A swarm of cave swallows churns the air in random circles outside the mouth of their namesake. We stand with a dozen other spectators and peer into the dark opening of the cave that extends a half mile into the limestone hill. Anticipation fills the small crowd as the sun approaches the western horizon of the Texas Hill Country west of San Antonio.

This day marks the end of a grueling turbine overhaul season followed by a very full five-day combustion turbine conference. The last two months have left me quite exhausted. My wife, Renee, joined me in San Antonio for the conference, but I hardly ever saw her. Now, she takes me on a small expedition to explore a natural wonder before we return to Colorado. As we drive through the rolling hills, I feel the stress begin to dissipate. This is the medication I need.

Within the dark recess of the cave, the swirling swallows fool me into thinking they are flying mammals and I blurt out, “I think the bats are getting ready.” A couple of the onlookers agree with me at first, but a man who has been here before corrects me. “Naw, they may look like bats, but those are the swallows.” Sir John Lubbock wrote, “What we see depends mainly on what we look for.” I think Lubbock had a broader context in mind, but what he had written in the nineteenth century described my situational perception on this evening of May 19, 2017.

We are on private property. We had been escorted up the mile-long dirt road on the ranch up to the cave site by a guide dressed in olive green safari clothing and a binocular strap draped around his neck. There is still an hour before the expected time of emergence, and he opens it up for questions and supplies some answers…

  • “The bats in this cave are called Mexican (or Brazilian) Free-tailed bats.”
  • “The population of this bat colony has been estimated to be 10-12 million, but the last couple years have had very good conditions and the population is probably well over 15 million by now.”
  • “They usually emerge a half hour before sunset, but sometimes it is much later.”
  • “The young bats experience a 50% mortality rate during their first year.”

He also supplies interesting information we hadn’t even considered…

  • “The bats pack themselves in there at a density of 200 bats per square foot.”
  • “Deep within the cave, the atmosphere is so toxic that scientists need to wear full Haz-Mat personal protective equipment in order to survive. Due to the large amounts of guano and urine, the ammonia levels are over 2000 parts per million.”
  • “In some places, the guano is ten feet deep.”
  • “Ever since the civil war (and probably before), the guano was mined from this cave and dried in that oven over there.” He points to a two-foot diameter, fifteen-foot long pipe with stones mortared onto the outside. “The dried guano, which contains Potassium Nitrate, was one of the ingredients of gunpowder.”
  • “The bats seem to be late this evening. Sometimes they are early. Sometimes they are late. You can never predict. They follow their own schedule.”
  • “These bats mostly eat moths and provide a huge benefit to farmers. They fan out over a thirty-five mile radius in search of food. They eat over 200,000 pounds of destructive moths every night which also means they drop a large amount of fertilizer.”

An article called, “The Economic Importance of Bats in Agriculture” (Science  01 Apr 2011: Vol. 332, Issue 6025, pp. 41-42) estimates that bats provide between 3.7 and 22.9 billion dollars each year in chemical-free pest control services in North America alone. That seems weird to me, as if for some reason agriculture is not supposed to work in harmony with nature; as if spraying our food supply with toxic chemicals is supposed to be our default course of action. And must we always provide economic justification for the existence or preservation of a species? Must their value be measured by their usefulness to short-term human needs, our entertainment, financial benefits, or how they contribute to the overall economy?

A week before, we had watched a smaller colony of 1.5 million Free-tails vacate the underside of the Congress Street Bridge in downtown Austin. A beer festival was in full swing on the adjacent lot, and the crowds were huge as the bats took flight. Also, there is Braken Cave just north of San Antonio that is said to house the largest colony of Free-tailed bats… over 20 million. This is also said to be the largest known concentration of mammals on earth. I guess everything’s bigger in Texas. In order to experience the bat emergence at Braken, one needs to buy a membership with Bat Conservation International which comes with various tour privileges depending on the level of membership.

At the Rio Frio cave, the surrounding landscape has gentle hills covered with a diversity of inhospitable brushes, prickly-pear cactus, and Ashe juniper (commonly called cedar). This vegetation is a product of generations of overgrazing. A lone Red-tailed hawk rides the thermals. Two male Mockingbirds compete for a female. A possum scurries across the dirt road running for cover within the bushes. A great-horned owl makes a surprise exit from the cave and its large flying mass startles those of us who are watching. Another two or three vehicles arrive. This group includes some kids, with three teen-aged girls who talk and giggle incessantly, seemingly more interested in some boy at their school than bat biology.

As the western sky begins to cast an orange tint, I hear someone call out, “Here they come!” Sure enough, a stream of erratic little black flyers spews from the mouth of the cave. The bats fly in a swirling column that is twenty to forty feet in diameter and as low as ten to twenty feet off the ground. The frantically flying procession heads over a small rise then down into the valley below. It reminds me of a dark, smoky plume on a windy day. I did not see a witch flying among them on her broom.

Bats 2

The Red-tailed Hawk had apparently been here before and was patiently waiting for their emergence. He soared above the black trail of bats then makes a graceful dive into the middle of the flowing stream of fresh bat meat. A few minutes later, I see the raptor go in for a second helping.

The scene is mesmerizing, enthralling, and somewhat surreal. The colony is a community, almost acting as a single complex organism. We try to capture the experience with iPhone videos, but as I watch the footage after returning home, it hardly compares to the sensation of being here where my corporal senses were fully engaged. In particular, the smell of guano still lingers in my mind (or is it that I haven’t washed my shirt yet?).

This winged parade continued for nearly an hour until it got too dark to see much. When we left, they were still coming and streaming out toward the dark eastern horizon with no sign of letting up. Fifteen million is an inconceivably large number.

Some of us can handle large amounts of stress for short periods of time. That is the gauntlet I pass through when I oversee turbine overhauls each spring and fall. At the annual conference in San Antonio, I was daily submerged into a large crowd of fellow engineers. This type of interaction with colleagues can be invigorating and energizing for some, but although it was enjoyable and rewarding, I am a “closet introvert,” and in the end, this week had been very draining for me.

Many of us seek relief and recovery by emerging into the natural world. The grand urban social experiment implemented in the last couple centuries has not been working out so well for the human race. There are those who say we have not yet evolved enough as a species to thrive within An Alien World of concrete canyons lined with glass windows. “It’s only a matter of time”… geologic time. In the meantime, however, to keep our sanity, we must regularly immerse our five senses and synchronize our souls with the pulse of nature in the environment God designed us to operate within. I need to at least imagine a world unmarred by human folly. Anyway, this strategy seems to work for some of us.

Sitting beneath a tree along the bank of a flowing river will usually do it for me. At Rio Frio, I discovered that standing beneath a flowing plume of flying mammals at the mouth of their cave will also calm my nerves and put my life back into perspective. Indeed, for me, the right medicine today was the bats of Rio Frio.


Click these links for a couple videos that are a meager attempt to capture the experience: https://youtu.be/vV_EsSIq-ZI, and https://youtu.be/hQhYI4-NV6k



4 thoughts on “The Bats of Rio Frio

  1. I love the poetry of the line “synchronize our souls with the pulse of nature.” I can picture the bats doing that, as well as the humans watching them. Thanks for an insightful post.


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