Because insects are not available as food during winter, temperate-zone bats survive by either migrating to warmer regions where insects are available, or by hibernating. Body temperature is reduced and heart-rate is slowed.
Bats usually lose from to their body weight during hibernation.
Several bat species hibernate in dense clusters on cave walls or ceilings. Clusters may consist of hundreds of bats per square foot. A few species reside in caves year-round, although they usually use different caves in summer than winter. Most cave bats are very loyal to certain caves and return year after year to the same caves, often to the exact location in the cave where they spent the previous winter. Tree bats seldom enter caves.