Description and Range
Townsend’s big-eared bat is a medium-sized insectivorous bat with very large ears connected at the base and two prominent lumps on either side of the nostrils. Five subspecies are recognized, with only C. t. townsendii present in Washington.
Food habits and foraging
More than 90% of the diet is usually comprised of moths. Smaller amounts of other prey such as beetles, flies, and lacewings are also eaten. Tissue moths (Triphosa haesitata), a hibernating moth that develops fat pads in fall, and other moths (e.g., Scoliopteryx libatrix) occur in some of the caves used by Townsend’s big-eared bats in fall and winter in Washington and may be an important autumn food source for these bats prior to hibernation.
Foraging activity extends from after sunset to before sunrise. Travel distances of 1-18 km between day roosts and foraging sites are probably typical in the West, although longer nightly foraging movements have been noted (e.g., more than 150 km). Individuals are often loyal to foraging sites and travel routes over successive nights. Townsend’s big-eared bats are characterized by slow and highly maneuverable flight, and feed mainly on flying insects caught near and among foliage. Gleaning has been observed, but the extent of this technique is unknown.
Mating peaks in late summer or early fall, although some breeding occurs during arousals from hibernation. Females store sperm through winter and delay ovulation and fertilization until spring. Length of pregnancy is quite variable, lasting 56 to 100 days depending on the frequency of torpor by females. Timing of births can therefore show considerable variation within and among colonies and years. For example, initial birth dates ranged between June 20 and July 26 and between early July and July 28 at two nursery colonies near one another in Okanogan County, Washington, over a three-year span. One pup is born annually.
Townsend’s big-eared bats occupy a broad range of arid and moist habitats. In Washington, this species is found in lowland conifer-hardwood forest, montane conifer forest, ponderosa pine forest and woodland, shrubsteppe, riparian habitats, and open fields. Caves, lava tubes, mines, old buildings, bridges, and concrete bunkers are commonly used as day roosts in Washington, with rock crevices and very large trees with basal hollows occupied in other regions.
This species occurs from southern British Columbia southward through most of the western United States to central Mexico. Isolated populations also exist in the Ozarks and Appalachians. Documented records exist for most counties in Washington, but are lacking for the southern Columbia Basin and Blue Mountains. Within the species’ range, distribution is often linked to the presence of suitable sites for maternity roosts and hibernacula located near foraging habitat.
Townsend’s big-eared bats generally occur at low densities across their range. Long-term population trends are difficult to assess for most western populations because of the scarcity of adequate count data, the species’ dynamic roosting behavior, and the use of multiple roosts under some conditions. In Washington, long-term count data are available for only a small number of roosts. Comparisons of bat numbers during the 1970-80s against those in the 1990-2000s can be made for six hibernacula, with four of these showing increases and two being stable during this period. However, two of the sites featuring increases experienced major declines (from >200 bats to 30 bats) from the mid-1960s to early 1970s, probably due to researcher activity. One of these has subsequently recovered, but the other remains at less than half its former size. Count data for the 1970s-1980s versus the 1990s-2000s are available for only two maternity colonies in the state, with one showing an increase and one a decrease. A third site that held a major maternity roost into the 1930s was abandoned by the 1960s and remains unoccupied by breeding bats. Townsend’s big-eared bats are typically rarely detected during capture and acoustic surveys in Washington. Because this species is difficult to capture in mist nets and has quiet echolocation, standard capture and acoustic surveys may be poorly suited for measuring abundance.
Most day roosts are in caves, mines, abandoned buildings, and attics, but bridges, rock crevices, and very large trees with basal hollows are also used. In Washington, lava tube caves, mines, old buildings, bridges, and concrete bunkers are commonly occupied. Large old-growth trees with basal hollows may have formerly been an important roost type in the state. Temperatures within potential roosting structures are particularly important in the selection of sites, as well as roost dimensions, sizes of openings, light quantity, and extent of airflow. Fidelity to roosts is high in this species, with individuals often returning to the same site or group of sites year after year. Use of multiple roosts within seasons throughout the year is probably common in many areas and may be related to colony size, roost type and availability, or other factors. These bats often aggregate in highly visible clusters on open surfaces within several meters of the ground when roosting at sites with cooler temperatures. Colonies are highly sensitive to human disturbance, but solitary individuals can be tolerant of moderate human activity when roosting in buildings.
Observations in Washington also indicate that these bats tolerate a wide range of temperatures at maternity colonies, especially those in buildings with structural features (e.g., A-frame roofs) that enhance daily temperature gradients. Cooler locations (either within a roost or at different sites) are preferred early in pregnancy, which allows females to enter torpor and save energy. Maternity roosts must also be fairly spacious.
In Washington, maternity colonies have been reported to form in April, begin to break up by mid-August or early September, and are vacant by September or early October. These roosts in Washington and elsewhere in the West usually range in size from about 10 to 250 bats, although large colonies can reach 450 bats. Of 29 recent maternity roost records for Washington, six held fewer than 50 bats, 11 held 50 to 100 bats, six held 101 to 200 bats, one held about 250 bats, and five held undetermined numbers. Maternity colonies appear to represent multi-generational groups of related females. Day-roosting adults spend most of their time resting and grooming.
Townsend’s big-eared bats use night roosts as resting places during foraging and for social interaction. Night roosting occurs in caves, mines, buildings, culverts, and bridges. Dropped insect parts, such as moth wings, can be used to identify night roosts.
Hibernacula occur mainly in caves, mines, lava tubes, and occasionally in buildings. Of the 61 hibernacula reported in Washington since 1980, 46 were in caves, 11 in mines, two in concrete vaults, and two in buildings. Western hibernacula commonly hold single bats or small aggregations of a few to several dozen individuals of both sexes, but rarely may exceed 1,000 bats. Bats begin arriving at hibernacula in October or early November. Abundance peaks in January and mid-February, then declines into April. Hibernating individuals roost singly or in small groups of multiple individuals, and hang in open areas with both ears often curled in the shape of ram horns. Areas near entrances are commonly used. Bats frequently arouse and shift locations within a hibernaculum or move to a different roost to seek suitable temperatures or to avoid disturbance.
Hibernacula feature moderate airflow and stable temperatures typically ranging from -3 to 13°C (27-55°F), with those below 10°C (50°F) preferred. In Washington, winter hibernacula temperatures vary from about -1 to 4°C (30-39°F) in the Cascades, but are about 3°C (5.4°F) higher at coastal Chuckanut Mountain in Whatcom and Skagit counties.
Sensitivity to climate change
Townsend’s big-eared bat is found in a variety of habitats across much of the western United States, including in both dry and moist regions. Distribution appears to be linked to the presence of roosting sites (including hibernacula) with suitable temperatures and nearby foraging habitat. In Washington, roosts include lava tube caves, mines, old buildings, bridges, and concrete bunkers. Climate change impacts to this species are speculative. Increased occurrence of wildfire and insect and disease outbreaks and changes in precipitation patterns could negatively impact Townsend’s big-eared bats by altering or eliminating foraging habitat and drinking sources and reducing the suitability of hibernacula (through changes in humidity, temperature, and air movement). This species feeds largely on moths, making it sensitive to prey availability. Thus, reductions in moth abundance resulting from changes in precipitation or increased spraying for forest pests could negatively affect Townsend’s big-eared bat. In drier regions, periods of drought near maternity sites could reduce reproductive output.
Exposure to climate change
- Changes in precipitation
- Increased temperatures
- Altered fire regimes
- Increased forest insect and disease outbreaks
- Actions to reduce human disturbance and destruction of roosts are considered the most important conservation measures for Townsend’s big-eared bats.
- Management of human access to roosts is strongly recommended, with appropriate activities including sign posting, seasonal road and trail closures, permanent gating, and enforcement of restrictions.
- This species appears to tolerate most types of gating. Roosts should be closed to human visitation during important periods of occupation.
- Surveys of old buildings are important and those with roosts should be repaired or maintained to preserve the structure and be protected through conservation easements, agreements, or acquisitions.
- Expanded survey coverage of mines and caves should be performed before any mine closure or logging is conducted in suspected occupied habitat.
- Timber harvest and associated road building within 400 m of roosts should be restricted during specific seasons to avoid disturbance.
- Alteration or removal of the forest canopy should be avoided above and within 150 meters of occupied caves and mines to prevent changes in temperature, humidity, and airflow in these sites as well as loss of foraging habitat.
- No burning of vegetation should be conducted within 2.4 km of roosts and spraying of insecticides on forests and farmlands should be avoided within 3.2 km of roosts.
Recovery plan: Washington State Bat Conservation Plan (2013)
Fact sheet: Townsend's big-eared bat fact sheet
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.