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  • Writer's pictureChloe Robinson

Species Spotlight: Big Brown Bat

To celebrate the run up to Halloween, we want to shine the moonlight on each of the eight bat species of Ontario. In this series of blog posts, we will be highlighting the key info for each species and will provide some exclusive recordings we have collected this year in our Batacea Guelph Bat Watch project.

Digital drawing of big brown bat
©Batacea 2020 (based on photo by Brock Fenton)

In this post, we will be spotlighting the most common and abundant bat - the big brown bat (scientific name: Eptesicus fuscus).

Species Description

Big brown bats are a medium-sized bat, with predominantly brown-gingery glossy fur on top with a lighter brown to grey underside. Their wing membranes and snout are hairless and they have a wingspan of up to 39 centimetres.

Range & Ecology

This species can be found across the majority of Canada, particularly around the Great Lakes. They can also be found from Alaska to northern South America and into the Caribbean islands.

Big brown bats are commonly found in and around urban areas, particularly in the winter months. They roost in a variety of locations, including houses, barns, churches, storm sewers, mines, bat houses, loose tree bark, tree cavities and rock crevices.

Their diet typically consists of a range of flying insects, especially beetles and moths. They consume a number of important pest species, including mosquitoes and agricultural pests such as western bean cutworm.

Female big brown bats form maternity colonies of sometimes up to 300 individuals. Mating occurs in the fall, however female big brown bats store sperm over hibernation and becomes pregnant in the spring. They have a gestation period of around two months before giving birth to one or two pups in May/June.

Conservation Status

Big brown bats are currently classified as 'Least Concern' according to the IUCN Red List, with current population trends suggesting an increase in numbers.

Despite their conservation status, big brown bats are protected in Ontario and still face many threats, particularly relating to human conflicts due to choice of roosting locations. White-nose syndrome, a fungus that infects the exposed skin of hibernating bats’ muzzles and wings, is less of a problem for big brown bats due to the fact they are medium-sized and tend to roost in warm locations.

Acoustic Recordings

Throughout our 2020 summer surveys, big brown bats have been by far the most common species we have detected. Out of 1100 recordings, 57% were from big brown bats.

Big brown bat and silver-haired bat calls overlap considerably, which can sometimes make distinguishing the two species difficult. Spectrograms from big brown bats form a smooth curve, beginning around 70 kHz and ending around 30 kHz.

Bat spectrogram
©Batacea 2020

They also produce harmonics, which can be seen above the main call. These higher-frequency harmonics form a narrower beam than the widespread low harmonic and diminish more quickly in the air (i.e. are weaker), which assists in focusing echolocation efforts when searching for insects.

To listen to the big brown bat call snippet above, download the MP4 file below.

Big Brown Bat_24 August 2020
Download ZIP • 68KB

More Info

For more information on big brown bats in Canada, visit the Nature Conservancy Canada (NCC), ON Nature Magazine's Bat Guide or Neighbourhood Bat Watch websites.

To explore more big brown bat spectrograms, check out the iNaturalist Bat Spectrogram project.

This is the first in a series of eight blog posts on Batacea. Check the Batacea blog page next week for exciting info on our next species highlight.


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