The hoary bat is a widely distributed migratory bat that is vulnerable to mortality from wind turbines during migration. It also faces threats from habitat alteration throughout its range.
Description and Range
The hoary bat is the largest bat in Washington . The fur is a mixture of yellowish-brown, dark brown, and white, giving it a distinctive frosty or “hoary” appearance. Individual hairs on the back have four distinct color bands, with blackish-brown at the base followed by yellowish-brown, blackish-brown, and silvery white at the tips. Yellow or white fur occurs on the shoulders and wrists. Yellow fur also encircles the ears and is present on the throat and underside of the wing membranes. Wing membranes are blackish-brown with paler brown strips along the forearm and metacarpals. Wings are long and narrow. The upper surface of the tail membrane is densely furred. Ears are short and rounded with a dark brown or black margin, and the tragus is short and broad. The calcar has a narrow keel.
Food Habits and Foraging
This species is commonly considered a moth specialist. The only dietary data for the Pacific Northwest come from Oregon, where several studies suggest a preference for moths, with leaf hoppers, true bugs, mosquitoes, and other insects consumed in lesser amounts. Feeding lasts all night, but often peaks during the middle of the night. Hoary bats are fast straight fliers with less maneuverability than most other bats, and have low frequency echolocation calls that are adapted for long range detection of prey. Thus, foraging occurs mainly in open areas, such as above the forest canopy, over clearings and other open areas, along roads with trees, over lakes and streams, and at street lights. Individuals may forage 1.6 km or more from their day roosts.
Mating likely occurs in fall or early winter before, during, or after migration, with ovulation and fertilization delayed until spring. Gestation lasts about 90 days. Females produce one to four pups in a single litter per year, with an average litter size of two. Breeding has not been confirmed in the Pacific Northwest.
Hoary bats have the broadest geographic distribution of any bat in the New World, including much of North America and South America, as well as Hawaii. This species has been documented in most of Washington’s counties, but is probably present in all. The sexes are relatively segregated within the species’ summer distribution, with males occurring primarily in the mountainous regions of western North America and females more numerous in eastern regions. This pattern may simply reflect that females tend to migrate farther distances than males from major wintering areas in California and Mexico. Among the few individuals captured during surveys in Washington, nearly all have thus far been males. Examination of bats killed at wind farms in Washington indicate that some females pass through the state during fall migration.
In Washington and elsewhere, hoary bats are mainly associated with a variety of forest types, but also occur in open cover types (e.g., grasslands, deserts, clear cuts, meadows), particularly when foraging and migrating. Urban areas are also used. Late successional forests are often occupied, perhaps because the presence of larger trees provides higher quality roosting habitat. One study suggests that hoary bats were attracted to old-growth forests with relatively open canopies because such habitat may offer improved foraging opportunities. Elevational range varies from sea level to at least 1,620 m in the Pacific Northwest, but reaches 3,100 m elsewhere.
Hoary bats roost alone or with dependent young primarily in the foliage of coniferous and deciduous trees at heights ranging from 3-16 m above the ground. Roost trees are commonly near the edges of clearings, and may or may not be taller than the adjacent canopy. Easy flight access and concealment from predators are other desirable roost characteristics. Rarely, roosting can occur in tree cavities, caves, buildings, and squirrel nests; beneath rock ledges and bridges; and other locations. Adult females and their nursing young commonly enter torpor while roosting.
Relatively little is known about the night and migration roosts of this species. Hibernating individuals have been found on tree trunks and in tree cavities, squirrel nests, and clumps of Spanish-moss. Other details of hibernation are poorly known.
Sensitivity to climate change
*NOTE: In general the CCVA methodology does not include human responses to climate change in sensitivity rankings. However because wind energy development is a direct and expanding response, and has an outsized impact on this species, it is included here. Without wind energy development as a factor, sensitivity would be LOW. Hoary bats are highly migratory. Collisions with wind turbines are a major mortality source for migrating individuals, thus the expansion of wind energy facilities in response to climate change will likely have a strongly negative impact on this species. Additionally, the hoary bat displays low physiological sensitivity with a generalist's diet and a broad geographic distribution in both coniferous and deciduous forests across a wide temperature gradient from 0 to 22°C at elevations from 0 to 1,620 m in the Pacific Northwest. Hoary bats are probably sensitive to the structural changes in forests caused by logging, fire, and increased insect outbreaks, which would reduce roosting and possibly foraging opportunities. Thus, altered forest fire regimes associated with climate change could degrade or eliminate roosting habitat on a local scale. Also, warmer, drier conditions could affect insect availability and reduce drinking sources for the bats, which could affect survival and reproductive output. Large numbers of individuals migrate through the Columbia Basin, thus hotter, drier conditions in this region could negatively affect the migration behavior of a significant portion of the population passing through Washington.
Exposure to climate change
- Wind energy development (human response)
- Altered fire regimes
- Changes in precipitation
- Increased forest insect outbreaks
- Pre-construction surveys of proposed wind energy facilities should be used to establish the timing and location of potential conflicts so that mitigation measures can be used to reduce mortality to this species.
- At existing wind farms, surveys are needed to document mortalities and measures are needed to further reduce mortalities.
- Retention of older forests with large trees may benefit hoary bats in the West.
- Documentation of the temporal and spatial distribution of this species throughout Washington, including important migratory pathways, will help inform conservation measures and the appropriate time to apply them.
For some people bats don't present a problem. For others, bats can be a worry, especially when they become unwanted guests in an attic, inside a wall of a home, or inside the home itself.
Unlike rodents, bats only have small teeth for eating insects, so they do not gnaw holes in walls, shred material for nests, chew electrical wiring, or cause structural damage to buildings. Damage caused by bats is usually minimal, but they can be noisy and alarming, and the smell of bats and their droppings can be offensive. It is possible to learn to coexist with bats, and to benefit from their presence. Learn more on our Living with Wildlife: Bats webpage.