In nature, bats do little harm; they are actually a protected species because they provide a beneficial service – insect control. But if they take up residence in a house, it's another story. Bats will migrate in cold weather to where food is plentiful. So if you discover you have bat problems in your own home, the best time to bat proof your house is before they return again when the weather warms up.
Bats will be attracted to areas where their food supply is plentiful and can take up residence in any home if the conditions are suitable. It's hard to say why a colony bats will take up residence in one house and ignore the next seemingly similar one. But accessibility is key. Bats can squeeze through holes as small as 3/4 inch in diameter, or less than the size of a finger.
Common entry and exit points are usually in the upper areas of the house, including any gaps between the house trim and roofing or siding. These gaps may be due to construction methods or decayed wood, squirrel damage, or the aging or drying of wood trim or other components. Sometimes the bats will not actually enter a house but take up residence in a suitably protected exterior gap.
Bats are social creatures -- where there is one, you’re likely to find a colony. They must exit every day or so to eat and drink. Most often they emerge around dusk -- just after sunset.
Besides happening to see the entering or emerging bats, their droppings are the most obvious sign. Droppings may accumulate at ground level below the openings. You may also see evidence on window edges or on top of window-mounted air conditioners or inside attics. The area around the bats' entrance and exit points may be stained a dirty brown from bat urine and/or the oil from fur constantly being rubbed off when the bat brushes against the surface.
The droppings are a little larger than a grain of rice. The color is black-brown and there may be a taper at one end. A bat dropping will crumble into dust if slight pressure is applied to it. This is because they eat nothing but insects and the bat dropping is made up of insect scales. With long-term infestation, the droppings will build up and when combined with the bats urine, a fairly unpleasant odor is likely.
The elimination of a bat problem is best left to a professional pest control company. To prevent the bats from returning, the primary as well as secondary exit holes must be identified and sealed. But the primary exit should not be sealed until it can be confirmed that all the bats are out.
Bats do not physically make the holes themselves; they only use what is available, so just about any sealant will work, e.g., caulk, expanding foam, 1/4 inch wire mesh, wood, steel wool, etc. Often, squirrels and birds are the ones who damage the structure, providing the openings bats need. So in some cases, the bat eradication will go hand and hand with eliminating a squirrel or bird problem.