Bats are truly wonders of nature. They represent the only flying mammals (flying squirrels glide but cannot fly). The sonic, directional, object detection ability not only serves them well while flying, but it was studied as a part of various research projects during the development of radar and sonar. Bat species in Minnesota are primarily insectivores.
Bats breed but once a year and depending on the species, females deliver one or two pups. By early August the young are flying along with the adults, but, until then, they are confined to the roost while the adult females fly in and out to hunt and feed their young. Males live separately from females until the fall migration when the sexes mingle and spend the winter together at their hibernation sites.
Only two species of bats the Big Brown Bat (Eptesicus fuscus) and the Little Brown Bat (Myotis lucifugus) are any threat to man. Individuals from these species will enter houses in the spring and fall. During the spring where attic and eve temperatures hover between 90 and 103 degrees F, female little brown bats can use the areas as nurseries for raising their young. Big brown bats prefer slightly cooler locations for their nurseries. Also big brown bats can tolerate much colder winter temperatures and are active later into the fall than are the little browns. Both species, however, require fairly constant temperature and humidity during the hibernation period. Little browns are likely to hibernate in caves where the temperatures stay above freezing. Big brown often successfully hibernate in attics, eve areas, abandoned bird houses, etc. where temperatures may fall to below 0 degrees F.
Both species of bats can enter very small openings. Any circular opening the diameter of a nickel will allow their passage. It is more common for them to enter buildings via more accessible routes such as broken attic windows, unprotected roof/gable vents, along side exterior masonry chimneys, sprung corners on eves, where dormers meet the main roof, etc.
If your house is currently under attack by bats it is wise to call a professional. It is very important that proper inspection techniques and execusion methods be utilized. If not, you could end up with numerous adult and/or young bats being closed in your walls, eves, or attic, causing a large sanitation problem from dead bats or even having bats come out into your house with you and your family.