What time of year do bats come into houses?

Bats come into houses at different times of year depending on species, climate, and whether they are seeking maternity roosts or hibernation sites, but the most common times are spring and late summer to fall. In spring, from March to May, bats become active after hibernation and females seek warm, dark, quiet places like attics, wall cavities, and chimneys to form maternity colonies to give birth and raise pups, with most pups born in May and June. This is when many homeowners first notice bats entering houses to roost and raise babies. In summer, bats remain in these maternity roosts raising pups until late summer. In late summer and early fall, from August to October, bats become more active as pups learn to fly and bats seek additional roost sites and build fat reserves for winter, and they may enter houses while exploring for hibernation sites. In fall, some species enter houses and other buildings to hibernate in attics and wall spaces where temperatures are stable for winter, from October to March. In winter, bats in hibernation may be found inside houses but are less active. The best time to humanely exclude bats is early spring before maternity season or fall after pups can fly, never during maternity season from May to August when flightless babies may be trapped inside.