While most normal people spend their Saturday evenings going out to dinner followed by either a movie or a show, I spent this past Saturday doing something entirely different: counting bats!

There is a local cave here in the Tucson area that is a maternity colony for one species of bats. During the summer the females gather here to gossip and then give birth to their babies, after which they move on until next year. Because of a desire to help these bats, a few years ago the managers of the cave decided to close it during the summer months. This way humans wouldn’t be there to disturb the mothers during their labor.

The managers also decided to do regular bat counts of the bats leaving the cave each evening to feed, in order to get an estimate of the population size. To everyone’s delight they found the numbers rising year-to-year, following the summer closure. The total population of bats isn’t actually going up, but it appears that bats are finding this cave to be a good place to give birth, so more and more of them are making it their summer residence.

In the end the situation will contribute to an actual rise in population, as providing the maternity colony a safe haven will allow for more successful births and more babies.

In the past three years the bat count numbers over each summer would exhibit a typical bell curve, going up and then declining as the summer progressed, with the largest numbers ranging between 75 to 150 each night. However, last year there was one evening in which no bats left the cave. The bat biologist leading the bat count, Sandy Wolf, has theorized that this might be because the mothers are synchronizing their labor so that everyone gives birth at the same time and, because of that, on that night no one exits for feeding.. She knows that some species do this, but for this particular species such behavior has never been documented.

Anyway, she decided to find out. This has required that someone be at the entrance counting the bats at least every other evening. (In past years the counts were only done about once a week.) This has required more help, and thus Sandy has called for volunteers to do the work.

And that is how I and fellow caver Jerry Isaman ended up hiking up the hill to the cave with digital camera, infrared lights, and monitor this past Saturday.

The camera and infrared lights were aimed at the entrance, with the monitor set up about thirty feet away, hidden from the entrance. We could watch the monitor and easily see bats entering and leaving, with me counting those going out and Jerry counting those going in. The camera meanwhile taped everything, so that our counts could later be confirmed and checked for accuracy.

I had expected this would be both boring and difficult to do. You start staring at the monitor unceasingly at around sunset, and nothing usually happens for the first fifteen minutes or so. Then the first bat flies out. I think “Whoopie!” as I click my clicker counter.

I found. however, that spotting the bats was far easier than expected, and that the work was not really boring at all. Once the first bat left things got busy for about a half hour, with bats flying out about 2 to 4 per minute. Then things quieted down, and after 90 minutes, when nothing flew out for 10 full minutes, we shut down and packed up.

The most challenging part of the count was when a bat decided to go back into the cave. It appeared that many of them had to plan their entrance and couldn’t do it on the first attempt. They would circle around several times, dive toward the entrance and then change their mind the last second, repeatedly faking Jerry out, who was about to count them for re-entering the cave and now could not. I meanwhile had to make sure I didn’t count them as exiting the cave.

The coolest thing about Saturday was the count itself: We counted 218 bats leaving, the highest count of the year. Though we were not there on the night no bats came out, confirming Sandy’s theory, our count once again indicated that the population using the cave is increasing.

I hope to do this again later this summer. It might not be the traditional way to spend an evening, with dinner and a movie, but it certainly is more interesting.