Little Brown Bats

For most of us who live in North America, the small bats that flutter overhead on summer evenings are little brown bats.  Residents of the Temperate Zone, from southern Alaska to the Canadian Maritimes and southward through all but the southernmost regions of the U.S., these common mammals are most active at dawn and dusk, strafing the treetops for a wide variety of flying insects.

In spring, pregnant females gather at large nursery roosts, usually located in abandoned buildings.  After giving birth to a single pup (twins are rare), the mother nurses her newborn for several weeks; by one month of age, the young bat is self sufficient and female pups may mate by the coming fall.  During the warmer months, little brown bats roost in attics, barns, tree cavities and man-made bat houses (among  other sites), leaving to hunt at dusk and during the predawn hours; they are active only 4-5 hours each day.  Promiscuous breeding occurs in late summer and early autumn but the sperm is stored and fertilization does not occur until the following spring; births peak in late May to early June (depending on latitude).

As their prey dies off with the autumn chill, little brown bats gather in hibernation caves or abandoned mines for the winter; those that summer in northern latitudes usually migrate to more southern areas for hibernation.  Throughout the winter, the bats stir during periods of mild weather (and may be seen flying about) but generally do not feed; indeed, like all true hibernators, they depend on stored brown fat to fuel their survival, an adaptation that may prove fatal in years when winter conditions extend well into spring.