They usually form roost colonies during summer in attics, barns, caves, or the walls of older buildings. A colony may consist of a few dozen up to several hundred individuals. In winter, these bats often roost in caves or abandoned mine shafts, but a few big brown bats may remain in attics. Most species of New York bats mate promiscuously during fall. The young of social species are born in maternity colonies during late spring.
Adult females have only one litter per year, and bear one to two young per litter.
Body heat from the colony maintains warm temperatures for the young bats. Juveniles develop rapidly, and are usually able to fly in about three weeks. In New York, little brown bats are born June 7 to July 10, and most will be airborne by early August. Big brown bats are born May 21 to June 7 and young will be capable of flying by mid-July. Bats have a relatively long life span, and may survive 10-20 years.