Winter Bat

By: Jim White, Associate Director, Land and Biodiversity

Last week, while cleaning out the rafters of the Coverdale Farm Education Building, my co-worker, Dave Pro and I discovered a bat hibernating in the rafters.  The building is slated for extensive renovations so we had to disturb the bat from its winter rest and relocate it to another building.  Bats can be tricky to identify and I wondered what species we had found.  I knew that most species of bats that occur in our area usually migrate away from Delaware in winter because of the lack of suitable hibernating sites such as caves and mines.  However, at least two species are known to overwinter here – the Red Bat (Lasiurus borealis) and the Big Brown Bat (Eptesicus fuscus).  Our bat did not have the distinctive reddish fur of the Red Bat, so I assumed it was a Big Brown Bat.  Over the years I have learned, sometimes the hard way, that assuming things when trying to identify wildlife can result in misidentification.  So my wife Amy and I examined the bat closely, taking body, limb, and ear measurements and photographing things like the hairs on the feet, and structures of the ear and wings.  These are all traits that can be used to identify bats.  Although we found that the identification was not as easy and straightforward as we were hoping, we were able to confirm that our bat was indeed a Big Brown Bat.

Big Brown Bat by Jim White.
Big Brown Bat by Jim White.

Big Brown Bats have adapted well to the presence of humans and use houses, barns, and other structures as communal summer roosts and nursery sites.  In winter, individual Big Brown Bats also use human structures for hibernation.  These sites, which are usually near their summer roosts, are warm enough to keep their bodies from freezing but cold enough to allow them to hibernate.  Attics and basements of heated structures often provide these conditions.  At these hibernation sites, Big Brown Bats enter into torpor as body temperature falls and metabolism is reduced.  However, this species is more tolerant of cold temperatures than most other bats and on unusually warm winter days it can become active and even fly to seek water.  In early spring the bats leave their hibernation sites to return to the communal roost sites to breed and spend the summer.

It is my hope that our bat will find its new hibernation site adequate, allowing him to survive the winter and rejoin his fellow bats in spring. 

On warm evenings from spring through fall, the Ashland Nature Center is a great place to watch Big Brown Bats as they swoop across the evening sky in search of their insect prey.

2 thoughts on “Winter Bat”

  1. Christine Hudson 302 5420117

    I need help I live in a lot house with metal roof 30 foot ceilings. Have a colony of 75 +living in t he roof in the spring as many as 24 per week fly in my log house. For the past 5 years I have paid various pest companies to rid my house of bats no success

  2. From Delaware Fish and Wildlife: We have a website with information regarding bats in houses: My guess is that the high-roofed, tin-roofed log cabin has holes that the nuisance wildlife control companies have not been able to find or patch for some reason. I wonder if they have hired people with bat experience. The website includes a list of people that are permitted to do bat work in Delaware.

    Let me know if you need anything further,

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