Townsend's Big-Eared Bat (Corynorhinus townsendii)

Townsend's Big-Eared Bat (Corynorhinus townsendii)



General Distribution
Townsend's big-eared bat occurs throughout the western United States, including California, Nevada, Idaho, Oregon, and Washington, from near sea level to elevations well above 10,367 feet (3,160 meters) (Nagorsen and Brigham 1993, Pearson and others 1952). In California, they are found from sea level along the coast to 6000 feet elevation in the Sierra Nevada Mountains (Dalquest 1947, Pearson and others 1952, Pierson and Rainey 1996). Two subspecies, Townsend's western big-eared bat (Corynorhinus t. townsendii) and pale Townsend's big-eared bat (C .t. pallescens), occur in California. The distribution map presented in Hall (1981) indicates that Townsend's western big-eared bat occurs north of Santa Barbara and pale Townsend's big-eared bat occurs south of Santa Barbara. However, the range of overlap of these subspecies is generally considered to be quite broad, and some level of sympatry is likely to occur within this zone (Pierson and others 1999).During 1996-1998, bat surveys were conducted at 76 sites located throughout the four forests of southern California. These surveys were in partnership with the Biological Resources Discipline within the U.S. Geological Survey. Townsend's big-eared bats were found at six of 76 sites on southern California National Forest System lands. One site occurred on the San Bernardino National Forest and the other five sites on the Cleveland National Forest. They were also found at 14 abandoned mine locations in the northeastern San Bernardino Mountains, with 55 individuals observed in one mine. Occurrences have also been documented in several abandoned mines in the Laguna Mountains (Stephenson and Calcarone 1999). Additional occurrences on the San Bernardino National Forest include Arrastre Creek at elevations of 6,400 feet, North Slope at 6,800 feet, Vaughn Spring and the Rose Mine area from 6,800–7,200 feet. In 2002, the U.S. Geological Survey began a series of bat inventory studies in San Diego County funded by several federal, state, and local agencies including the National Park Service, United States Forest Service, California Department of Fish and Game, and the County of San Diego. Four study areas are being surveyed including the Cabrillo National Monument, Descanso Ranger District of the Cleveland National Forest, Santa Ysabel Ranch Preserve, and the Multi-Species Planning Area (MSCP) of eastern San Diego County. Townsend's big-eared bats are suspected to use other mine sites, bridges and buildings on National Forest System lands (Stokes pers. comm.).
Systematics
Two subspecies of Townsend's big-eared bat occur in California: Townsend's western big-eared bat and pale Townsend's big-eared bat.
Habitat Requirements
The distribution of this species is strongly correlated with the availability of suitable caves and cave analogues (mines, rock shelters, tunnels, building) for roosting. Population centers occur in areas dominated by exposed, cavity forming rock and/or historic mining areas (Sherwin 1998). Abandoned mines are particularly important as roost sites in areas where there are not suitable caves (Stephenson and Calcarone 1999). A high degree of site fidelity (more than 80 percent) has been noted for this species (Humphrey and Kunz 1976, Pierson and others 1999). Townsend's big-eared bat can be found in a variety of habitats throughout California, from the moist coastal redwoods to the mid-elevation mixed conifers to the dry deserts, but are most commonly associated with desert scrub, mixed conifer, pinyon-juniper, and pine forest. Within these communities, these bats are most commonly associated with limestone caves, mines, lava tubes, buildings and tunnels (Dalquest 1947, 1948; Dobkin and others 1995; Graham 1966; Kunz and Martin 1982; Pearson and others 1952; Pierson and Rainey 1991). During hibernation, Townsend's big-eared bats typically prefer habitats with relatively cold (but above freezing) temperatures in quiet, undisturbed places. These areas are often in the more interior, thermally stable portions of caves and mines (Barbour and Davis 1969, Dalquest 1947, Humphrey and Kunz 1976, Pearson and others 1952, Zeiner and others 1990). Hibernating bats are often found in ceiling pockets (Pierson and Rainey 1991). In central California, solitary males and small clusters of females are also known to hibernate in buildings (Kunz and Martin 1982, Pearson and others 1952). Females may roost in colder hibernacula than males (Pearson and others 1952). During spring and summer, females establish maternity colonies in the warm parts of caves, mines, and buildings (Dalquest 1948, Pearson and others 1952, Pierson and Rainey 1991, Twente 1955). In California, some maternity roosts may reach 86 º F (30 º C) (Pierson and Rainey 1991). Favored roost locations for the females and young are often in a ceiling pocket or along the walls just inside the roost entrance (Pierson and Rainey 1991). This makes them very vulnerable to disturbance. Interior dimensions are an important factor in roost selection. The majority of the roosts examined in California were at least 100 feet long with a ceiling height of 4 feet (Pierson and others 1991). The proximity of good foraging habitat also appears to be a determining factor in roost selection. In a recent survey in the Panamint Mountains, mines with suitable temperatures were occupied by maternity colonies only if they were within two miles of a canyon with water (Brown 2002). Night roosts may include buildings or other structures (e.g., bridges, tunnels, and mines) (Philpott 1997, Pierson and Rainey 1998).
Reproduction
Female Townsend's big-eared bats form maternity colonies in early spring, usually returning to the same site every year. In maternity colonies, females form tight clusters to preserve body heat. In California, maternity colonies have been found to contain 17–40 adult females (Philpott 1997). They give birth to a single offspring in late spring or early summer after a gestation period of approximately 3 months (Pearson and others 1952). Timing varies by temperature and latitude (Brown 2002, Kunz and Martin 1982). In California, young are born over a 3–5 week period beginning in late May. Townsend's big-eared bats are large at birth, weighing approximately 25 percent of the mother's post-partum mass (Kunz and Martin 1982). The young grow rapidly, reaching adult size in approximately 1 month, and are capable of flight in 2.5–3 weeks. They are fully weaned by 6 weeks (Pearson and others 1952). Maternity colonies disperse in the fall, and mating occurs in the fall and winter. The peak copulation period occurs in November–February, although some females apparently mate earlier, before arriving at hibernacula (Kunz and Martin 1982). Females store sperm, and ovulation does not occur until early spring (Pearson and others 1952). Ovulation may occur either before or after females leave hibernation. Female Townsend's big-eared bats are sexually mature in their first autumn. However, the sex organs of males do not mature until the second year; accordingly, males are not reproductively active their first year.
Daily/Seasonal Activity
Townsend's big-eared bats emerge from the roost approximately 45 minutes after sunset (Clark and others 1993, Pierson and others 1999). Netting studies indicate two peak activity periods during the night (Cockrum and Cross 1964, Pierson and others 1999). Seasonal activity includes three basic periods: 1) fall swarming and copulation; 2) winter hibernation; and 3) spring and summer gestation, birth, and pup rearing. Seasonal activity may involve use of multiple sites. Hibernating bats are known to change their location within the cave or even move to another cave in response to temperature changes or disturbance.
Diet and Foraging
Townsend's big-eared bat feeds primarily on small moths, but also takes other insects including flies, lacewings, dung beetles, and sawflies (Kunz and Martin 1982). This bat flies slowly and is highly maneuverable, foraging both above and within forest canopies (Findley and others 1972, Hayward and Davis 1964, Pierson and others 1999). Townsend's big-eared bat has been observed gleaning insects from vegetation (Howell 1920); however, the extent to which this foraging strategy is used is unknown (Pierson and others 1999).
Territoriality/Home Range
Townsend's big-eared bats often stay within a year-round radius of 20 to 30 kilometers.
Predator-Prey Relations
Little is known about predation on Townsend's big-eared bats. In California, house cats and black rats are suspected to predate on this species (Pearson and others 1952). Black rat snakes, spotted skunks, and ringtails may also have been reported as predators of this species in other parts of its range (Pierson and others 1999).
Literature Cited
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Clark, D.R.; Leslie, D.M.; Carter, T.S. 1993. Foraging activity of adult Ozark big-eared bats (Plecotus townsendii ingens) in summer. Journal of Mammalogy 74: 422-427.
Cockrum E.L.; Cross, S.P. 1964. Time of bat activity over water holes. Journal of Mammalogy 45: 635- 636.
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Philpott, W. 1997. Summaries of the life histories of California bat species. Unpublished document. USDA Forest Service, Sierra National Forest, Pineridge Ranger Station.?Pierson, E.D.; Rainey, W.E. 1991. The importance of mines as roosting habitat for bats: Plecotus townsendii in California. Bat Research News 32: 83.
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Information gathered from California DFG - California Interagency Wildlife Task Group