For the last few years the building I live in has been home to a large colony of bats. Hopefully readers will find my experiences of removing the bats and my observations of bat behavior helpful and educational. For brevity I have omitted many events and details.
I live in a multifamily condominium in the northwestern part of South Carolina. My unit consists of two floors at one end of the building. For two summers I heard noises in my outer wall. I assumed the noises were made by birds, since birds getting into the attic has been a recurring problem here. One evening in late August 2006, neighbors noticed bats emerging from under the vinyl flashing at the roof's peak. I went out and watched. There were hundreds of bats. Neighbors contacted the company that manages the condominiums. The management company hired a local pest-control company that wanted to hang bags of BAT-A-Way (i.e. naphthalene, moth balls) in the attic.
I am sensitive to moth balls. Whenever I am exposed to moth balls I get a bad headache, my eyes burn, and my stomach feels queasy. I would not allow the pest-control company to put naphthalene in the attic. I researched naphthalene and discovered it not only causes headaches and nausea, but it can also damage the liver and kidneys, and may cause cataracts. Here are two material safety data sheets on Naphthalene: MSDS 1, MSDS 2. Many Internet sites sell naphthalene bat repellent, however they list the states to which they can not legally ship the product, which seems to confirm the hazardous nature of the product. I found several sites that say bat repellents do not work. One such site, Bat Repellent - Does It Work? says the following, "There is no such thing as an effective bat repellent. Period.... The internet is full of bat deterrent products, but none of them work.... I've seen people dump OVER 50 POUNDS of mothballs in an attic where bats are living, and they don't care in the slightest. They keep using the area."
There is an effective and humane way to get rid of bats (sometimes called "bat exclusion"). The beauty and cleverness of getting rid of bats through exclusion is that the bats leave on their own. There's no attempt at bat removal. You make it so bats can't get back in after they have left. The next time the bats go out to feed on insects they can't get back in.
Exclusions should not be attempted if there are baby bats in the building. Like all baby mammals, baby bats are nursed by their mothers until they can fly to hunt for insects on their own. In this part of the United States the nursery period seems to be late May to late August. You should wait until all the babies can fly before excluding the bats.
Bat Exclusion Steps
1. Discover all the openings bats can use to get in and out of the building.
2. Seal all the openings except for the most-used opening.
3. Make it so bats can exit the most-used opening, but can't get back in.
4. Wait until all the bats have left.
5. Permanently seal the last opening.
Hundreds of bats left a couple of days after we discovered them. In the following days only 20-30 bats left to feed each night. A man came and hung sheets of black plastic attached to the flashing with duct tape. The plan was that the bats would drop from behind the flashing and not be able to find their way back in. It didn't work. More often than not the plastic was standing out from the building due to the breeze, then it blew onto the roof so that it wasn't hanging at all, then the duct tape came loose and the plastic fell to the ground. There continued to be 20-30 bats every night. Hundreds of bats returned during the last week of September. I watched and counted for a half hour before dark one night and counted 1,409 bats exit. A neighbor whose night vision is better than mine said at least as many bats emerged after dark as before dark. That meant our colony had close to 3,000 bats. The next night I took pictures with my digital camera as the bats flew out.
Since the management company wasn't getting rid of the bats, and since I was the one who forbade them to use naphthalene, I decided to take action. On September 29 I went to Lowe's and Home Depot in search of ¼-inch polypropylene screen. Neither store had it, nor anything close. Both stores had window screen and bird netting (to protect fruit trees), but no screen size in between. The holes in the bird netting were too large (at least ½-inch), plus the netting was so light and flimsy I was afraid it would work like a fish net and entangle the bats. I settled for the cheapest nylon window screen. I designed and partially built my screen before going onto the roof. I straightened out metal clothes hangers and threaded them along the bottom of the screen for weight. I covered the hanger ends with duct tape to prevent slippage. During the nightly watches neighbors had seen bats exiting from the base of a chimney. I inspected the base of the chimney and discovered a metal plate had popped up leaving a large opening into the attic through which I could smell bats. I got nails and hammered the plate down securely. I installed my screen. Duct tape does not adhere to screen well. So I put duct tape along the upper screen edges and stapled the duct tape to the screen. Duct tape sticks to vinyl flashing well and it sticks to itself. I used duct tape to hold the screen to the flashing. (I stapled the top edge of the screen to narrow wooden boards which I laid along the edge of the roof and duct-taped to the shingles. I didn't want to make holes in the shingles with nails. Duct tape doesn't hold well through wind and rain. Later I used one screw in the center of each wooden board affixing it to the flashing.) The screen concept was good, but I hadn't made it wide enough. The next day I added an extension and wedged duct tape into the crevice under the flashing on both sides of the screen.
The screen worked! The nightly bat count went from over 1,400 to 2 bats in one day. The two bats that continued to come and go were seen landing on the brick wall below the screen and crawling up the wall behind the screen to the opening. I don't know if those two bats were a different species, smarter, or just the most persistent. A few days later those two bats appeared to be gone too. We decided to wait for cold weather, remove the screen, and seal the crevice.
On January 17, 2007 men came, removed my screen, and hammered the flashing. I alerted management that the crevice still looked too wide. Men came and hammered the flashing again. The crevice still looked too wide. During February a few bats returned and on March 3 I counted 27 bats come out from under the flashing. I had gotten rid of the bats only to have the management company remove my screen, not seal the crevice, and let the bats back in! If 27 bats could get in, so could 3,000.
Men came and installed a new exclusion screen, but they didn't make it hang straight, they didn't weight it, and they didn't seal the crevices on either side of the screen. More bats came until there were hundreds again. On March 9 a man came and removed the flashing, in order to install new flashing and seal the attic. Neighbors saw what he was doing, knew there were hundreds of bats in the attic, and made the man leave without sealing the opening. With the vinyl flashing gone I could see the opening the bats had been using. It was perhaps eight inches by one inch. The bats had been coming out of that hole and crawling behind the flashing to drop out from behind the flashing.
Finally the management company did what should have been done the previous summer. They hired a company that knows how to get rid of bats, Critter Control. The two guys that came from Critter Control were great. They examined the roof for other possible openings and asked me a lot of questions. They understood the problem, knew exactly what to do, and had the right tools. They sealed the narrow crevices between the wood and the brick with a polymer foam resin. They left one hole at the peak to which they attached an exit pipe. They said the bats would exit through the pipe but not be able to find their way back in through the pipe.
That night some bats discovered they could squeeze out from under a roof shingle at the corner. After four days all the bats had found their way out, either through the pipe or the corner. The following week the Critter Control guys sealed the corner under the shingle and decided to leave the pipe up a few more days in case bats had been getting back in through the corner. All the bats are gone now. The Critter Control guys came and removed the exit pipe on April 10. On May 8 new flashing was installed along the roof's edge.
I watched the bats almost every night for several months. I made several interesting observations. I do not know if these observations were true only for the spiecies I watched, or if other species of bats act the same way.
The colony contained at least two distinct groups, possibly different species. Each group had its own preferred exit time. One group exited 30-40 minutes before the second group. The second group exited for about ten minutes right before dark. In February I noticed the bats began their exit each day about three minutes later than the previous day.
Bats do not feed every night. During an exclusion the exclusion device should be left up for several days to make sure all the bats have exited.
Bats come and go all night. During the nights when there were only two bats I noticed they would feed for two to three hours, return, wait a few hours, and leave again.
Bats do not only feed at night. During February when the nights were cold bats left to feed in broad daylight and returned around sundown before the temperature dropped.
Bats often urinate immediately after they exit. I was hit several times during my first few bat watches and wore a wide-brimmed hat thereafter.
Bats can fly and feed in the rain. Some days when it was drizzling I assumed the bats wouldn't feed. I assumed the rain would interfere with their echolocation navigation. I assumed the rain would down the flying insects on which bats feed. My assumptions were wrong. I saw bats leaving to feed several times during gentle rains.
I read on at least one web site that bats don't fly when the air temperature is below 45 degrees Fahrenheit. That's not true. I saw bats leaving and returning several times when the temperature was about 40 degrees Fahrenheit.
Bats are programmed to enter a structure exactly where they exited. This behavior became apparent to me as I fine-tuned my exclusion screen. When I put the screen up the bats didn't want to exit through the opening behind the screen because they couldn't get back in that way. Even though there was a wide opening for them to drop through behind the screen they crawled behind the flashing until they found a spot where they could drop out from behind the flashing. Later, they returned to the spot where they had dropped out of the flashing rather than to the opening behind the screen. When Critter Control installed the exit pipe the bats refused to exit through the pipe for hours. Instead, the bats searched for another way out. That's when they discovered they could push up a corner shingle to get out.
With the occasional rare exception bats land directly on the opening from which they exited. If the opening is difficult to get into, a bat may make many landing attempts before successfully entering the hole. A bat will try to land on the hole, fail, fly another loop, and repeat the landing attempt. In general a bat will not land near a hole and crawl to the hole.