Yes, bats are mammals—not a subspecies of birds. They are the only mammal capable of flight (flying squirrels only glide). They are the second largest mammal group in the world, roughly 950 species. Only rodents have more species. Bats generally have one offspring per year. They live up to 30 years. Bats migrate, showing up in the spring and leaving by fall. They hibernate in caves or travel to Mexico or Central America for the winter.
All fourteen Northern California species are insect eaters, flying insects and some ground insects. Yes, some insects, such as Potatoe Bugs, Stink Bugs and Centipedes are favorites of Pallid bats, which catch them on the ground, and can hear them crawling from ten feet away! Bats also feed on moths, cucumber beetles, and leafhoppers.
The majority of local bats are flying insect eaters. Probably, the most common item eaten by weight, is moths. The Codling Moth, in season, is a favorite. Mosquitoes can be consumed at upwards of 600 per hour. In essence, bats are nighttime swallows; they take over when the birds go to roost. To be practical and realistic about having bats as allies, an average bat house colony of 200 bats is capable of consuming 5 lbs. of insects in a night. This becomes quite impressive when you consider the number of insects it takes to make a pound, and that bats do this every night.
To cover one misconception about bats, they are not blind. They do have good eyesight, but the key to their insect gathering ability is personal sonar. Their vocal squeaks, mostly above the human hearing range, bounce off an insect and are picked up by their super sensitive large ears. Bats then home in on the insect to capture it. Using the skin between their legs as a basket, bats scoop up insects, do a belly tuck, and then lower their heads to eat.
To put these valuable mammals on your payroll, bat houses and roost enhancement are the two routs to go. It is illegal to capture bats to put in a bat house, so cross this approach off your list. Also, it would be like capturing a homing pigeon; the first time you release it, it is back to it’s original roost.
Basically a bat house is two boards 3/4-1 1/2" apart. The most successful bat houses usually have four cavities, are 20-24" wide by 24" high, with a 6" lower landing pad, and are 5-6" deep, depending on wood thickness and slot width. Field mounting is usually on a 4x4" wood pole, 16' long, put in a hole 31/2' deep. This elevation gives a bat room to gain flying speed when it leaves the bottom of the box, which, incidentally, is open on the bottom, and closed with a rain proof top. Location is critical, but it also can be open to debate and trial and error.
Temperature is the biggest factor. To protect the hairless babies (pups), 75-95F° is ideal. Hitting this range is the problem. Basically Napa Valley is somewhat of a mid range, hotter than the coast, cooler than the inner valley. In Sacramento Valley, the most successful location is on the north side of a barn or tree shaded area, but in Napa Valley it is probably on the east side, where it can get morning sun, but little noon or afternoon sun. Two unsuccessful years in one location is reason to review your choice. The best advice is to contact Bat Conservation International in Austin, TX. 78716 PO Box 162603, Phone 512-327-9721. To order their Bat House Builders Handbook, call 1-800-538-2287. This book has the best available information for building and locating a bat house. They also have bat books for general information to broaden your bat knowledge.
The other approach to bat housing is roost enhancement. This is a varied problem, depending on the location of an existing roost in an attic, loft, inner wall, etc. inside or outside of a building. Also, there are other considerations such as smell, guano accumulation, human contact etc. Some of you could very well take this route since there are many old and sometimes loose buildings in the valley, housing bats.
The appeal of this route is that of hitting a jackpot if all the conditions are right. A large colony could be established in an old unused building if someone were to dedicate the time and effort to the project. The ultimate jackpot is to have a big enough colony where the public could view it, while sipping a little wine in the evening on your open-air veranda. The 1.5 million bats in the Congress Avenue Bridge in Austin Texas, generate eight million tourist dollars from bat enthusiasts having come to watch the evening flight.
I’m not in this crazy business professionally, since I am retired, but if you are seriously contemplating a project and have questions I can be reached through Astrid C. Bock-Foster at the Napa County Resource Conservation District’s office at 707-252-4188 or nswg@naparcd.org, or Rachael Long at the Farm Advisor office in Yolo County at 530-666-8734 or rflong@ucdavis.edu