Northern Prairie Wildlife Research Center
Among the 40 species of bats in the United States, only a few cause problems in buildings. The most common house bats congregating in groups or colonies are called colonial bats. Other species live a secluded or solitary existence and are known as solitary bats.
The three species most often encountered by humans are the little brown bat, big brown bat, and Mexican free-tailed bat.
The little brown bat is one of the most abundant species, often forming nursery colonies in buildings during the summer. Adults and young vacate the buildings in the fall to hibernate in caves and mines. Colonies may be as large as 2,000 (Humphrey and Cope 1976).
The big brown bat is undoubtedly the most familiar to humans and the only species for which buildings are ideal for both raising young and hibernating. Colonies are small, ranging from 12 to 200 (Barbour and Davis 1969).
The Mexican free-tailed bat is the most colonial of all bats. Its habits vary in different parts of the country. Primarily a cave dweller in the Southwest, a colony may include thousands of individuals. In Florida this bat never enters caves and thousands have been found in a single building. Some populations migrate 1,600 km (1,000 miles) to overwinter in Mexico whereas others are year-round residents (Davis et al. 1962).
Solitary bats live alone in tree foliage or under bark, but never in caves. The red bat, the hoary bat, and the silverhaired bat (Lasionycteris noctivagans) may occasionally enter buildings during spring and fall migrations as transients but do not permanently roost in buildings (Barbour and Davis 1969).
A number of other bat species are occasionally found in buildings but, because they infrequently cause problems, they are not discussed. Appendix A contains an identification key to the bats most often associated with houses, including detailed descriptions and photographs.