For most of the twentieth century, bats steadily declined in numbers across the United States. Their disappearance was related to the fact that bat are more likely to suffer large population loses than any other animals. One of the major factors in the bats decline has been their habit of roosting together in large numbers. Unlike other small mammals, some bats live together in colonies that may number in the millions. This behavior has resulted in thousands of bats dying from several natural disasters such as flooding, or from human disturbances.

Another factor in the disappearance of bats is their need to sleep during the day. Because the bats sleep in caves and buildings where they can be easily found, they are at greater risk for attack than other animals. Sleeping bats are easy prey for animal predators or people with the intent of harming them. Peoples fear of bats, based on the belief that bats infect humans with deadly diseases, such as rabies, led people to kill bats. Many communities, believing they are ridding themselves of dangerous animals, organize campaigns to destroy any bats found nearby.

Animals such as snakes, owls, hawks, skunks, and raccoons occasionally eat bats. But man bears the responsibility for bats disappearing at a rate that threatens them with extinction. Vandalism of bat roosting sites, and killing bats for sport, was considered acceptable for much of the twentieth century. Even when people have no intention of harming bats, they can still do harm. People exploring a cave during a certain species hibernation (vampire bats do not hibernate), or when females are having their young, has resulted in the death of many bats.