Big Brown vs. Little Brown Bats: An Ontario Homeowner's Guide

Two bat species turn up in Ontario homes most of the time: the big brown bat and the little brown bat. Most homeowners do not know which one they have — and most of the time it does not change what to do. But the species matters for protected status, roosting patterns, and a few practical decisions that shape the inspection and the exclusion plan.

Quick visual ID

The two species look similar at first glance, but the size difference is real and easy to spot once you know what to compare.

Little brown bat. Smaller — roughly 9 cm wingspan and 7 to 9 grams. Glossy brown fur, often with a slight sheen in good light. Round face, small ears, and a compact body. This is the bat most people picture when they hear the words “small brown bat” — the classic Ontario barn bat.

Big brown bat. Larger — roughly 14 cm wingspan and 16 to 18 grams. Slightly darker fur, sometimes with a more rust-toned or chocolate cast. Stockier face with a more pronounced muzzle, and a heavier body overall. About double the body mass of a little brown bat, even though the wingspan is only about fifty percent larger.

In flight. Big brown bats fly slower and lower. You will often see one cruising over a yard, a porch, or a streetlight at dusk, working flying insects at roof-line height. Little brown bats fly faster and higher, often above tree-line, and twist more sharply between insects.

In hand — but please do not. Little brown bats are much smaller in the hand, about half the body mass of a big brown. We mention this so the difference is concrete, not because it is something to verify yourself. Never handle a bat with bare hands. The rabies risk is small but real, and any direct contact warrants medical attention. See our post on bats in your attic for the bat-in-bedroom protocol if a single bat ends up inside the home.

Where each species roosts

This is the most important behavioural difference for homeowners — and the reason species ID actually shapes the inspection.

Little brown bats prefer warm, open spaces. Attics, hot roof voids under sun-baked shingles, and large barns are their favourite Ontario roost types. They form bigger colonies — sometimes a hundred or more animals during maternity season — and the colony tends to be concentrated in one area of the attic. If the attic is safely accessible, a little brown colony is often visible: clusters of bats hanging from rafters, distinct guano piles, and oily stains on the wood beneath them.

Big brown bats prefer enclosed wall cavities, soffit voids, and small attic pockets. Their colonies are smaller — often ten to thirty animals — and the roost is hidden inside the structure rather than in an open attic. You can have a big brown colony in your home for years without ever seeing a bat indoors, because the colony stays sealed inside the wall and emerges directly to the outside through a vent or soffit gap.

Practical implication. If your attic looks clean and the inspector signed off, but you are still hearing scratching at dusk or seeing single bats indoors, the colony is more likely to be big browns in a wall than little browns in an attic. We covered the full pattern in our post on bats in walls vs attic — it is worth reading if you suspect an in-wall colony.

Why little brown bats are protected differently

The little brown bat is listed as endangered under Ontario’s Endangered Species Act. The big brown bat is also protected — under the Fish and Wildlife Conservation Act — but it is not endangered.

The reason for the difference is white-nose syndrome, a fungal disease that has destroyed millions of little brown bats across North America since around 2010. The fungus thrives in cold humid hibernation sites, and the little brown bat — which hibernates in caves and abandoned mines in large groups — has been hit the hardest. Some Ontario hibernation sites lost more than ninety percent of their populations within a few years. The Government of Ontario maintains a species page for the little brown myotis that covers the listing in detail.

For homeowners, the practical effect is straightforward. Both species are protected, so neither can be harmed, trapped, or relocated without a licence. The endangered listing on the little brown bat adds extra protection for its habitat — including buildings used as maternity roosts — and shapes some of the regulatory framework around exclusion timing. We covered the full legal picture in our post on bat protection law in Ontario.

You do not need to verify endangered status before calling. Compliance is our responsibility on every job.

What both species eat (and why bats matter)

Both species eat insects in flight — primarily mosquitoes, moths, beetles, midges, and other flying insects that are active at dusk and through the night. A single bat eats well over a thousand insects in a night. A maternity colony of one hundred little brown bats can clear hundreds of thousands of insects per night across a few-kilometre radius around the roost.

That is why bats matter ecologically, and why preserving healthy populations is in everyone’s interest. The exclusion approach we use does not harm the colony — it relocates it humanely back to a natural roost, where the bats keep doing the work they have always done. Replacing a colony costs the surrounding ecosystem something real; we work in a way that keeps that math intact.

How species affects what we do

The exclusion process is similar for both species, but several details shift based on which one is present.

Inspection focus. We confirm species during the on-site inspection, and the answer changes where we look hardest. Little brown signs send us into the attic first — guano piles, staining, audible activity, sometimes visible animals. Big brown signs send us along the exterior envelope first — vent flashings, soffit corners, dormer trim, wall-roof transitions — because the colony is inside a wall rather than over the ceiling.

Timing. Maternity season restrictions are the same for both species — May through early August in Ontario. Exclusion during that window is illegal because flightless pups would be trapped inside. See our post on maternity season for the full timing detail.

Cleanup. Bigger little brown colonies usually mean bigger guano accumulations, more contaminated insulation, and more decontamination work — handled through our attic cleanup and decontamination service. Big brown wall colonies generate less guano per colony but it accumulates inside the cavity, which sometimes requires opening drywall to reach.

Exclusion approach. The one-way valve method works for both species. The difference is in entry-point geometry. Big browns squeeze through gaps under five-eighths of an inch; little browns through gaps under half an inch. Both gaps need to be sealed during the bat-proofing phase, so in practice we seal to the tighter standard either way. Every active entry point gets a one-way valve during the exclusion window, and every gap on the structure gets sealed once the colony has cleared.

Warranty. The lifetime warranty applies regardless of species:

If a bat re-enters through any point we sealed, we come back and do all the work necessary — at no extra cost. Forever.

Every home is different — the inspection determines the exact sealing plan and the materials used.

What you do not need to worry about

Plenty of bat content online makes species identification feel like the homeowner’s job. It is not.

• You do not need to identify the species yourself. We confirm it during the on-site inspection.
• You do not need to count the colony or estimate its size. Colony size affects the cleanup quote, not the exclusion approach.
• You do not need to verify endangered status before calling. We handle ESA and Fish and Wildlife Conservation Act compliance on every job.
• You do not need to memorize the timing rules. We schedule the exclusion to the legal window automatically.

The legal requirements are the same in practice for both species: do not harm, do not trap, schedule humane exclusion, wait until the maternity window closes if pups are present. The species changes how we work the inspection, not what you have to know going in.

When to call

If you have any of the signs — bats flying out at dusk, scratching in the wall or attic, single bats indoors, a sharp ammonia smell, or a previous inspection that came back clean — book a free quote. No fee, no obligation, written quote at the end. Get a quote from the home page and our team will be in touch within three to five business days. The FAQ and about page have more on how we work.