Townsend’s Big-eared Bat (Corynorhinus townsendii)

Description: Townsend’s Big-eared Bat can be distinguished from all other vesper bats by the presence of prominent, bilateral nose lumps (glands) and outlandishly large ears. The ears may reach a length of 38 mm and when laid back they extend to the middle of the bat’s body. Their fur is pale gray or brown above and buff colored on the underside.

One of our Central Coast Bats

Western North American Range & Distribution: In the west, Corynorhinus townsendii occurs throughout and is distributed from southern British Columbia south along the Pacific coast to central Mexico and east into the Great Plains. In California, it is found (virtually) throughout except at the state’s highest elevation mountaintops. Along the Central Coast, we most often observe or detect Big-eared Bats in and around oak riparian habitats as well as dense oak woodlands, along riparian corridors, and other mesophytic habitats. Foraging associations include edge habitats along streams, adjacent to and within a variety of wooded habitats. The species is rare or absent from areas with intense agriculture.

Roosting: Colony roosts are strongly correlated with the presence of caves and cave-like roosting habitat. Population centers – that is, where the species is most common – are typically associated with areas dominated by exposed, cavity-forming rock and/or abandoned mines. Although these big-eared bats prefer open roosting areas in large rooms or caverns and typically do not sequester in cracks and crevices, individual males and small bachelor colonies may take up residence in a wider variety of roosting habitats and host structures.

Feeding: Big-eared Bats emerge early in the evening and appear to utilize regular, predictable feeding routes – during our surveys, we have numerous sites where single or small numbers of individuals can be regularly found. The Big-eared Bat diet consists entirely of insects, captured in flight. This is a moth specialist with over 90% of its diet composed of lepidopterans. However, and not specific to the Central Coast, Corys may also feed on beetles, stoneflies, Mayflies, true bugs, nerve-wings, scorpionflies, caddisflies and probably only occasionally, bees and wasps. These bats often travel large distances while foraging; movements of over 150 kilometers during a single evening have been documented.
Migration: Local (Central Coast) meta-populations appear to overwinter. Individuals may be active (albeit periodically) throughout colder months and probably do not truly hibernate. In more northerly (or higher elevation) climes, Corys may migrate or seek hibernation quarters in caves or buildings. Overwintering colonies are typically composed of males & females and such colonies can range in size from a single individual to colonies of several hundred individuals.

Breeding: Mating generally takes place between October and February. Maternity colonies – from a few individuals to several hundred individuals in size – form between March and June (w/individuals at higher elevations and in colder climates favoring later dates) and females give birth, typically, to a single pup born between May and July. Because of their non-migratory status, there is a quite high roost fidelity to roost sites along the Central Coast. Males appear to be solitary during the maternity period and we have rarely observed more than one or two (assumedly male) individuals during surveys at any one locale away from maternity roosts.

Threats: The primary threat to Townsend’s Big-eared Bats in California is related to disturbance and/or destruction of roost sites; for example, recreational caving or mine exploration, mine reclamation, and renewed mining activities. Population declines (and possibly site reclamation) may result from reduction or elimination of proximate human activities and roost protection (e.g., by installing human exclusion features including grating and gates of several types including ladder gates and culverts w/full gates. Other factors may adversely affect Corys in other parts of its range including reduced genetic diversity where meta-populations become isolated, poisoning due to abuse of insecticides, and loss and reduction of foraging habitats.

In general, the long-term persistence of North American bat species is threatened by:
• Loss of clean, open water;
• Chemicals in the environment that affect bats or their prey;
• modification or destruction of roosting and foraging habitat; and, for hibernating species,
• disturbance or destruction of hibernacula