California Department of Fish and Wildlife

Bay Delta Region

Studies and Surveys

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Main Office
  2825 Cordelia Road, Suite 100
  Fairfield, CA 94534
  (707) 428-2002

Stockton Office
  2109 Arch Airport Rd
  Stockton, CA 95206
  (209) 234-3420

Acting Regional Manager:
Scott Wilson

Related Programs

Stanislaus River Report

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Stanislaus River Basin and Calaveras River Water Use Program
Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept of Fish and Game

Townsend's Big-eared Bat

Plecotus townsendii
Category 2 candidate for listing by the U.S. Fish and Wildlife Service as a Threatened or Endangered, and a California Department of Fish and Game "Species of Special Concern"
Two subspecies are recognized in California by E. Raymond Hall (1981) P.t. pallescens and P.t. townsendii.

Life History

The Townsend's big-eared bats have large forward facing ears measuring 25 mm long and are joined across the forehead. The forearm is 39.2 to 47.6 mm, and body weight is 9 to 11 g. There are two glandular lumps between the nose and eyes. The upper and lower body parts are grayish-brown in color with the hairs darker at base. Big-eared bats have a pointed tragus and no keel on the calcar.

Townsend's big-eared bats' roosting sites are restricted to caves and cave-type dwelling such as tunnels, mines, and bridges. This species is perhaps the most characteristic of bats to dwell in caves and abandoned mine tunnels (Barbour 1969). In California, nursery colonies can be found in mines and sometimes in the attics of buildings. They are rarely captured in mist nets (Barbour 1969) and even though they are found throughout California their distribution is poorly known and numbers have reportedly declined steeply in California (Zeiner 1990).

This bat is thought to be a moth specialist, foraging by gleaning insects from shrubs and trees while feeding along habitat edges. Peak activity occurs in the late evening. From October to April they are in their hibernicula roosting singly or forming small clusters. Kunz and Martin (1982) show the longest movement of 1500 banded bats was 20 miles. This species shows a high site fidelity if undisturbed. The males are solitary in the spring at which time the females form maternity colonies. Reproduction occurs (some before hibernation) from October to February and births occur from May to June. One young is born, weaned in six weeks, and flies by three weeks of age.

Big-eared bats are found in all habitats except subalpine and alpine. P.t. townsendii is found only along the inland half of the west coast. In California, Hall (1981) lists records at Happy Camp; Pope Creek, 8 miles north west of Monticello; 4 miles southeast of Mission San Jose; Hernandes; 6.5 miles southeast of Shanson; 5 miles south-southwest of Adelaida; and along the entire pacific coast. P.t. pallescens is found over the majority of the western half of the United States except along the Pacific coast north of the Channel Islands. Hall's 1981 records in California include: Potholes; Duluzura; San Clemente Island; Santa Catalina Island; San Nicolas Island; Santa Cruz Island; Old Fort Tejon, near Lebec; 4 miles south east of Porterville; Auburn; mouth of Battle Creek, near Bloody Island; Lava Beds National Monument; and 5 mi. southwest of Tule Lake.

Findings and Conclusions

Reasons for decline of this species include loss of suitable roosting habitat which includes destruction and disturbance, and to some degree, pesticides. They are extremely sensitive to disturbance at roosting sites and all known nursery colonies in California's limestone caves have been abandoned. Few maternity colonies have been found in buildings.

The Stanislaus River flows through the ranges of both subspecies, thus they could occur within the project site.

Our limited surveys did not result in finding big-eared bats. Elizabeth D. Pierson Ph.D. did, however, locate one animal using an irrigation tunnel in the Oakdale area (BioSystems 1994). Dr. Pierson concluded that these tunnels offer potential roosting habitat and more bats could be using these tunnels seasonally throughout the year.

Since big-eared bats are known to the area there is potential for a maternity colony in caves or tunnels. Any project involving flows of the Stanislaus River should not interfere with any of the big-eared bats roost sites or foraging areas and should have little to no impacts on big-eared bats. Those projects affecting foraging areas may have impacts and would warrant further studies.