The Sixth Extinction: An Unnatural History by Elizabeth Kolbert:
The best time to take a bat census is the dead of winter. Bats are what are known as “true hibernators”; when the mercury drops, they begin looking for a place to settle down, or really upside down, since bats in torpor hang by their toes. In the northeastern United States, the first bats to go into hibernation are usually the little browns. Sometime in late October or early November, they seek out a sheltered space, like a cave or a mineshaft, where conditions are likely to remain stable. The little browns are soon joined by the tricolored bats and then by the big browns and the small-footed bats. The body temperature of a hibernating bat drops by fifty or sixty degrees, often to right around freezing. Its heartbeat slows, its immune system shuts down, and the bat, dangling by its feet, falls into a state close to suspended animation. Counting hibernating bats demands a strong neck, a good headlamp, and a warm pair of socks.