To better serve site visitors, Government of Alberta ministry web content is being centralized on Webpages on this ministry site will be either relocated to or removed over the next few months. Messaging and redirects will help guide you to updated content during and after this transition. Scheduled completion date for this project is March 31, 2019. Thank you for your patience as we proceed with these changes.

Big Brown Bat (Eptesicus fuscus)

Big Brown Bat


  • Adult big brown bats weigh 15 to 25 grams
    (0.5 to 0.8 ounces).
  • Wingspan can measure 30 centimetres (12 inches).


  • As its name describes, the big brown bat is brown, the upper side being darker than the underside.
  • The flight membranes are black and unfurred.
  • The big brown bat is probably the most common bat in southern Alberta, particularly in the vicinity of Lethbridge and Medicine Hat.
  • The number of individuals gradually declines in the northern regions with only a few reports from the Peace River region and Wood Buffalo National Park.
Natural History


  • Big brown bats roost in colonies, often in schools, houses, and barns. In the southern part of the province, red brick buildings more than 40 years old are preferred.
  • Big brown bats are relatively sedentary. They use an aggregation of roosting sites within a small local area and usually forage within three or four kilometres (two to two-and-a-half miles) of their day roost.
  • Big brown bats are able to tolerate cold better than most bat species. A few individuals have been found in environmental temperatures of -15°C (5°F); however, usually they hibernate before such extremes are reached.
  • Although few details are known about the hibernation of big brown bats in Alberta, it appears that buildings are the preferred sites. (They are the only species of bat known to hibernate in buildings in Alberta).
  • As the number of big brown bats found in Edmonton is greater in the winter than in the summer, it is thought that the bats may move into the city to hibernate, often in old warehouses where temperatures remain just above freezing.
  • These bats seldom move more than 120 kilometres (75 miles) between nursery and hibernation sites. Some individuals may use the same site as a nursery roost and a hibernaculum (place to hibernate); however, this is unusual.
  • Hibernacula are characterized by low humidity, variable temperature and often, considerable air movement. If the temperature in the hibernaculum drops below -4°C (24.8°F), big brown bats will arouse themselves from torpor and seek a warmer site. This behaviour may explain many of the reports of bats seen during the winter in Alberta.
  • Males and females in the United States appear to hibernate separately in caves and buildings, respectively. It is not known whether this pattern is true for big brown bats in Alberta.


  • Flight patterns and feeding activity are consistent among members of the local population and often are associated with concentrations of night-flying insects. For example, bats roosting two kilometres (1.2 miles) south of Stony Plain, Alberta, fly into town each night to feed on insects gathered around street lights.
Reproduction and Growth

Breeding Behaviour

  • A colony in Alberta may consist of 10 to 80 individuals; southern colonies are slightly larger than those in the north.
  • Summer colonies consist predominantly of adult females; juveniles are present in June, July and August.
  • There is strong site fidelity, with females consistently returning to the same nursery roosts each year, arriving in early May.
  • Parturition (birth) is not synchronized and may be any time from May through June.
  • Litter size in Alberta is usually one young per female.
  • The pregnancy rate is approximately 92 percent in adult females but many yearling females do not produce young until their third summer.
  • The sex ratio at birth is approximately 1:1 and the rate of mortality appears similar in males and females.

Growth Process

  • Juveniles are tended only by the adult females. Birth weights range from 2.5 to 3 grams (0.09 to 0.10 ounces).
  • The young can fly and forage for themselves after four weeks while adult size is attained by ten weeks.
  • Juvenile males may mature and breed in their first fall, but as indicated earlier, in northern populations females rarely produce young until their third summer.
  • Life expectancy is one to two years; however, some individuals live up to 20 years.
Conservation and Management


The big brown bat is classified as Secure in the current General Status of Alberta Wild Species report. See:


Page Information

Updated: April 29, 2009