From late fall bats prepare for hibernation storing the fat reserves they will need to last until spring. In Michigan there is a shortage of food for insectivorous animals. Bats will either migrate to warmer climates or go into hibernation. Most male brown bats in Michigan will choose hibernation, most of the time this will be a big brown bat. Hibernating bats require temperatures to be 45 degrees or above. Summer nurseries or maternity colonies need temperatures close to 70 degrees or above therefore they will not hibernate in Michigan. A bat will drop its heartbeat from 400 beats per minute when active to 25 beats per minute when in hibernation. Even in hibernation, a bat must periodically arouse to drink or urinate, or sometimes find a warmer spot within the dwelling area as temperature fluctuates. Moisture on the walls or condensation on their fur provides drinking water during this time. A brown bat may awake every 12-19 days, but could stay in hibernation for up to 83 days. As the winter goes on, the fat a bat has stored during fall is slowly metabolized. These limited reserves must last it anywhere from five to eight months. When winter temperatures rise above 55 degrees bats will arouse and feed, often in the brief warmth of mid-afternoon, in order to capture the few available insects.
The length of time it takes bats to exit your home varies depending on the season. During summer months, getting rid of bats usually takes between one and three days. During winter, or other times of extreme cold, bats often hibernate and have very low activity levels. Getting rid of bats during these seasons may take several weeks or months.