Quick Answers
Bats often seek shelter in human homes during the winter months when it gets cold outside. They may enter attics, wall voids, chimneys and other areas to hibernate for the season. While some bats migrate to warmer climates in the winter, many species go into a dormant state known as torpor. During this time their heart rates and body temperatures drop dramatically as their metabolisms slow down. They live off stored body fat until spring arrives and insects become available again.
Bats don’t necessarily leave human homes in the winter if they have already roosted there and conditions remain suitable. However, there are steps homeowners can take to encourage bats to leave before winter sets in, such as sealing up entry points, installing one-way exclusion devices, and using lights, sounds and repellents. Persistent bats may need to be removed by a professional wildlife control expert. Preventative action is best taken in late summer/early fall before bats start swarming around homes seeking shelter.
Why Bats Enter Homes in Winter
Bats enter human homes in winter for several key reasons:
Seeking Shelter
Bats need a place to hibernate safely for the winter months when prey is scarce and temperatures drop. Structures like attics, barns and porches provide protection from the elements. Inside, bats can find dark, quiet areas with stable temperatures and humidity. These conditions allow them to enter torpor and conserve energy when insect activity declines.
Accessibility
Many human homes have access points that make it easy for bats to get inside. Gaps around chimneys, vents, eaves and roof shingles may allow bats to squeeze through. Attics with open rafters or walls with spaces between studs also provide accessibility. Bats may return to the same winter roosts year after year if entry remains simple.
Established Roosts
Some bats choose attics, dead trees and caves as summer maternity colony roosts to raise pups. By winter, pups are grown and these roosts transition to hibernation spots. Bats return to familiar roosts already scouted out for safety and insulation. Established roosts have a temperature and humidity buffer during severe cold.
Lack of Natural Sites
Human activities like logging, construction, mining and caving have diminished undisturbed winter hibernation sites. Cave openings and abandoned mines may be blocked off or disturbed by people. Hollow trees are cut down during land clearing. With fewer natural options, bats are more likely to seek out buildings for winter shelter.
Urban Areas
Bats living in urban or suburban environments have access to more human structures. Rural bats may migrate farther or hibernate in cliff crevices, rock outcrops and remaining large trees. But city bats have adapted to utilize manmade roosts year-round within their habitat. Houses, office buildings, stadiums and other structures provide winter refuge.
Do Bats Leave Houses in Winter?
Whether established bat colonies leave houses in winter depends on several factors:
Species
Some bat species migrate long distances to warmer regions with adequate food supply, like Mexican free-tailed bats. But many bats simply enter short distance migration, move between roosts or stay put to hibernate. Species like big brown bats, little brown bats and Eastern pipistrelles are year-round residential bats active in colder climates. They stubbornly remain in human homes where conditions are favorable.
Cold Tolerance
A bat species’ ability to survive freezing temperatures impacts its winter roosting habits. Cave bats accustomed to deep cold are more likely to tough out winter inside houses. But Crevice roosting species and Southeastern populations may move to nearby hibernacula or migrate from frigid locations entirely. Each species has a different threshold for the coldest temperature and length of torpor sustainable.
Roost Suitability
If a house or building provides an ideal roost already, bats have no reason to abandon it in winter. They return to familiar roosts that offer protection from wind, light and external noise while moderating temperature extremes. Attics, walls and chimneys mimic cave-like conditions. With sufficient insulation and nearby heat sources, some roosts maintain above-freezing temperatures that prevent deadly cold exposure.
Energy Conservation
Swarming a new winter roost and scouting new hibernacula uses precious energy bats need to survive hibernation. If current shelter remains suitable, bats opt to stay put and immediately enter torpor. Leaving may actually decrease their chance of survival if new roosts are unknown or inferior. Frequent winter arousals and relocations could be fatally taxing.
Outside Conditions
During fall transition, unseasonably warm spells may delay bats departure until cold weather truly sets in. Early winter cold snaps force bats to quickly take shelter. Bats respond to temperatures, precipitation and availability of insects in their decision to stay or leave. Drastic plunges in degrees and snow/ice storms prompt bats to enter hibernation where they already roost.
Roost Fidelity
Many bat species exhibit roost fidelity – the tendency to return to the same roosts year after year. Familiar shelter is preferred over the unknown. Bats revisit former maternity and hibernation roosts where they bred successfully in the past. Unless forced out by major disturbance, bats stick to consistent roosting patterns. So bats using a house one winter are likely to reuse it the next.
Getting Rid of Bats
While bats may remain in human homes over winter if conditions allow, homeowners usually want to evict their unwelcome guests well before hibernation begins. Here are some common methods used:
Exclusion
This involves carefully sealing up all possible exterior entry points so bats can’t get back inside in fall. Any gaps wider than 1/4 inch must be closed. Exclusion is most effective from late summer through early autumn before temperatures necessitate bats seeking shelter. All bats must be out of the building prior to sealing it up.
One-Way Funnel Devices
These tube devices let bats leave through a one-way funnel exit but not re-enter. They are placed over entry points in summer and left until fall so bats can exit but are excluded thereafter. By winter, bats should have left to find alternate roosts.
Ultrasonic Sounds
Emitting high frequency noises uncomfortable to bat hearing may drive bats away from a roosting site, especially if deployed soon after bats take up residence. However, bats tend to habituate to sounds and lose effectiveness over time. Sounds must be variable and unbelievable annoying to work.
Repellents & Deterrents
Various smelly, sticky or irritating chemical repellents exist to irritate bats and alter roost conditions. Things like ammonia, naphthalene, pyrethrins, bleach, vinegar, capsaicin and moth balls may be used. Some deterrent devices create uncomfortable electric fields or mimic predatory bird calls. Success depends on timing/persistence of use.
Lights
Illuminating dark roost areas convinces bats a site is no longer suitable. Both interior and exterior lights can be used to disturb bats. However, lights must run 24/7 in spaces bats prefer completely black. Humans forget to maintain this consistently, reducing success.
Predators
Installing plastic owl decoys or recorded predatory bird calls around roosts may frighten bats away. Bats interpret the threat of predators nearby as a sign of danger. Scaring bats is most effective before hibernation starts. Predator models must remain in place or bats acclimate.
Smoke/Fire
Used carefully, smoke or fire applied close to roosts may temporarily irritate bats and cause evacuation of a roost. However, smoke also damages buildings and risks dangerous uncontrolled fires. Approach with extreme caution.
Removal Service
For large, stubborn colonies, or when humane exclusion methods fail, calling a professional bat removal service is advisable. They humanely trap and relocate bats offsite, clean contaminated areas, and properly seal up buildings. Though costly, removal services are often the quickest, most thorough option.
Preventing Bat Colonies
To avoid issues with bats attempting to roost long-term in human homes, pest-proofing measures should be taken proactively:
Seal Up Openings
Any access points on your home’s exterior should be sealed with caulk, foam, wood, metal mesh or other durable materials. Pay special attention around eaves, vents, chimneys, attic fans, windows and underneath wood siding. Trim back any nearby tree branches touching the building.
Install Exclusion Devices
One-way exclusion tubes can be placed over potential entry points before bats start seeking shelter in fall. This prevents colony establishment while allowing any individual bats already inside a chance to leave.
Add Lighting
Illuminate the exterior of darkened attic vents, chimneys, crawl spaces and other potential entry points. Install motion-activated lighting to surprise scouting bats. Keep lights on dusk to dawn when bats begin roost shopping.
Use Repellents & Deterrents
Applying smelly chemicals, sticky substances or sound-emitting deterrent devices during peak swarming season may prevent bats from colonizing a new space. Constant variability is key for repellents.
Keep Roosts Unappealing
Avoid having attics, crawlspaces and wall voids that mimic the dark, quiet conditions bats seek. Where possible, install vents to increase air flow and light penetration. Insulate poorly to keep temperatures fluctuating.
Install One-Way Vents
Specialized vents containing mesh or flap barriers allow warm air to escape attics but prevent bats from entering. This makes roost sites less thermally attractive to bats during summer inspections.
Try Predator Models
Placing owl effigies or other fake predators on a home’s roofline and yard deter bats from approaching. The looming threat convinces bats the area is unsafe for roosting. Move models frequently to seem realistic.
Stay Vigilant
Check the exterior of your home regularly each summer for any new gaps or damage that could allow bats entry. Monitor for signs of bats like guano pellets or insect parts around possible access points. Address problems immediately before they establish winter roosts.
Conclusion
Bats commonly seek out buildings to roost in throughout the winter where they can safely enter torpor. Whether established bat colonies vacate homes during winter depends on species characteristics, roost suitability, outside conditions influencing shelter needs, and roost site fidelity. Persuading bats to leave human homes requires employing exclusion methods before winter temperatures arrive. Preventing winter colonies involves proactively modifying buildings to become undesirable to roosting bats during warmer months. With proper timing, vigilance and humane removal techniques, homeowners can successfully evict bats and avoid repeat invasions in the future.