Corynorhinus townsendii
(Townsend's Big-eared Bat)

Order: Chiroptera
Order Description: Bats
Family Description:

The dorsal fur is brownish to grayish brown. The ears, which may be curled like ram horns during torpor Click word for definitionand hibernation Click word for definition, are brown and long (30-39mm); the tragus Click word for definitionis about one-half the length of the ear. Glandular lumps occur on the face between the nostrils and eyes.

A map of the state-by-state distribution of this species showing maternity sites, hibernacula Click word for definition, mist-net capture sites and museum voucher specimen sites was prepared in 1995 and is referenced below. This bat is found in British Columbia, east through Montana, south into western south Dakota, Nebraska, Kansas, Oklahoma and Texas, south into Mexico and along the Pacific Coastal States of California, Oregon, and Washington. In Idaho, it has been collected or observed in hibernal sites in 17 counties. Four maternity sites have been found in Boundary, Bonner, and Butte Counties. Many abandoned mine sites, often extremely dangerous to enter, are used by nonbreeding females and males for roosting in summer.

Townsend's bats occur in a variety of habitats from desert shrub to deciduous and coniferous forests at a wide range of elevations. In Idaho, some individuals likely migrate to hibernal sites to overwinter and disperse to forested areas during summer when the sexes separate. Other individuals found near Lake Pend Oreille seem use the same mine during both summer and winter. This species is captured or observed in abandoned mines and both unoccupied and actively used old buildings where it may be considered a pest as a result of deposition of guano and urine. In California, two radiotagged females have been relocated in tree cavities where they remained for several days. It is probable that hollow cavities in large trees or snags may constitute an important undocumented resource for maternity colonies of this species, a condition that may be displacing some nursery colonies to man-made structures.

On West Coast, found regularly in forested regions and buildings. In Texas, ranges from shrub steppe to pinyon/juniper woodlands, but is consistently found in areas with canyons or cliffs.

C. townsendii are moth specialists. Their diet consists mostly of moths in the family Noctuidae. Beetles and flies and lesser amounts of other insects may also be consumed.

The winter ecology and summer distribution of this species is under intensive investigation in Idaho as a result of concern about declining numbers observed at winter hibernal sites. Mines and caves that have been surveyed by netting during summers usually yield from one to six individuals. In northern Idaho, winter hibernal sites contain fewer than a dozen individuals. The largest maternity colony, containing a total of 50 females and pups, occurs in a actively used outbuilding in Bonner County. Thermal profiles for this structure suggest pups are delivered in cooler conditions that documented in other states. In southern Idaho, several lava tube caves exist that contained from 150 to 400 overwintering individuals. The larger population has declined significantly, but it is unclear whether the decline resulted through human induced mortality as a result of disturbance, displacement, or as a result of a natural tendency for this species to shift site locations.

In western range, species seems to prefer cool, damp sites for hibernation Click word for definition; hibernacula Click word for definitionaverage 38° -54° F. Hibernates Click word for definitionsingly, or in clusters in some areas. Maternity and hibernation colonies occur exclusively in caves and mine tunnels. Often moves between caves, even in coldest weather. Does not use crevices or cracks; hangs from ceiling, generally near zone of total darkness (in Idaho, individuals hang in exposed, open areas of cave). Occasionally uses buildings, bridges, and tree cavities for night roosts. Forages near foliage of trees and shrubs; foraging activity usually begins well into night. Population densities of western populations are approximately 1 bat/139 ha. In Idaho, individuals are sedentary and have high degree of site attachment.

Mating takes place during the fall, and ovulation Click word for definitionand implantation in spring. A single pup is produced by each female in June in northern Idaho. Maternity colonies may require warmer temperatures to speed pup development, but more research is needed on the variation in temperatures required for successful rearing and recruitment throughout the range of this species.

Mating begins in autumn and continues into winter. ovulation Click word for definitionand fertilization Click word for definitionare delayed until late winter/early spring. gestation Click word for definitionlasts 2-3.5 mo. Female produces 1 young, in late spring/early summer. Young are weaned Click word for definitionby 6-8 wk, and fly at 1 mo. Females reach sexual maturity in first summer; males are sexually mature in second year (California). Females form nursery colonies, of up to 200 (western range) or 1000 (eastern range) individuals; males roost Click word for definitionseparately, in small groups, or singly during summer.

Status: Unprotected nongame species
Global Rank: G4
State Rank: S2

Important State References:
Wackenhut, M.C. 1990. Bat species overwintering in lava-tube caves in Lincoln, Gooding, Blaine, Bingham, and Butte Counties Idaho, with special reference to annual return of banded Plecotus townsendii. M.S. Thesis, Idaho St. Univ., Pocatello. 63pp.

Information written by Barry Keller,© 2000
Map image provided by
Stephen Burton,© 2000
Design by Ean Harker©1999, 2000.