May 04,2007 00:00
Q: We heard a little bit about bats and thought we might want to attract them to our lake house so they will eat the mosquitoes in the yard. However, we do not want them in our house. What can we do?
A: Do you remember the movie "Field of Dreams"? A voice in a cornfield tells Kevin Costner's character, "If you build it, they will come." Well, that was a movie. In real life, they might not come. But, if you don't build it, they can't come.
In some parts of the world, bats are the pollinators of many flowers. Hundreds of commercial products come from bat-dependent plants, including bananas, figs, dates, mangoes and avocados. Many savanna and rain forest trees use bats to spread their seeds. Worldwide, about one-quarter of all mammal species are bats. There are about 1,000 total species of bats. They are the only flying mammals. All North American bats are insect eaters.
Bats may have a bad reputation with some people, but are benign to have around. Panicking at the sight of a bat is more likely to cause harm than will the bat.
Bats are not blind and do not become tangled in people's hair. Many use sonar, just like dolphins.
Bats do not get rabies any more so than other animals. It is just in the news more often when they do.
Many more people get sick from mosquito-borne diseases than from bat-borne diseases. Because bats eat mosquitoes, it is a net benefit to have bats.
Finally, bats are no more likely to nest in your attic if you put up a house for them. But they might move out of the attic if the bat house is more suitable.
If there are bats in your attic, wait until the young can fly and watch where they leave the house. Tape a piece of netting over the entrance with an opening at the bottom. They will crawl out and leave through the bottom opening. When they return they can't get in through the net and won't remember the gap at the bottom.
In the winter, most bats in North America migrate south to warm areas or to caves to hibernate. Some may stay and hibernate in bat houses.
There are several species of bats native to North America. Here is some information about common North American bats:
The little brown bat is a very common bat. It is 1 inch to 2 inches long from head to tail and has a wingspan of about 8 to 10 inches. The little brown bat eats as many as 600 mosquitoes per hour. It will take up residence in a bat house in the summer and migrates south to caves and mines for the winter. It is heat-tolerant and can have a body temperature of 129 degrees.
The big brown bat is about twice the size of the little brown bat, about 4 to 5 inches long from head to tail with a wingspan of about 14 inches. It seems to prefer beetles, whose larvae are the grubs in our lawns. It will take up residence in a bat house and sometimes will share the house with little brown bats.
The evening bat, or vesper bat, commonly takes up residence in bat houses and is between the previous two bats in size. Little is known about where the evening bat migrates in winter. In summer, it is commonly found in urban areas eating night-flying insects.
Another bat that comes to houses is the Eastern Pipistrelle. It is about the same size as the other bats, but it has fur that is three colors of yellowish brown. It migrates to caves for the winter.
Bats are becoming less common. Scientists don't know why. One problem is the loss of roosting areas. Nest boxes can help replace that need.
It is important for bats to have safe places to raise their young, because a female bat can only raise only one pup per year, unlike most animals and birds.
Much is yet to be learned about what each bat species requires for a nest site, but the current information is useful. Of the bat houses in one study, some have not been occupied in more than five years. The overall total was 52 percent occupancy.
If the bat house was within one-quarter mile of a stream of any size or a still body of water over 3 acres in size and the house received more than four hours of direct sunlight, the occupancy rate was 83 percent. The more sunlight the house receives the faster it is occupied.
Research shows that the best shape for a house is a wide, flat front exposed to the sun for warmth. The house sticks out from the wall only a few inches, but is about 2 feet tall and almost as wide.
Thicker houses with more than one chamber get more bats faster than single-chamber houses. A 3-inch to 6-inch landing area on the bottom of the house and front ventilation that prevents overheating also help attract bats more quickly.
Bat houses that maintain daytime temperatures in the 80s and 90s had more bats than those that were cooler. Those that were painted dark colors were more successful, and a south or west exposure was best.
More than one crevice (less than 1 inch thick) or a built-in attic space allow for a range of temperatures in one box and give the bats the ability to move around inside and find the temperature that suits them.
The house should be a minimum of 12 feet above the ground, the higher the better. Those mounted on buildings attract bats faster than those in trees, probably because they get more direct sunlight.
Bats do not leave much in the way of droppings near the house, but do not install a bat house over a patio. Painting the house with bat guano doesn't increase bat usage, but aging the wood by filling it with dirt for a few days does seem to help.
After each summer that a house goes unoccupied it should be moved to a new location. Putting up more than one house in more locations helps in determining what the bats need from you.
You can get more information on bats from the Organization for Bat Conservation at www.batconservation.org.
E-mail questions to Jeff Rugg at firstname.lastname@example.org.
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