A medium-sized, nearly black bat with dorsal surface of interfemoral membrane densely furred at least on the basal half and usually to near margins; upper and lowerparts sooty brown or black with white tips of hairs producing a frosted appearance; membranes and ears sooty brown or black. Dental formula: I 2/3, C 1/1, Pm 2/3, M 3/3 X 2 = 36 (upper incisors and first lower premolar very small and easily overlooked). External measurements average: total length, 100 mm; tail, 40 mm; hind foot, 8 mm; ear, 16 mm; forearm, 41 mm. Weight, 8-12 g.
Habits. These bats are denizens of forested areas and seldom are observed in xeric areas except in migration. Cavities in trees and spaces under loose bark are favorite daytime retreats but these bats may also use buildings.
This species is migratory, at least in part. It spends the summer in northern latitudes and winters toward the south, even crossing several hundred kilometers of ocean to reach Bermuda. Surprisingly few winter records are available; thus, the mystery of just where these bats spend the winter is still not completely solved. It is likely that many of them winter on their breeding grounds because occasional individuals have been found hibernating as far north as New York and British Columbia. Interestingly, most summer records of this bat across the southwest are of males, suggesting that geographical segregation of the sexes may occur during the warmer months. Females appear to move north in spring and summer to bear young, whereas the males remain behind at more southern locales. A small population, apparently comprised entirely of males, appears to be resident in the Guadalupe Mountains (Culberson County) during summer.
This bat typically forages in or near coniferous and/or mixed deciduous forests adjacent to ponds or other sources of water. It is a relatively late flier that often appears after other bats have begun feeding. As with most other insectivorous bats, Lasionycteris is opportunistic in its feeding habits and takes a wide variety of small to medium-sized insects including moths, bugs, beetles, flies, and caddisflies.
The one or two young are born in June and July. Small maternity colonies may form in hollow trees and abandoned bird nests. The young are black and wrinkled at birth and are able to fly when about 3 weeks old.