Bat Flight from Rose Guano Cave

One evening this past week I had the opportunity to go to Rose Guano Cave in eastern Nevada to watch the bat flight. This cave is an important migratory stop for Brazilian free-tailed bats (also called Mexican free-tailed bats; Tadarida brasiliensis), with over a million using it each year. They usually stay for one to four nights, heading out to feed on insects that are especially prevalent over the nearby agricultural fields.

From the highway, the cave can be seen, near the base of the cliffs (left-center of photo above).

The road towards the cave is rough and definitely requires high clearance and four wheel drive. We parked next to a trailer that is being used by graduate students to study the bats (more on that later), and then hiked the old road towards the cave.

The late afternoon sunlight was superb against the limestone. The first attraction we noticed was a huge limestone arch. Then we could see the gaping mouth of the cave. The old road ended at the base of a tailings pile. This was from an adit built in the 1920s to mine guano out of the cave. The guano is rich in phosphates and nitrates and was used to make gunpowder. Because the adit upset the natural airflow in the cave, it was sealed in the 1990s.

Inside the cave mouth it says "Positively No Trespassing. Rose Guano Mining Claim."

It's steep to get up to the cave, and a rope was installed as a handline to make it a bit easier. We could smell that guano as we got closer to the cave.

On the way we passed a thermal-imaging camera. This was installed earlier this year to record the bat flights so more accurate counts can be done. The bats in the cave have received a lot of attention in recent years due to nearby wind farm proposals.

A sign outside the cave entrance provides more information about the bats (click on the photo to enlarge it).

Below the cave entrance, sitting in a chair with a camera by his side was Peter, one of two field technicians helping two graduate students learn more about the bats. The two graduate students had spent the previous summer counting the bats every night, and they liked it so much that they decided to do further studies and return again. They work closely with the Nevada Department of Wildlife and the Bureau of Land Management to not only count the bats, but also find out where they go. One hundred transmitters were attached to bats over the past couple of years to track where they go and how long they stay in the area.

Even though there is a camera that records the entire bat flight now, they are continuing to count the bats nightly to compare the past method counts to the new counts.

We climbed up to a rock next to the cave entrance. From there we could see that the cave entrance had some special lights in it (that we couldn't see when they were on) to help the camera images.

Jason, an NDOW wildlife biologist, came out to explain to our group more about bat biology and their use of the cave. As he was talking, we saw the first bat come out of the cave--and then turn around and head back in, presumably to tell the rest that it was time to start heading out.

Then more bats started coming out. They looked a little like a stream, flowing by quickly. Then the number of bats increased, and instead of flying straight out, some swirled a bit--the stream was bigger and had some whitewater.

As it got darker, we found that our vantage point was a bit high because the bats blended in with the rock behind them. So we moved down next to Peter and saw the bats silhouetted against the sky.

Jason pulled out a camera and showed us a video of the bats from inside the cave. They come from a deeper chamber in the cave and swirled around twice to gain elevation before they flew out of the cave. It looked really neat.

He also had a thermal-imaging camera with him. Using that, I thought the bats looked like fish swimming in a fast current in the ocean. Jason estimated that about 2,000 bats per minute were flying out of the cave.

By 8:20 p.m., it was too dark to see the bats with the naked eye. Jason said they would continue until about 11:00 p.m. The bats primarily use the cave from July into October.

I couldn't get a photo of the bats flying out, but you might be able to see them in the video below. They are really amazing animals, and with such strange life histories. I can't wait to learn more about them.