by Lewis Young
The Silver-haired Bat (Lasionycteris noctivagans) is well named. The body fur is black or dark brown but the hairs on the back are silver-tipped. The distinctive appearance makes it easily identifiable unlike several other species of bats in Montana. It is a medium sized bat with a wingspan of 10-12 inches and a weight of 8-12 grams (0.25-0.45 ounces). The membrane between the hind legs and the tail is furred on the top surface, and the ears are black, naked, and rounded with a distinctive light-colored area at the front edge of the base. The lifespan is not known, but one researcher estimated they can live up to12 years, which is considerably shorter than some species that may live 2-3 times longer.
First described in 1831 in the eastern U.S., the word Lasionycterius means” hairy bat”, and noctivagans means “night wandering.” Considered slow flyers, occasionally some individuals emerge well before dark and may be observed relatively easy. They require open, still water for drinking which they do on the wing. Echolocation at frequencies above human hearing is used to find prey and navigate.
Foraging is often over still and running water and in openings surrounded by forest but sometimes at tree-top level. Although characterized by some researchers as “moth specialists,” Silver-haired Bats consume a wide variety of relatively soft-bodied insects and occasionally spiders and harvestmen. Silver-haired Bats have been documented to feed on many insects perceived as pest species to humans and/or agriculture and forestry.
In summer, males and females are segregated. Males and non-reproductive females tend to roost singly and primarily utilize loose bark of trees but less frequently have been found in tree cracks and crevices, and tree cavities formed by other animals such as woodpeckers. These males and non-reproductive females change roosts frequently. Reproductive females form small maternity colonies in tree cavities or small hollows where they typically give birth to twins, usually in late June and July. Mating takes place in early fall, and fertilization is delayed until the following spring. Gestation takes 50–60 days, so that parturition of pups occurs in early summer when insect availability is high. Just before birth takes place the female will begin to roost with her head facing upward. She will hold her tail membrane forward to form a cup-shaped basket which will catch the pups as they are born. The pups are able to fly 3 to 4 weeks after birth.
In winter, Silver-haired Bats appear to hibernate alone mainly in forested areas, though they may be making long migrations from their summer forest to a winter forest site. Typical hibernation roosts for this species include tree cavities, beneath exfoliating bark, beneath leaf litter, in wood piles, and in cliff faces. Occasionally, Silver-haired Bats will hibernate in cave entrances, especially in northern regions of their range. No large aggregations have been documented. They are true hibernators in that their heart rate drops from 200/per minute at rest in summer to 10-20/minute during hibernation, and their body temperature can drop to near freezing.
Silver-haired Bats are found all across Montana including the prairies. Prairie populations rely on trees in riparian areas and around farms (shelterbelts). Across the continent they range from southeastern Alaska, across southern Canada, all of the lower 48 states (except parts of the southern states), and into northeastern Mexico. Although range maps describe Silver-haired Bats as permanent residents in Montana, they are believed to be mostly migratory with some individuals possibly remaining year-round. There are no confirmed winter records in Montana to-date, but acoustic monitoring indicates some are likely present here in the winter.
Conservation concerns include wind energy facilities, forest management practices, and possibly white-nose syndrome. The Silver-haired Bat is one of the 3 bat species most commonly killed at wind energy facilities (Hoary Bat and Eastern Red Bat are the others). Research has found that high snag numbers (more than 8/acre) and large diameter snags are important but often forest management practices fall short. Managing forests for diverse age structure and maintaining forested corridors are important to these bats. The causative agent of white-nose syndrome, Pseudogymnoascus destructans, has been detected on a Silver-haired Bat in Delaware, although the same mass mortalities observed in smaller-bodied hibernating North American cave bats has not been observed.
Besides being tremendous consumers of insects, Silver-haired Bats are just plain fascinating with their appearance, ecology, and behavior. Some evening before dark, if you see a medium-sized bat flying back and forth over water or along the edge of a forest opening, you just may have seen one!