To the bat cave!

I like the winter garden just fine, but among the things I miss from the summer garden are my bats.

They are not technically “my” bats, but rather a small colony that roosts in the eaves of my home in the non-winter months. The colony has grown over the past three years from just a few shadowy, smaller-than-fist-sized, dark-brown creatures to perhaps 10 or 12 that return each year.

Each sunset in spring, summer and fall, the bats unfold themselves out of a crack less than a quarter of an inch wide and, apparition-like, disappear into the gloaming over our garden and into the countryside beyond. Each morning in the dim before sunrise, they return and put themselves carefully back to bed in a seemingly choreographed flight pattern, circling beneath the deep eaves of our back porch. Each bat takes its turn, swiftly but carefully folding itself back up into the eave to rest for the day. Their daily patterns are mesmerizing to me and my family.

Northern California is relatively rich with bats. Seventeen species are known to live in our region, such as the Mexican free-tailed bat and the Pallid bat. Bats primarily navigate by echolocation—a biologically based built-in sonar. They are not actually blind. They generally see as well as humans, although in black and white. While some species migrate in winter, several types of Northern California bat remain in the area in secure winter roosts with good protection from cold weather.

After attending a ranger program on bats at Craters of the Moon National Monument and Preserve in Idaho this past summer and learning of a fungal disease known as white-nose syndrome affecting bat populations in much of the United States, I was determined to find out more about what I could do to help the bats who so vigilantly patrol my garden for bugs much of the year.

Among other resources, I talked at length with regional bat researcher Ray Miller, of Mount Shasta.

I asked Miller—a retired career Navy man—what kind of bats I have, after describing the smallish, brown-winged animals that live in my eaves.

“You have just described most of the 17 species that live in Northern California,” Miller answered, “but if they don’t have a strong smell, they are probably not the Mexican free-tailed bat, which stink. The largest bat in your area is the Western Mastiff bat, but they are very uncommon and generally cliff-dwellers, so they are not likely.”

I asked whether I should I worry—in terms of human health or safety—about having bats living in the eaves of my house. (Some people choose to erect bat houses for their bats to live in.) He said the diseases bats carry are few, and the parasites—such as mites—they carry on them are fairly specific to bats, so there is not much to worry about.

“Cold is a problem for bats,” Miller offered, “and when I capture bats for ongoing studies and to collect data, I frequently put them inside my coat against my skin to keep them warm. In 25 years, I have never been bitten by a bat’s fleas or mites.” He has been bitten by the bats themselves on occasion, but is protected by a rabies vaccination.

Miller asked if my bats are bothersome. “Are they damaging the structure of the house or do they have a strong smell?” he asked.

My answer was no. My small colony roosts in an exterior knee wall of a second-story deck, so they are never in our house’s internal structure near wires or air vents. Their guano (feces) does drop on the ground beneath the entrance to their roost, but I have solved the mess of this issue by placing a wide container in the spot that I empty every now and then into compost bins.

As for white-nose syndrome, Miller said that “the catastrophic fungal white-nose disease is not in our region’s bat community. Research is ongoing, but from what has already been learned, the spread of this disease to our area is unlikely due to environmental and bat behavior factors.”

As far as what I can do to help them, Miller had several suggestions. He said if my bats seemed happy and healthy, leave them alone and just enjoy their many benefits, such as feeding on insects.

“A healthy adult bat will eat around its own body weight in bugs each night,” Miller said, “the equivalent of us eating 150 hamburgers a night.”

He said one of the best things one can do for the health of bats is reduce or eliminate pesticides from one’s home and garden.

“Pesticides will accumulate in the insect populations and begin to harm the bat populations,” he said, “which in turn will create a much larger insect problem in our region.”

Finally, he suggested that if a person wants to encourage bats into the garden with a bat house—as is becoming increasingly common—a few things should be kept in mind.

“[T]emperature is critical for them,” he said. “Their average body temperature is about 107 degrees, so they need it nice and warm.”

He pointed out that most commercial bat houses are fairly thin and the temperature inside of them fluctuates rapidly. If you are going to install a bat house, the best ones are those that are tall and have an air vent half way up to help the air circulate and allow for the bats to adjust themselves up and down inside the house to find a warm spot.

“The bats come home in the morning, cold and tired from the night’s feeding,” he said, so it is important to orient a bat house against a flat surface, such as the side of a building, facing southeast so the morning sun will provide needed warmth.

“Bats will rarely roost in a bat house placed in a tree,” he warned, “because the temperature is too variable and predators such as cats and snakes are able to access the house.”

It isn’t necessary to clean a bat house, Miller added, as most of the guano falls out the bottom, and, anyway, “a little poop and pee in the house makes it smell more like home.”

Jewell hosts Northstate Public Radio’s weekly gardening program In a Northstate Garden.