Bats in winter in Wyoming

Wyoming bats survive the winter by hibernating, and some do it inside buildings. Here is where they go, what it means to find a bat in the house in midwinter, what Wyoming Game and Fish asks you to report, and what you can safely do before spring.

The short version

Wyoming bats hibernate. With almost no insects to eat in winter, they slow down and live off fat stored in late summer and fall.
Some bats spend the winter in buildings. Big brown bats are known to hibernate in attics and walls, and a bat that turns up indoors in winter usually came from inside the house.
Do not touch it. If anyone may have been bitten or touched, or woke up with the bat in the room, call your doctor or county public health office. Rabies precautions apply all year.
Report bats flying outside in winter to Wyoming Game and Fish. It can be a sign of white-nose syndrome.
Winter is for planning, not exclusion. Hibernating bats may not be able to leave through one-way devices. Inspect now, and exclude in spring or late summer.

Where Wyoming bats spend the winter

Wyoming Game and Fish lists 18 bat species in the state, and every one of them eats insects. Once the insects are gone in fall, bats have to hibernate to make it to spring. Game and Fish explains that a hibernaculum is a winter roost where bats can wait out the long cold season without food.

In the eastern U.S., bats gather by the thousands in caves and mines. The West is different. Game and Fish biologists say western bats rarely use those kinds of sites and do not gather in big winter groups the way eastern populations once did. Wyoming’s large hibernacula are not well known. Game and Fish says there is evidence bats hibernate in non-cavernous rocky habitat, such as cracks in rock or talus slopes. That makes Wyoming bats harder to count in winter, and it is part of why the department tracks bats with radio telemetry and winter acoustic surveys.

Buildings are another option. Research published in the Journal of Mammalogy documented big brown bats hibernating in buildings. The CDC notes that most bats leave buildings in fall or winter, which is one reason fall is a good time to bat-proof. A bat colony that is still in a building once hard cold sets in may be hibernating there.

Why a bat shows up in the house in January

In midwinter there are no insects to hunt outside, so a bat flying around the living room in December or January most likely came from inside the building. It was probably hibernating in the attic, a wall void, the space around a chimney, or a gap near a heated room. A warm spell, a furnace or heat leaking into the attic can wake a torpid bat. A bat that is awake and looking for a cooler or more stable spot can slip through gaps around attic hatches, recessed lights, ductwork, plumbing chases and fireplace dampers.

One bat does not always mean a colony. But a winter bat inside the house is a good reason to have the building inspected, especially if you have seen bats there more than once, or noticed droppings, staining at a gap, or bats leaving the roofline at dusk in summer. Our guide to signs of bats in your house explains what each of those looks like.

What to do if you find a bat inside in winter

Treat it as a health question first. Do not touch it with bare hands. The CDC says bat bites can be so small you may not know you were bitten. If a bat bit, scratched or landed on someone, if someone woke up with it in the room, or if it was found near a young child or a pet, call your doctor or county public health office. The Wyoming Department of Health’s 24/7 public health emergency line is 1-888-996-9104. In those cases, the CDC says to contain the bat for rabies testing if you can do it safely, and not to release it until you have talked with public health. Wear leather work gloves, wait until it lands, cover it with a box or container, slide cardboard underneath and tape it shut. Our bats and rabies in Wyoming guide has the full steps and county contacts.

If nobody had contact with the bat, Wyoming Game and Fish’s usual advice is to close off the room, open a window with the screen removed, and let the bat fly out on its own. In a Wyoming winter, a bat let outside on a sub-freezing night may not find shelter, and Game and Fish asks people to call about injured bats. If the bat seems weak or the weather is bitter, call your Game and Fish regional office or the Nongame Program in Lander at (307) 332-2688 before you do anything. Do not keep or try to care for the bat. Game and Fish says that is illegal without a permit.

Bats flying outside in winter and white-nose syndrome

Wyoming Game and Fish says bats flying during the winter could be a sign of white-nose syndrome and asks the public to report those sightings. White-nose syndrome is caused by a fungus, Pseudogymnoascus destructans (Pd), that grows on the bare skin of hibernating bats. Infected bats wake up too often, burn through their fat too early, and may die of starvation or exposure after leaving their winter roost to look for food.

The fungus was first found in Wyoming in May 2018, on a little brown bat at Fort Laramie National Historic Site in Goshen County. In 2021, white-nose syndrome itself was confirmed in bats at Devils Tower National Monument, making Wyoming the 37th state with the disease. Game and Fish reported in 2024 that eight bat species found in Wyoming have proven vulnerable to white-nose syndrome, including the little brown bat, and six more have carried the fungus without symptoms. Game and Fish says people and pets are not affected. If you find a sick or dead bat, report it to Game and Fish and do not touch it.

Game and Fish also works with the USGS National Wildlife Health Center and the University of Wyoming to test bats for the fungus after they have been submitted for rabies testing and come back negative. A bat submitted for a rabies test can also help track white-nose syndrome.

Why winter is the wrong time for exclusion

Humane exclusion depends on bats leaving on their own through one-way devices and finding they cannot get back in. Hibernating or torpid bats may not leave at all. If the openings are sealed while bats are hibernating inside, they can die in the walls or find their way into the living space instead. Wyoming Game and Fish says one-way doors are best used in late summer or early fall. Standard national practice calls for leaving exits up through several nights warm enough for bats to fly (above about 50°F). Most of Wyoming does not see nights like that between late fall and spring.

So we use winter for inspection and planning. We can find and document the entry points, look for staining and droppings, check the attic, and schedule the exclusion for spring, once bats are active and nights warm up, or for late summer, after the pups can fly. See when to exclude bats in Wyoming for the timing rules and the June–mid-August maternity season.

What you can safely do this winter

Protect living spaces. Close the gaps between the roost and the rooms you use: attic hatches, recessed light housings, ductwork and plumbing chases, and fireplace dampers. This keeps wandering bats out without trapping them in the attic or walls.
Do not seal the outside entry points yet. Leave the exterior gaps the bats use alone until they can be excluded.
Do not disturb the roost or use poisons, foggers or sticky traps. Disturbance burns the fat bats need to survive the winter, and poisons scatter sick bats into homes.
Keep pets’ rabies vaccinations current, and keep pets away from any bat found on the ground.
Leave old droppings alone. Bat guano can carry the fungus that causes histoplasmosis. Do not sweep or vacuum it without protection; see guano cleanup.

If you have had bats in the attic or walls, or a bat showed up indoors this winter, call (307) 522-1433. We will tell you whether it sounds like a colony and when the work can be scheduled. Learn more about Wyoming bat species, bat-proofing and our Wyoming bat removal service.

This guide summarizes published guidance from Wyoming Game and Fish, the Wyoming Department of Health and the CDC for homeowners. It is not medical advice. For a possible bat exposure, call your doctor or county public health office.

Common questions

Do bats hibernate in Wyoming?

Yes. There are almost no insects to eat in a Wyoming winter, so bats hibernate to survive. Wyoming Game and Fish says western bats seldom gather in big caves or mines the way eastern bats do. Many likely hibernate in less obvious spots, such as cracks in rock or talus slopes. Big brown bats are known to hibernate in buildings.

Why is there a bat in my house in the middle of winter?

A bat indoors in winter most likely came from inside the building, from an attic, wall void or chimney where it was hibernating. A warm spell or heat moving through the house can rouse it, and it may wander into the living space. One winter bat can mean more are roosting inside, so have the building inspected.

I saw a bat flying outside in winter. Should I report it?

Yes. Wyoming Game and Fish says bats flying during the winter could be a sign of white-nose syndrome and asks people to report those sightings. Report sick or dead bats too, but do not touch or pick them up.

Can bats be removed from a house in winter?

Usually not. Hibernating bats may be unable to leave through one-way exclusion devices, and sealing the openings could trap them inside. Wyoming Game and Fish says one-way doors are best used in late summer or early fall. We use winter for inspection and planning, and we schedule the exclusion for spring once nights warm up, or for late summer.

Can white-nose syndrome make people or pets sick?

No. Wyoming Game and Fish says humans and pets are not affected by the fungus or the disease it causes. Rabies is a separate concern: any bat that may have touched a person or pet, in any season, should be reported to your doctor or county public health office.

Related Wyoming bat guides and cities

Sources

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