Media & Education
BATS Magazine

Volume 36, Issue 3, 2017

Species Spotlight: Townsend’s Big-Eared Bat

When it comes to Townsend’s big-eared bats, size matters

By Michelle Z. Donahue


Destruction of habitat is one of the primary threats to this bat species, which have
relocated into thousands of abandoned mines in the western U.S.
Photo: J. Scott Altenbach

My, what large ears you have! The size of its ears, of course, is the first thing one notices about the Townsend’s big-eared bat (Corynorhinus townsendii). Aptly named, this species is widespread across the western United States, Canada and Mexico, dwelling primarily in abandoned mines and caves. When laid flat against its body, the bat’s cartoonish ears reach almost to the middle of an adult bat’s back. Even newborn pups’ ears are prominent, folded down to cover their eyes until they open several days after birth.

Large as they are, those ears are never still: while asleep, bats roll them up, so they look like rams’ horns; while in flight, they can point them straight out, almost parallel to their bodies. As well as the obvious benefits for hearing and echolocation, their mulish ears are also thought to aid in temperature regulation and providing lift during flight. Perhaps this explains why they’re such agile flyers, able to hover and maneuver deftly around obstacles.

Diet-wise, Townsend’s big-eared bats primarily hunt moths, but they also consume flies, lacewings, beetles and other small insects. They emerge later than most other bat species to forage, leaving their roost sites once darkness has already or almost fully descended.

A relatively stationary species, Townsend’s do not migrate, but hibernate in colonies of several dozen to several hundred individuals, keeping their dun- to cinnamon-hued fur fluffed to conserve body heat. Mating occurs during the winter, and females store sperm until the weather warms, gestating for 56 to 100 days before giving birth to a single pup. Females form maternity colonies in caves, mines and human structures, dispersing in late summer to early fall after their pups are weaned and flying on their own.

Photo: Michael Durham / Minden Pictures

Townsend’s prefer cool, rather dark roost sites, which is one reason why BCI’s Subterranean Program team members often encounter them during their work assessing abandoned mines.
“One species we always expect to see is the Townsend’s big-eared bat, and it’s the most common species we encounter,” said Shawn Thomas, BCI’s Subterranean Program Manager. “Their guano is conspicuous—this bright golden-brown color. We’ll see it and say, ‘oh yeah, that’s a Townsend’s.’”

Mines are particularly important for bats like the Townsend’s big-eared, which prefer broad, open surfaces on which to roost. They’re poor crawlers, and won’t creep into cracks and crevices after landing, like other species. They’re also particularly sensitive to human disturbance: left alone, they’ll use a roost site for years on end, but the entire colony will abandon a roost and relocate with even a seemingly minor disturbance.

In California alone, the state’s Department of Fish and Game found that in the late 1980s, the population had declined by an estimated 40 to 60 percent compared with the previous three decades, primarily as a result of loss of roosting sites in mines and abandoned buildings. Only three maternity colonies, all located in national parks, increased in size during that period.

Over the last eight years, BCI’s Subterranean Program teams have been helping federal land management agencies as they systematically identify abandoned mines as candidates for closure. Program Director Jason Corbett, Thomas, and their teams are looking at which specific sites provide important habitat for bat species like the Townsend’s, as well as a range of other wildlife.
When a site is found to host bats, BCI works with its partners to install bat-accessible gates over the mine portals, or shore up collapsing structures. Otherwise, mines are often sealed off or destroyed. With gates, wildlife can still come and go, with the benefit of reduced human disturbance or risk to human safety.

“Thanks in part to efforts from BCI employees and volunteers over the last 30 years, it’s widely recognized mines are important to bats,” Corbett said. “It’s standard practice now when a mine is to be closed that a bat survey is conducted, and if we find it is important habitat, we make sure the bats aren’t harmed in a destructive closure.” 

 

All articles in this issue:

Stay up to date with BCI

Sign up and receive timely bat updates

BCI relies on the support of our amazing members around the world.

Our mission is to conserve the world’s bats and their ecosystems to ensure a healthy planet.

Please join us or donate so our work can continue.