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Fall 2010

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

October 26, 2010

Bats: Halloween’s angels

While many other bat species have gone into hibernation, robust big brown bats may still be out hunting for treats on Halloween night

Recovering from an age-old image problem of being miscast in the company of malevolent witches, monsters and ghouls, bats are in fact do-gooders.

"Bats are such an important part of agricultural pest control," says John Griffin, who helps resolve wildlife conflict issues in the Washington area for Humane Wildlife Services, an arm of the Humane Society of the United States. On top of that, Griffin says, "bats reduce our exposure to mosquitoes carrying blood-borne diseases, such as West Nile virus."

Of course, people don't like finding bats roosting in their attics. "If they are not in a living space, they aren't really a problem," Griffin says.

Large colonies and long-term occupation can cause a buildup of guano, which in some environments promotes the growth of a fungus that can cause a lung disease. Attics tend to be a poor environment for the fungus.

Although rabies in bats is rare, people are cautioned to never handle a bat.

Big brown bats and little brown bats are the most common species found in house attics. Little browns tend to migrate to caves, where they hibernate in constant, cool temperatures. But the hardier big browns are able to withstand freezing temperatures and often stay put in buildings, sometimes emerging from their torpor for a drink of water during winter warm spells.

Homeowners who want bats out of their attics can hire Humane Wildlife Services to locate any spots where bats may be entering and install check valves, netting that allows bats to leave but prevents them from reentering. After all bats have departed, HWS will seal any openings, bat-proofing the house. If a homeowner still wants the mosquito-devouring mammals around -- just not in their attic -- HWS will put up an outdoor bat house for an additional fee.

Griffin doesn't recommend installation of check valves between March and September. That's when dependent young bats might be inside, waiting for their mothers to return.

Sources: Humane Society, Bat Conservation International

Big Brown Bat

White nose syndrome is a disease that is decimating cave-hibernating bat populations in the Northeast. First described in New York four years ago, the disease has rapidly spread as far north as Ontario and as far west as Oklahoma. Of the country's 25 species of hibernating bats, six have been affected by the disease (including big brown and little brown bats) and another three species have been detected with the fungus, which covers the nose of affected bats.

Diseased bats show compromised immune systems, damaged wings and abnormal behavior during hibernation -- emerging from their caves too early, depleting their fat reserves and either freezing or starving to death.