Colony

The Orient Mine is a different story. Its entrance is too big to cover.

The now-abandoned football field-size hole was an iron mine owned by CF&I Steel in Pueblo. The miners carved great hollow rooms called stopes as they followed the ore. One day in 1893, the roof of the highest stope collapsed, leaving a jagged, rust-colored wound in the mountainside.

Miners continued to work the mountain until 1938, when the ore ran out and the mine shut down. Sometime later — no one knows quite when — the bats discovered the mine.

The first report of a large outflight, as the nightly commute is called, came in the early 1960s. A real study of the cave didn’t take place until the late 1970s, Navo said. That’s when scientists came up with the 250,000 count by timing the outflight to estimate the population.

“It’s a true estimate, though,” Navo said. “There’s no easy way to get a better number.”

Much knowledge about the bats is a rough guess. Where do they feed? Somewhere in the valley, probably. Where do they go in the winter? Mexico, but no one is sure exactly where. How did the bats find the Orient Mine and spread the word of the cushy roost to 250,000 friends?

“We don’t know,” Navo said. “These guys are small. They’re fast. They’re out only at night, covering at least 50 miles. They’re very hard to study.”

Trying to count them during the day, while they’re sleeping on the roof of the mine, would mean a long rappel down into a waist-deep marsh of bat litter cloaked in a cloud of ammonia.

“I’ve been down there. It’s not very easy or nice,” said Neil Seitz, director of the Orient Land Trust, which owns the mine.

He was waiting for the bats with Navo.

“One thing we do know,” Navo said, “is that this colony is almost entirely males. Most of the other big free-tailed colonies (in Texas and New Mexico) are all females raising young.”

Why is the Orient a giant bachelor pad? Again, Navo shrugged. He scanned the darkening sky. A line of thunderheads dragged gray veils across the distant valley floor. Lightning flickered. “We think this may be some kind of migration hub,” he said. “A place where bats stop over to rest before they . . .”

He never finished his thought because suddenly a rush of bats swirled out of the mine’s jagged maw — first just a few, then thousands flapping just overhead in a bustle so furious it created its own wind. The sound was like the rush of distant whitewater, pierced by the high chirps of crowds of bats heading out to start their night. (Their hunting calls are much too high for humans to hear, but some social calls are audible.)

It was 8:18 p.m. Right on time. The group flew in tight formation, meandering toward the valley like a river bound by invisible banks. About 25 people had made the 1-mile trek up a rocky path to see the outflight.

“They look like they’re computer generated,” said Alec Albright, 17, as he looked up, slack-jawed. “Put a moth on your head and see what happens,” said his father, Jacob.

The torrent of black wings kept going: five minutes. Ten minutes. Twenty minutes without a break. At 30 minutes, the rush slowed to a steady stream. Most of the human visitors groped down the trail in the last hints of daylight.

Friends to Farmers

The bats stay out all night, hunting insects, then gradually stagger back in.

“It’s like any group of males coming back from an all-nighter,” said Seitz, just before he turned to walk down the trail. “Some come home at midnight, some at 2, some at 4, some just before dawn.”

They fly at up to 60 mph. Some cover 100 miles in a single night of foraging, resting for long periods of time between frenzied feeding bouts. Free-tailed bats may eat more than half their body weight in insects each night, making the nightly total for the bats from the Orient Mine about three tons of bugs.

No one has studied what effect the squadrons of hunters have on agriculture in the San Luis Valley, Navo said, but an ongoing study in Texas suggests that a healthy bat population saves local farmers millions of dollars.

Using radar to track bats, and DNA analysis to determine what they’ve been eating, a team of scientists found that free-tailed bats living in the area around San Antonio actively track and feed on outbreaks of crop pests. One of the biggest banes of farmers in Texas is the corn earworm, which feeds on corn as a larva, then becomes a moth and flies off to lay eggs in nearby cotton fields.

“The bats are migrating north into the area in June — just as the moths are emerging — and hit them hard. Then, when the next generation emerges from the cotton, they hit them hard again,” said Gary McCracken, a biologist at the University of Tennessee who is part of the Texas study.

Scientists have used radar to see thousands of bats zeroing in on moth swarms migrating in prevailing winds up to a kilometer above the ground. Preliminary findings indicate that bats in one eight-county region save farmers $1.7 million a year in lost crops and pesticide costs. But, McCracken said, that’s only the mouth of a much bigger cave.

“These guys are extremely beneficial on a broad scale. The bats and the moths both migrate,” he said. “Basically, the bats at the border are eating the granddaddies of moths that would otherwise eventually end up eating corn in Nebraska. What the bats are doing is protecting the crops all the way up through the breadbasket.”

In the San Luis Valley, bats probably play a similar role controlling worms in wheat, corn and barley fields. Right now, the Orient Mine is the northernmost large colony of free-tailed bats. But, Mc-Cracken said, if global climate change warms the region, as many think it will, the bats will spread throughout Colorado.

Dwindling Population

For all the work Mexican free-tailed bats do for people, people haven’t done much in return.

Though getting precise counts of bat populations is next to impossible, evidence shows a vast decline in bat numbers over the past 100 years. At one time people deliberately destroyed cave openings because they feared bats, said Barbara French, conservation information specialist for Bat Conservation International. Other cave roosts have been damaged by tourists and people mining nitrogen-rich bat droppings. Pesticides, especially DDT, now banned in the United States, may have dealt the biggest blow, killing baby bats and their parents alike.

The famed bats at Carlsbad Caverns in New Mexico, estimated to number 8 million in the 1930s, now number only about 250,000. Other former roosts now have no bats at all.

To protect the bats, groups such as Bat Conservation International are working to close off caves in Mexico and the United States, and limit pesticides. McCracken said conservation efforts may aid the bats’ recovery.

“We’ve made a lot of efforts to protect habitat because we know if you protect the habitat, the bats will come back.”

The bats of Colorado’s Orient Mine have been lucky. The population apparently has been steady for 40 years. The mine has remained undisturbed, and the agricultural and ranching practices in the San Luis Valley seem to suit the bats just fine.

The director of the Orient Land Trust hopes to do more. Neil Seitz has lived in the valley since 1974. For most of that time, he and his wife have run the Valley View Hot Springs, a clothing-optional hippie hideaway where native fireflies float over naturally warm pools.

In 2001, they created the Orient Land Trust. The goal was to preserve the 2,100 acres around the mine and the springs as undeveloped land. But now it’s expanding. The Land Trust hopes to serve as a catalyst to get other large-holding farmers and ranchers in the region to preserve their open spaces.

“We’re just getting started, but the hope is that we can convince others to keep this valley like it has been for so long,” Seitz said.

The bats would benefit from the farmers’ actions, just as the farmers, whether they realize it or not, benefit from the bats.


An estimated 250,000 bats leave the Orient Mine every night at dusk from mid-June through mid-September. The mine can be reached by a 1-mile hike.

To get there: Drive west on U.S. 24 to U.S. 285. Go south over Poncha Pass to the tiny town of Villa Grove. Continue four miles and turn left on County Road GG. Drive seven miles to the end of the road.

Rules: Visiting the mine is free, but visitors are required to check in at the land trust office. Donations are suggested. For more information: Visit  www.olt.org

Mexican Free-Tailed Bat (Tadarida brasiliensis)

Weight: .45 ounces, about as much as two quarters

Wingspan: 12 inches

Top speed: About 60 mph

Nightly flight range: Up to 100 miles

Lifespan: 10-15 years

Food: Primarily small moths

Migratory range: Colorado in the north to Chile in the south.

Largest colony: Bracken Cave, north of San Antonio, with nearly 20 million bats.

Reproduction: Females give birth to a single “pup” in June, which can weigh a quarter of the mother’s body weight.

*Editor’s Note: This article was originally published by The Gazette in August 2007.



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