Bats generally mate in the fall and winter but females retain sperm in the uterus until spring, when ovulation and fertilization take place. Pregnant females may congregate in maternity colonies in buildings, behind chimneys, beneath bridges, in tree hollows, caves, mines, or other dark retreats. They do not build nests. Birth typically occurs from May through early July. Young bats grow rapidly and can fly within 3 to 4 weeks. Weaning occurs in July and August, after which nursery colonies disperse.
Bats prepare for winter around the first frost. Some species migrate relatively short distances, whereas certain populations of Mexican freetailed bats may migrate up to 1,000 miles. Bats in the northern US and Canada may hibernate from September through May. Hibernation for the same species in the southern part of their range may be shorter or even sporadic. Some species fly during warm winter spells (e.g., big brown bats in the northeastern US).
The human-made structures that bats tend to inhabit usually are in locations with significant exposure to sunlight.
Bats in North America are insectivorous, feeding on a variety of flying insects. Many of the
insects are crop pests. Although there are some limitations, such as body size, flight capabilities, and jaw opening, insectivorous bats apparently consume a wide range of prey.
The diet of little brown bats includes mayflies, midges, mosquitoes, caddis flies, moths, and beetles. An individual bat can consume insects equal to 1/3 its body weight in ½ hour of foraging. Big brown bats may fill their stomach in about an hour (roughly 0.1 ounce per hour) with prey including beetles, moths, flying ants, true bugs, mayflies, caddis flies, and other insects. The nightly consumption of insects by a colony of bats can be extremely large.