The rule of thumb is (albeit imperfect): If you find two bats in your living area during one summer, then the odds are you have bats residing in your house. If you find a bat during the winter time, then you have definitive proof that bats are residing in your home. For rabies protection guidelines, visit the FAQ on bat rabies.
You can also perform a bat watch, which consists of monitoring your home at dusk (one person looking at each of the four corners) to see if bats emerge. This should be done on a clear night during the summer months.
Other ways to determine bat presence can be seen below. Look for:
Rub Marks
Surface areas on walls, under loose woodwork, between bricks, and around other bat entryways often have a smooth, polished appearance. The stained area is slightly sticky, may contain a few bat hairs, and is yellow-brown to blackish brown in color. The smooth gloss of these rub marks is due to oils from fur and other bodily secretions mixed with dust, deposited there as many animals pass repeatedly for a long period over the same surface. Openings marked in this way have been used heavily by bats.
Noise
Disturbing sounds may be heard from vocalizations and grooming, scratching, crawling, or climbing in attics, under eaves, behind walls, and between floors. Bats become particularly noisy on hot days in attics, before leaving the roost at dusk, and upon returning at dawn. Note that rustling sounds in chimneys may be caused by birds or raccoons, and scratching and thumping sounds in attics and behind walls may indicate rats, mice, or squirrels.
Guano and Urine
Fecal pellets indicate the presence of animals and are found on attic floors, in wall recesses, and outside the house at its base. Fecal pellets along and inside walls may indicate the presence of mice, rats, or even roaches. Since most house bats north of Mexico are insectivorous, their droppings are easily distinguished from those of small rodents. Bat droppings tend to be segmented, elongated, and friable. When crushed, they become powdery and reveal shiny bits of undigested insect remains. In contrast, mice and rat droppings tend to taper, are unsegmented, are harder and more fibrous, and do not become powdery when crushed (unless extremely aged). Feces of the larger common cockroach species are similar to mouse droppings but, under magnification, are blunt ended with ridged markings (American) or have ends that are rounded or slightly tapered (Oriental). Feces of the smaller common cockroaches (German and brown-banded) are pepper-like specks.
The droppings of some birds and lizards may occasionally be found along with those of bats. However, bat droppings never contain the white chalky material characteristic of the feces of these other animals.
Bat excrement produces an unpleasant odor as it decomposes in attics, wall spaces, and other voids. The pungent, musty, acrid odor can often be detected from outside a building containing a large or long-term colony. Similar odor problems occur when animals die in inaccessible locations. The odor also attracts arthropods which may later invade other areas of a building.
Bat guano may provide a growth medium for microorganisms, some of which are pathogenic (histoplasmosis, for example) to humans. Guano accumulations may fill spaces between walls, floors, and ceilings. It may create a safety hazard on floors, steps, and ladders and may even collapse ceilings. Accumulations also result in the staining of ceilings, soffits, and siding, producing unsightly and unsanitary conditions.
Bats also urinate and defecate in flight, causing multiple spotting and staining on sides of buildings, windows, patio furniture, automobiles, and other objects at and near entry/exit holes or beneath roosts. Bat excrement may also contaminate stored food, commercial products, and work surfaces.
Bat urine readily crystallizes at room temperature. In warm conditions under roofs exposed to sun and on chimney walls, the urine evaporates so quickly that it crystallizes in great accumulations. Boards and beams saturated with urine acquire a whitish powder-like coating. With large numbers of bats, thick and hard stalactites and stalagmites of crystallized bat urine are occasionally formed.
Although the fresh urine of a single bat is relatively odorless, that of any moderate-sized colony is obvious, and the odor increases during damp weather. Over a long period of time. urine may cause mild wood deterioration. As the urine saturates the surfaces of dry wood beams and crystallizes, the wood fibers expand and separate. These fibers then are torn loose by the bats crawling over such surfaces, resulting in wood fibers being mixed with guano accumulations underneath.
The close proximity of bat roosts to human living quarters can result in excreta, animal dander, fragments of arthropods, and various microorganisms entering air ducts as well as falling onto the unfortunate residents below. Such contaminants can result in airborne particles of public health significance.
Ectoparasites and Other Arthropods
Several arthropods (fungivores, detritivores, predators, and bat ectoparasites) are often associated with colonies of bats in buildings. Their diversity depends on the number of bats, age and quantity of excreta deposits, and season. Arthropods such as dermestid beetles (Attagenus megatoma) contribute to the decomposition of guano and insect remnants but may also become a pest of stored goods and/or a nuisance within the living quarters. Cockroaches (for example, Blatta orientalis) attracted to guano may invade other parts of a building. Bat bugs (Cimex spp.) are sometimes found crawling on the surface of beams or around holes leading to secluded recesses used by bats. Bat ectoparasites (ticks, mites, fleas, and bugs) rarely attack humans or pets and quickly die in the absence of bats. Ectoparasites may become a nuisance, however, following exclusion of large numbers of bats from a well-established roost site. Area fumigation with a total release pyrethrum-based aerosol may be an appropriate solution for arthropod knockdown within an enclosed space but only after bats have departed. For long-term arthropod control, lightly dust appropriate surfaces (affected attic beams, soffits) with boric acid powder or diatomaceous earth; carefully read all product labels before using any pesticide. Note that neither rabies nor Lyme disease is transmitted by any arthropods associated with bats.
Resources: (online)
Bats
