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Bats Northwest

"helping bats in Washington State"

Meet the Silver-haired Bat (Lasionycteris noctivagans)

by Carrie Carlstead

Ah...summertime in the Pacific Northwest. A time when we humans emerge from our form of hibernation against the rain and gray, to go forth and explore the mountains and lakes in our ultimate backyard. If you will be doing any camping along any of the Northwest's many lakes and streams, the first bat you may see will most likely be the Silver-haired Bat, Lasionycteris noctivagans. This bat emerges just before or right at sunset to prey on aquatic, night-flying insects. Clocked at just 11 MPH, L. noctivagans is one of the slowest-flying bats of our region, making it easier to identify on the wing.

The Silver-haired Bat can be identified by its rich chocolate brown fur with silver-frosted hairs forming a cape-like appearance on the back and belly. The wing membranes are brown and the short, broad ears and blunt tragus are black. At first glance the Silver-haired Bat can be confused with the Hoary Bat, Lasiurus cinereus, but closer examination shows L. noctivagans lacks the cream and yellow swatches of L. cinereus, who is also notably larger. The total body length for this bat averages 100 mm (~ 4 inches) with a wingspread of 270-310 mm (11 inches) and a weight average of 9 grams (1/3 oz.). Using echolocation to hunt for its meals, insects are captured in flight its teeth, a wingtip, or by snagging one in the pouch formed by bringing the tail membrane toward the belly. By flying slowly and quietly over ponds and streams, this stealthy hunter will capture and devour hundreds of delicious insects each night.

The Silver-haired Bat can be found in southwestern Alaska, southern Canada and across most of the U.S. Although preferring to roost under the loose bark of trees or to burrow into a large bird's nest, L. noctivagans can also be found in rock crevices and an occasional building structure. The roosts are almost always situated near lakes and streams around wooded areas. L. noctivagans, who generally lives a solitary life during the summer, was one of the first bats recognized as having a seasonal migratory pattern. It is thought to congregate with others of its kind only in late August and September when they migrate from colder latitudes to more temperate climes to hibernate in winter hideaways. Hibernation in caves has been observed, but this bat tends to hibernate in tree snags, abandoned woodpecker holes and occasionally in an outlying structure. Silver-haired Bats are year-round residents in many regions.

Mating is also believed to occur in the fall. The sperm will be stored inside the female until spring, when it is thought that ovulation peaks. The embryos begin their 50-60 day gestation as the female awakens from her hibernation to migrate farther north for the summer. These mothers will bear their pups alone or in small groups - there is no firm evidence that large nursery colonies are formed. The female commonly gives birth to two young in late spring or early summer when the insect population is flourishing and there is plenty to eat. By three weeks of age, the pups are taking brief, awkward flights. And by fall, the youngsters are skilled enough flyers to join mature bats on their migration flight to hibernation sites, when the cycle begins again.

Clear-cutting, accompanied by the disposal of dead trees (snags), continues to shrink the habitat that this bat requires for survival. While keeping an eye out for this silver-haired beauty when you are out enjoying our NW wilderness this summer, remember to be a conscientious and courteous guest in the place this bat calls home.