Some 1.6 million gray myotis - almost the entire North American population - spend the winter together in the same nine caves, deep in hibernation. Some of them migrate long distances to reach these caves. Although 1.6 million sounds like a lot of bats, the total population has probably fallen by about 50 percent since 1965, and the species was listed as endangered in 1976. The bats depend on caves not only to hibernate, but also for summer maternity roosts. Hibernating bats can die if they are awakened when no food is available. Females need warm caves for maternity colonies, often choosing caves that have streams running through them, because the streams prevent some predators from entering the cave. The caves are always near a major river or lake, where they can forage for insects over water. Caves that are opened for tourism, vandalized, or flooded, are lost to the bats  and so are caves that are sealed up, unless bat-friendly gates are installed. Awareness of the bats' needs seems to be helping halt the decline of gray myotis and some other cave-dependent species.

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account