Bats in Winter in Colorado: Where They Go, What a Winter Bat Means, and What to Do Before Spring

By November most of the bats that spent the summer in Colorado attics have left. That makes winter the quiet season for bats, and the best time to fix the openings they used. It also means a bat you do see in winter deserves a closer look. Here is what Colorado wildlife and health agencies say about bats in the cold months.

Where Colorado bats go in winter

Colorado Parks and Wildlife says roughly 20 bat species live in the state. Some stay year-round and some only pass through on migration. The resident species survive winter by hibernating, and in Colorado they mostly do it away from houses.

The best evidence comes from Fort Collins. In 2002–2004, researchers from Colorado State University and the U.S. Geological Survey radio-tagged 56 big brown bats at maternity roosts in Fort Collins buildings during August and September. Of the 55 bats in the analysis, 24 were later found in rock crevices at higher elevations in the Poudre River watershed, 24.5 to 87.4 kilometers (about 15 to 54 miles) from the city limit and 322 to 1,369 meters higher than their summer homes. One bat traveled 56 km (about 35 miles) from its maternity roost to a mountain crevice in a single night. The deeper, steadier-temperature crevices were used for a week or more and are presumed to be hibernation sites.

The Colorado Bat Working Group, hosted by the Colorado Natural Heritage Program at Colorado State University, sums up the follow-up work: when buildings with known summer bat use were searched in winter, bats were not found hibernating in them to any notable extent. Telemetry and surveys have instead documented Colorado bats in caves, mines and rock crevices in winter. The Working Group notes that bats do use buildings in winter in the Midwest, likely because natural roosts are scarce there, and that Colorado has plenty of winter roost options, with the possible exception of the Eastern Plains.

The Colorado Department of Public Health and Environment (CDPHE) describes the same pattern from the homeowner's side. Summer colonies form in homes to raise young, the bats migrate away in the fall, and that is when the outside entrances can be sealed so the colony doesn't come back next year.

What a bat in winter can mean

Because most bats are hibernating somewhere else, a bat flying around in January or lying on a sidewalk in February is unusual. There are a few explanations:

A bat woken from hibernation. Hibernating bats live off stored fat. CPW explains that bats infected with white-nose syndrome rouse from hibernation more often, burn through their fat too early, and can starve. A bat out in midwinter may be one of them.
A bat in a building on the plains. The Working Group's exception for the Eastern Plains means a winter bat in a barn or outbuilding there is less surprising than one in a Denver suburb.
A migrant or a late straggler. CPW says that during migration it is normal to find a bat hanging under an eave, behind a shutter or under a porch roof, and that it will usually move on if left alone.
A sick bat. CPW notes that daytime activity is not always a sign of rabies, but abnormal behavior can be a sign of illness, and any bat acting abnormally should be treated as a suspect.

Report winter bats to CPW's white-nose syndrome line

White-nose syndrome is caused by a fungus that grows in the same cold range found in hibernation sites, roughly 14 to 59°F according to CPW. It affects bats only while they hibernate, when their immune systems are suppressed. The fungus was confirmed in Colorado in the summer of 2022, and the first bat in the state to die of the disease, a Yuma myotis, was found at Bent's Old Fort National Historic Site in spring 2023.

CPW currently lists the disease as confirmed in Boulder, Douglas, El Paso, Grand, Jefferson, Larimer, Otero, Routt, Teller and Weld counties, and the fungus alone has been detected in counties including Baca, Jackson, Montezuma and Pueblo. Species confirmed with the disease in Colorado include the big brown bat, little brown myotis, Yuma myotis, fringed myotis, long-legged myotis, western long-eared myotis and western small-footed bat.

To track the spread, CPW asks the public to report any active or dead bat seen from December through June (December through July at high elevations) by calling 303-291-7771 or by email. For bats you find from July through November, or any time someone has touched a bat without protective gloves, CPW says to call your county health department right away instead. Do not handle the bat; CPW suggests placing a box or can over it so it isn't disturbed.

A bat inside in winter: the rabies rules don't change

Rabies doesn't take the winter off. CDPHE names bats as the main rabies carrier throughout Colorado, and CPW notes that a bat bite can be a pin-sized puncture or leave no visible mark at all. If a bat turns up indoors in winter, the same rules apply as in summer: if it touched anyone, landed on someone, was in a room with a sleeping person, a young child or a pet, don't release it. Close it in the room, call your county health department, and if you are told to collect it, use thick gloves and a container. Our guide to finding a bat in your house lists county phone numbers.

A bat in a living space in winter is also a clue about the house. It may mean a bat got in through an opening while moving between roosts, or that some part of the structure, such as a chimney, a basement, or a wall void near a heat source, is being used. Either way, it is worth a professional inspection before spring.

Why winter is the time to bat-proof

CPW asks property owners not to seal bats out of buildings during pupping season, June through August, because trapping bats and their young can push them into living spaces. The Working Group's advice is similar: if the work can wait until the maternity colony disperses in autumn, wait, and avoid the June to mid-August birthing window. CDPHE says the external entrances can be sealed once the bats have migrated away in the fall. El Paso County Public Health spells out the calendar: do bat-proofing in winter, November through March, after the bats have left, using materials such as quarter-inch hardware cloth, screening, sheet metal and caulk.

Winter work still has to be done carefully:

Confirm the colony is gone. Look for fresh droppings, staining at entry points and, on a mild evening, watch at sunset for anything exiting. The Working Group recommends identifying every exit used before any work starts.
Use one-way devices where there is doubt. If a gap may still hold bats, a one-way door lets them leave without letting them back in. The Working Group recommends running one-way devices for 5 to 10 days before final sealing, and longer if stormy nights keep bats inside.
Seal every thumb-sized opening. The Working Group says bats can use any gap a thumb fits into and will switch to a new entrance when the main one is blocked. Ridge caps, gable vents, fascia gaps, chimney flashing and the joints where additions meet the original roof are the usual suspects.
Clean up what the colony left. With the attic empty, guano and soiled insulation can be removed. Bat and bird droppings can harbor the fungus that causes histoplasmosis, so NIOSH guidance calls for respiratory protection and dust control, not a shop vacuum and a dust mask.
Use clean gear. The Working Group asks wildlife operators to follow White-nose Syndrome National Plan decontamination guidance so equipment doesn't carry the fungus between sites.

If the bats return in April or May and find every opening closed, they move on to another roost before the pups are born. That's the goal.

A winter checklist for Colorado homeowners

Walk the outside of the house on a clear day and look for brown staining or droppings below gaps at the roofline, gable vents and chimney.
Check the attic for guano piles and stained insulation, and note where they are. They mark the summer roost.
Make sure chimney caps are on and window screens are intact before spring.
Keep dogs and cats current on rabies shots. CDPHE says a vaccinated pet exposed to a rabid or suspect animal needs a booster within 96 hours.
Report a bat seen outdoors between December and June to CPW at 303-291-7771, and any bat that touched a person or pet to your county health department.
Book exclusion and repairs for the November–March window so the work is done before the colony comes back.

Frequently asked questions

Do bats hibernate in Colorado attics?

Rarely. The Colorado Bat Working Group says searches of Colorado buildings with known summer bat use found that bats do not hibernate in them to any notable extent. A Fort Collins study tracked big brown bats from city maternity roosts to rock crevices in the mountains for the winter. Buildings on the Eastern Plains are a possible exception.

I saw a bat outside in January. What should I do?

Don't touch it. CPW asks people to report active or dead bats seen from December through June to its white-nose syndrome line, 303-291-7771, because winter activity can be a sign of the disease. If anyone or any pet touched the bat, call your county health department right away.

Is it OK to seal bat entry points in winter?

Yes, that is the recommended time. CPW asks owners to avoid sealing during June–August pupping season, CDPHE says outside entrances can be sealed once bats have migrated away in fall, and El Paso County Public Health recommends bat-proofing from November through March. Confirm no bats remain first, and use one-way devices on any opening where you are unsure.

Where is white-nose syndrome in Colorado?

CPW lists the disease as confirmed in Boulder, Douglas, El Paso, Grand, Jefferson, Larimer, Otero, Routt, Teller and Weld counties. The fungus alone has been detected in counties including Baca, Jackson, Montezuma and Pueblo. It was first confirmed in Colorado in 2022.

Can a bat that's inside my house in winter have rabies?

Yes. Rabies can occur in any season, and CDPHE names bats as the main rabies carrier in Colorado. If the bat may have touched someone or was in a room with a sleeping person, child or pet, don't release it; close it in the room and call your county health department.

Sources

See our bat removal service

Bats in your attic this fall? Schedule an inspection.

Licensed exclusion timed to Colorado's fall window, from Denver to Boulder and the foothills.

Call (720) 248-8581 Schedule online