Bats are nocturnal. They sleep in roosts during the daytime, and emerge at dusk. If it’s a colony of bats living in a building, they crawl to the edge, and fly out. After a while they get full and head back to the roost in order to rest. They may make several trips per night. These colonies are composed primarily of females. The males roost alone in solitary areas, such as trees. The females form huge clusters, very frequently in man-made architecture such as church towers, attics, bridges, etc. They tolerate and even prefer very high temperatures.
The primary concern involves large colonies. If it’s just a few bats, it may not be a big deal. However, if you’ve got a typical maternity colony of bats in your home or building, it can be a big problem. A large colony is not only noisy and unsettling at dusk as swarms of bats fly in and out, but the main problem is that they leave their droppings and urine behind. With a large colony of bats, this really adds up. After a while large piles of droppings form. Not only do the droppings and urine corrode wood/metal, but the weight of them can collapse the ceiling below the attic. The waste has a foul odor, but it can also grow fungal spores that people can breathe in, leading to the lung disease Histoplasmosis.