Time line of a bat proofing and mitigation project:
Concentrate on excluding the bats from your attic. This is the only permanent way to solve your problems, and is often easier than you first expect. It does take time. Usually a bat exclusion may take 2 or 3 seasons (tries) unless a more aggressive bat proofing campaign is undertaken. Briefly, these the general steps:
1) Install at least one Five Chamber bat box nearby where receives proper sun exposure. This is best placed on a post and can be several hundred feet away from your existing colony. It should be installed at least before July so that bats will become familiar with it.
2) In June-August watch the attic carefully at dusk (from 8:30-9:30 EST or 15 to 45 minutes after your local sunset) and the bats will exit, using their primary exit points.
3) The next winter (November-March) seal the exits you observed during the summer and any other entrances which you may find while on the roof, etc. The reason for this time lag is to give all bats (including newborn) the chance to leave the attic for the winter.
4) When bats return in mid-May, they will already be aware of the nearby available bat house. Bats will find the attic more difficult to enter, and may choose the bat box over your attic (and over your neighbors attic, etc.). A few pesky bats may find secondary, less desirable entrances to your attic space and use them. Repeat this procedure of observing and sealing until bats are in the bat box, and out of the attic.
Bat maternity colonies within structures
Confusing other animals with bats
Before attempting any kind of exclusion is essential to verify that a nuisance is caused by bats in particular. Twittering and rustling sounds in old chimneys, often attributed to bats, may be caused by chimney swifts. If the "bats" swarm and ENTER the chimney at dusk, most likely these are swifts and can be evicted with a chimney cap in the winter. Scrambling, scratching, and thumping sounds coming from attics and walls may be caused by rats, mice, or flying squirrels. Bats often become noisy before leaving their roosts at sunset and may chatter on hot days when they move down attic walls to seek refuge from heat. Thus, an increase in chirping noises about dusk probably indicates bats. Any grouping of bats this noisy should easily be seen exiting the structure 15 minutes after local sunset in midsummer.
Droppings from insectivorous bats are easily distinguished from those of small rodents because of their friability. They are easily crushed with a pencil eraser which reveal shiny bits of undigested insect chitin (the exoskeleton of the insect). In contrast, rodent droppings are unsegmented, harder, and more fibrous. They do not contain shiny fragments.
Occasionally the droppings of birds that feed on insects may be found along with bat droppings. Bat droppings never contain the white chalky (uric acid) material characteristic of the feces of these other animals.
Yes, there are bats in the house!
Attractive openings are found in old frame structures where boards shrink, warp, or otherwise become loosened. Bats commonly enter buildings through the overhang of the roof made by overlapping sheeting or drop siding. They are most often found in attics, between roofs and ceilings or roof spaces, in cornices, fascia, or other crevices around the roof, in walls, behind chimneys, around drainpipes, behind rafters and sheathing in open barns, between a window and screen, and occasionally in crawl spaces. Depending on the size of the space and on the species, bats will be found singly, congregated in groups of a few individuals, or in colonies of hundreds and occasionally thousands. Often bats will not be seen in attic spaces except on very hot summer afternoons when they must move into the open to cool off. The droppings they leave in the attic will reveal their presence much more readily than just looking for bats themselves.
What about repellents?
Usually there are only a few openings to a house bat roost, and bat proofing is relatively simple because visible bat accesses can be easily located and blocked. However, some very old homes having large attics with dormers may have many small, obscure holes will be more difficult to bat proof. On any structure, generally assume that the soffit and fascia will need sealed on all sides. On some tin and slate roofs, all edges may need sealed (along the gutters especially). Finally, any place there is a change in roof pitch could be a bat entrance as well.
Bats should be out of the building before bat proofing begins. The best time for bat proofing is in November through March (northern North American latitudes) after the young bats have learned to fly. If the structure must be sealed before in summer months, the time of day is important so that holes can be blocked in the early evening after the bats have departed to feed. Also, some sort of exclusion device must be installed to allow remaining bats to escape over the next few days. Most bats start to leave a building about 15 minutes after sunset; however, some species of bats leave their roosts later than others, some when it is dark. It is not recommended to seal a structure at all during April-August as this will trap flightless young inside.
Blocking and sealing holes
If the structure must be sealed before November, care must be taken as to not seal bats within the building. All obvious accesses should be sealed except one or two of the principal openings. For several days, bat counts should be made as the holes are closed, leaving the main exit open. On the night of the final count when all the bats have left, rags, tissue paper, or cotton should be pushed firmly into the main hole to prevent reentry. Returning bats will cluster or flounder outside the plugged entrance, and some will probably find another obscure entrance to the inside. Early the following evening, the plugging should be removed so as to allow any further bats to escape before sealing the last hole. If any bats are seen within the structure, the routine must be repeated. The house should be watched for several evenings at dusk (and later, if necessary) to make certain that bats have not found an overlooked access. Do not be discouraged if this process takes a week or longer. Properly installing an alternate roost will significantly speed the eviction time since the bats will opt for the more easily accessible bat box. The number of bats counted will dictate the bare minimum number of boxes needed. Do not underestimate the intelligence of these animals, as you can be sure they will return next season to see if your sealing job has held over the winter. If a more casual approach is taken, such as initially sealing the structure in the winter, it may take several seasons to learn where the bats are still gaining access.