Lasiurus cinereus
hoary bat

Hoary bats are the most widespread of all bats in the United States. Though not yet recorded in Alaska, these bats are thought to occur in all 50 states. They range from the tree limit in Canada down to at least Guatemala in Central America, and throughout South America. They are the only bats found in Hawaii. There are records of migrant hoary bats on Southampton Island off of Northern Canada, and from Iceland, Bermuda, and the Orkney Islands off Scotland. They are rare in most of the eastern United States and northern Rockies and common in the Pacific Northwest and prairie states. They are abundant in California, Arizona, and New Mexico, where they winter. They winter in southern California, southeastern United States, Mexico, and Guatemala, but have also been found in Michigan, New York and Connecticut during December and in Indiana during January. This suggests that some may winter farther north than was previously expected.

Sexes are generally only found together in parts of Nebraska, Montana, and the Badlands of South Dakota. Males and females are usually separated during the warmer months in North America, except during the mating season. Females appear to be more concentrated in the western part of North America. There is evidence for an altitudinal separation of sexes in California, with females concentrated in the lowlands and coastal valleys and males higher up in the foothills and mountains.

Authorities disagree as to the bat's preference for coniferous versus broadleaf trees. Hoary bats are thought to prefer trees at the edge of clearings, but have been found in trees in heavy forests, open wooded glades, and shade trees along urban streets and in city parks.

The body of hoary bats is about the size of a fat mouse. Hoary bats weigh 20 to 35 g. The length from the tip of the nose to the end of the tail vertebrae is 13 to 15 cm. The wingspan is 43 cm. These bats have blunt, rounded noses and small, beady eyes. The ears are short, thick, broad, and rounded. When laid forward they do not reach the nostrils. The tragus in the ear is short and blunt. The hindfoot is half as long as the tibia and has thick fur on the dorsal side. The thumbs are long. The calcar is twice as long as the hindfoot and is narrowly keeled on the posterior edge, bearing lobes on the tip. These bats have four mammary glands.

Thick, long, soft hair covers the entire dorsal surface extending to the elbow, the median ventral border of the undersides of the wings, the ventral side of the long bones that make up the upper arm and forearm, and the basal part of the lower surface of the interfemoral tail membrane. The coloring of the dorsal area (including the tail membrane) is a mixed brown-gray with a heavy white tinge, giving these bats a frosty appearance. In fact, these bats' name means "frosty or ash colored hairy tail." The individual silky hairs are basally dark, medially yellowish, and distally black with white tips. The belly of these bats is not heavily frosted. The throat has a distinct yellow patch. The hair on the elbow, at the base of the clawed thumb, and the upper arm is yellowish as well. The ears are yellow with black edges. Brownish fur extends out on the underside of the wing nearly to the wrist.

The skull is large (16 mm long) and broad (13 mm wide), with a large auditory bullae. These bats have large, strong teeth, with the first premolar located at the inner junction of the large canine.

Juveniles appear nearly grayish, but still have a frosty appearance.

Hoary bats are solitary. They roost 3 to 5 m above ground during the day, usually in the foliage of trees. They prefer dense leaf coverage above and an open area below. They also prefer trees that border clearings. They have been seen roosting in a woodpecker hole in British Columbia, in the nest of a gray squirrel, and under a driftwood plank. Occasionally they are found clinging to the overhangs of buildings and in caves in the latter part of the summer. They often have trouble finding their way out of the caves and die there.

Hoary bats reach their peak activity at about five hours after sunset, although they may occasionally be seen flying on warm winter afternoons. Their flight is stong and direct, reaching speeds of thirteen miles/hr. While hunting, they soar and glide. They forage about the tree tops, along streams and lake shores, and in urban areas where there are lots of trees. These bats stop to rest between meals at night. Feeding is the only time that hoary bats appear to associate with other bat species. Hoary bats often form groups when hunting for insects.

These bats wrap their hairy tail membrane around their curled up bodies for insulation while resting during harsh weather conditions. They become torpid when inactive during the day, as well as between feeding flights at night. When they are resting and non-torpid, they have a metabolic rate of 1.19 cc of oxygen per gram of body weight per hour.

Hoary bats can be seen flying in large groups in spring and autumn, during the time of breeding and migration. They are believed to migrate through Florida from late October to late November and from February through early May. Autumn migration occurs in waves, whereas spring migration appears to be less organized. Some hoary bats are believed to remain in the north and hibernate for the winter, rather than moving south of the United States like most do.

Like all microbats, hoary bats use echolocation while flying. They make a shrill, hissing sound when disturbed. Lasiurus cinereus is one of the only vespertilionid bats which makes an audible chatter during flight.

Moths (Lepidoptera) make up the bulk of the diet of hoary bats. These bats are also known to feed on flies (Diptera), beetles (Coleoptera), small wasps and their relatives (Hymenoptera), grasshoppers (Orthoptera), termites (Isoptera), and dragonflies (Odonata). The bat approaches the insect from behind, taking the abdomen and thorax in its mouth and biting off and swallowing this area of the insect, while dropping the wings and head. In comparison to other bats, hoary bats feed on relatively few orders of insects. On rare occasions, these bats have been observed to feed on leaves, grass, shed snake skin, and eastern pipistrelles.

The main enemies of hoary bats are hawks and owls. American kestrels and rat snakes have on rare occasions been reported to feed on hoary bats. These bats are also known to become entangled in barbed wire fences. Another important source of mortality is females falling out of their roost with attached young, thus becoming easy prey for terrestrial passers by.

Hoary bats have an important ecosystem role as insect consumers. These bats are often infested with mites (Pteracarus chalinolbus and Chiroptonysus americanus), helminths (Longibucca lasiura, Oochoustica taborensis, and Physocephalus), and protozoa (Distoma).

Hoary bats occasionally hang out under overhangs of houses and garages, but this is only menacing and they rarely cause any true disturbance to homeowners.

These bats have a relatively high incidence of rabies. In some years, 25% of sick bats collected were found to be rabid.

Hoary bats prey on many insect species that are considered to be pests.

Hoary bats are widespread and secure over much of their range. One subspecies, the Hawaiian hoary bat (Lasiurus cinereus semotus), is listed as endangered by the U.S. Fish and Wildlife Service.

There are three subspecies of Lasiurus cinereus: L. c. cinereus in North and Central America, L. c. semotus in Hawaii, and L. c. villosissimus in South America.