Two types of bat problems. 1 solution.

Ontario homeowners call me about two types of bat problems

1. Bats inside The House

The most common calls I get are about bats showing up inside houses.
It usually only happens occasionally, with homeowners reporting one or so per year (primarily in summer), or even only one every few years. I have also had clients who lived for decades in a home with no bat encounters and were shocked the first time one showed up.

Was It A fluke?

Homeowners believe the first few bats to be “flukes” that came in through an open door or window.

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It usually takes several “flukes” before they believe otherwise. Many homeowners will then do a “bat watch” to figure out where the bats are getting in, and how many they have. They are greatly confused if they see and hear nothing, and bats continue to show up inside.

This happens because they are dealing with big brown bats.

Most are gangs of males.
Unlike larger maternity colonies, which are more obvious (and where males are not welcome), male big brown bats form colonies with a handful of members and are rather transient, moving quietly (and frequently) back and forth among several houses.

In the majority of cases where homeowners have bats (especially in urban settings), they are dealing with these male colonies. They are nocturnal, secretive, and live quietly in millions of North American houses. Most homeowners never know they have bats.

That’s because big brown bats typically enter roofs and attics high up and can usually enter your living space only by coming down interior walls beside pipes, chimneys, and ducts to unfinished basements and furnace rooms.

Other potential access points are through chimneys and vents, but the basement remains the No. 1 place where bats first get out of the wall and into your living space.

Fortunately for most Ontario homeowners, houses usually have sufficient barriers and framing within the wall to stop the bats getting down to the basement. They are also generally quiet in walls and are not too often heard, with any noises being attributed to the far more abundant common house mouse.

But if you are one of the unfortunate homeowners where bats can get out of the walls….

Once Inside Bats Can Move From Room To Room By Crawling Under Doors.

Most of the bats that show up inside are exploring, especially first-year immature bats checking out their environment. Others are drawn to the cool air of the basement during summer, and some mistake the cool air of the basement as being outside air.

Bats that live in houses nearby will scout out the walls and attics of neighboring houses looking for other potential roosting and hibernation spots. Big brown bats are typically year-round residents, and where they choose to live in your walls and attics will depend on the time of year, as an ideal summer roost spot in one house will not be good for hibernation.

In winter, big brown bats will hibernate in exterior walls, under attic insulation, and in finished basements (they love finished basements).

They will wake up from hibernation every few weeks to relieve themselves, to mate, and stretch their wings. They most often relieve themselves in attics (attics are mostly a giant toilet for big brown bats) or basements. The majority of bats encountered inside houses in winter are looking to relieve themselves in your basement, first-year bats that will not survive their first winter (all too common), and older, sicker bats that come out because they are dying of starvation and dehydration.

Other than winter, if you encounter a bat in your bedroom in the middle of the night, it is probably a lost bat that cannot figure out how to get back up into the wall from where it came. It’s now going from room to room, desperately looking for a way out of the house before it dies of dehydration. Don’t be surprised if after encountering a bat inside you soon can’t find it anywhere. Bats have a maddening habit of showing up inside a house and disappearing very quickly. There are very good reason why they vanish which I talk about in detail in this video.

The majority of big brown bats that show up inside houses are not rabid (big brown bats have the lowest known rabies rate). As the oldest living bat species in North America, they appear to have lots of natural resistance to rabies and often survive it. It also appears that they don’t get the brain encephalitis that drives other species mad and may cause them to attack.

As of 2026, there are no known cases of rabies deaths attributed to big brown bats in Canadian history. None in Mexico and only 3 suspected cases (not definitive, but likely) in U.S. history.

Conclusion….

If you are going to have bats, you want them to be big browns, as they are the least statistically dangerous, and mostly live in small transient groups that regularly move to other houses and therefore the amount of damage they can do is minimal at worst.
Unless you have one of the less common but much larger maternity colonies, which brings me to my next point.

The other type of bat problem I am called about is..

Bats in the Attic.

The largest colonies of residential big brown bats are the female maternity colonies where somewhere between 12 and 30 females (on average, based on my observations) will gather together and raise their babies collectively.

By late summer, the population will have nearly doubled. And because of their larger numbers, maternity colonies are more easily discovered, with more bats witnessed entering and exiting the building and more guano observed both in attics and outside

Still, of the many houses we work on each year, few have maternity colonies, and I have rarely seen a colony with more than a hundred big browns.

So if you have an unusually large colony of bats, you may be dealing with Ontario’s only other species that lives in houses: the little brown bat.

Little brown bats

Of the two bat species that live in Ontario houses, only little brown bats consistently form large colonies capable of doing enormous damage to houses with their indiscriminate guano and urine production. Up until 2013, a residential colony of 200-500 little brown bats was considered small, and we regularly worked on houses with thousands.

Little brown bats are the worst

They literally get into everything!

For Ontario homeowners, little brown bats are a punch in the face. There is nothing subtle about them. They get into everything. Attics, chimneys, walls, additions. They would live behind your siding and shutters and even show up roosting in patio umbrellas and barbecues.

If you put up a bat house? Sure, no problem, they would infest that too, but it would not cause them to move out of your house.

For the first 20-plus years of my career, the majority of the houses we worked on had little brown bats. They made up roughly 80% of our jobs. And they were a nightmare to deal with. Especially with the stench of urine and bat guano everywhere on the building.

However, dealing with little browns proved to be an excellent education because after we did our batproofing work, and set our excluders, they would show you every small, stupid, ridiculous, idiotic spot they could possibly use to squeeze back into the house.

If you missed anything, they would find it, and hundreds would get back in. They never took being excluded lying down. We often found dead and near-dead bats, stuck trying to squeeze into some impossible spot.

This dance would go on for years as they would return each spring from theri hibernation caves and try once more to get back in. And if there was any winter damage to the house? Bam! They got back in. A woodpecker or squirrel chewed a small hole? Boom! They found it.

It forced us to level our game up to heights we didn’t believe necessary at first.
The sheer amount of prevention work needed to keep little brown bats out still astonishes us to this day.

However, by 2013, a European fungus (first identified in 2006 in New York State) that killed bats in their hibernation caves had spread across Ontario and devastated little brown bat populations. The declines were believed to be greater than 90%.

Spread Of White Nose Syndrome 2006-2013.

Most of our little brown bat jobs were in Ontario cottage country (particularly Canadian Shield and boreal forest areas, as well as along Ontario rivers, streams, and lakes).

But by 2013, that work largely vanished, giving home and cottage owners across Ontario a break from the extreme damage and costs associated with having little brown bats in their buildings. (A typical little brown bat job involved not only excluding thousands of bats, but major prevention work and then the abatement of large quantities of guano and contaminated materials.)

We personally had believed that little brown bats were gone for good; there was legitimate concern they would go extinct. We spent the next decade focused on urban big brown work. Which was, strangely enough, a relief. It was only when we had no little brown bat work that we realized just how stressful they were compared to urban big browns. And we ended up being busier than ever, so we didn’t really miss them at all.

They’re back!

However, by 2020, we noticed a marked rebound in little brown bat calls, and by 2025, fully half of our jobs once again involved little brown bats. Still, few of the colonies we dealt with reached more than a hundred or so. Tiny compared to what we dealt with historically. But they appear to be well on the way to recovery, rebounding at a tremendous rate, which is great news for the environment but lousy for Ontario homeowners.

Little browns are currently on the endangered species list. It’s illegal to exclude them during maternity season. How government can enforce this edict, I am not sure. It’s still common for homeowners who can’t afford proper bat control, or resent having to spend any money, to resort to inhumane attempts to kill the bats rather than deal with them properly. As these are early days in their recovery, I imagine it will only get worse as their numbers continue to grow. When I talk to homeowners with little browns now, many don’t want to believe me when I tell them it used to be so much worse.

Wait a few years, I tell them, and watch how bad it gets.

Ultimately regardless of species, and regardless whether you have the odd bat showing up inside, or thousands infesting your walls and attic, the cure is always the same. Bat-proof the house and humanely exclude the bats.

Explaining that requires a whole blog to itself, the link to that is below.

Benjamin Vaughan/Bat Specialist

Bye Bye Bats! How We Get Them Out Of Your Home

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