Big Brown Bat

Eptesicus fuscus

What does a big brown bat look like, and what else might it be?

A robust, glossy brown bat with black wings and ears — and, more usefully, the one you are most likely to be dealing with when a colony has taken up residence in a house here.

Held at a distance or seen against a dusk sky, the big brown bat is noticeably solid for a local bat, with a broad blunt muzzle, short rounded black ears and a coat that ranges from a rich chestnut to a dark glossy brown. The wing membranes and the face are black and largely hairless. The flight is strong and relatively direct compared with the fluttering of the smaller species.

Against the little brown bat, which is the other species people name. It is smaller and lighter in build with a more delicate face. The practical distinction for a homeowner is not the identification itself but what it implies: the big brown bat tolerates cooler and drier roost conditions than most, which is why it is the species most often found in buildings across this region and — importantly — the one most likely to spend the winter inside a structure rather than departing for a cave or a mine.

Against birds, which is the usual confusion at dusk. A bat’s flight is erratic and the wingbeat looks like flickering rather than stroking. Swifts and swallows working the same airspace at the same hour fly in long smooth arcs and disappear well before full dark; bats keep going.

Against a moth, at close range in a room. People genuinely mistake the two in poor light, and the difference is obvious once the animal turns.

None of the identification changes the method. Every bat species here is protected, so the approach is exclusion in all cases, and the species question mostly affects what to expect: a colony that overwinters in the structure behaves differently through the calendar from one that vacates each autumn, and that changes when a survey will actually find anything.

How does a bat get into a roof, and how little space does it need?

Through a crevice rather than a hole, at the top of the building, and the dimension is smaller than almost anyone believes.

A bat does not fly through an opening. It lands at the edge of a gap, grips, and crawls in. That single behavioral fact explains why bat entries are so hard to find: they are slots and seams rather than round holes, they are frequently on a plane you cannot see from the ground, and they show almost no damage because the animal is not chewing or forcing anything. The gap required is narrower than the width of your thumb — think of the slot you would get where two pieces of trim have shrunk apart, or where a shingle course sits proud of the underlayment.

The productive locations are consistent, and they cluster at the top of the building because that is where the warm, dry, undisturbed cavities are.

Ridge lines. The gap beneath the last shingle course at the peak, a compressed ridge vent baffle, a torn ridge vent mesh, a lifted end cap. On older roofs the ridge is very often the primary entry, and it is invisible from a driveway.

Gable ends. The joint where the rake board meets the siding or the sheathing, shrunken trim joints, the head of a gable louver, a louver with corroded or absent screening behind it, and the small gaps around a gable-end vent frame.

Roof-plane junctions. Valleys, the point where a dormer cheek meets the main roof, the junction where a lower roof plane runs into a taller wall, and the edge of a step flashing. These details rarely close perfectly at their edges, and the resulting fraction-of-an-inch slot leads directly into a rafter bay.

Flashing and drip edges. A lifted counter-flashing at a chimney, a curled drip edge, a corner where two pieces of flashing overlap without sealing.

Fascia and soffit joints. The channel where a vinyl soffit meets the fascia, the gap behind a fascia board where it has pulled away, and the seam at a soffit return.

On older housing stock specifically. North Shore frame houses with fieldstone foundations and roofs that have been through several coverings tend to accumulate these slots at every re-roof. Pre-war buildings in the city offer cornice work with open joints, parapet copings that have shifted, and mansard assemblies that are effectively a rack of crevices. Barns, garages and outbuildings on estate properties are frequently the roost, with the house merely reporting the sightings.

Once inside, a bat uses the attic void, the space between the roof deck and the top of the insulation, and the wall cavities below it. Bats in a wall void rather than an open attic are common and are the reason people hear chittering in a bedroom wall on a hot afternoon.

What is the roosting year here, and why does the pup season stop the work?

Because for a period each summer the roost contains young that physically cannot leave, and a one-way device cannot tell the difference between an adult and a pup.

Spring. Females gather into maternity colonies in warm, protected cavities. A south- or west-facing attic, a roof cavity above an occupied house, or a sun-warmed wall void meets that specification well. Males are more solitary and roost separately in smaller numbers or singly. Activity at the entry rises as the colony assembles.

Early to mid summer. Pups are born and are entirely dependent. They cannot fly, they do not follow the adults out at dusk, and they remain in the roost while the females leave to feed each night. This is the constrained period, and it is absolute. Installing a one-way device now lets every flying animal out and seals the pups inside, where they die in the cavity. The consequences run past the welfare question: decomposition inside an inaccessible roof or wall void, weeks of odor, staining through finished surfaces, a secondary insect problem feeding on the carcasses, and adults working hard at the structure trying to get back to their young. It is a bigger, longer and dirtier problem than the original one.

Late summer. Once the young are flying, the colony leaves and returns as a body each night, and exclusion becomes possible. This is generally the most productive window of the year for the work, and it is when an emergence count gives the most accurate picture of how many animals are actually in the roost.

Autumn. Numbers at the roost fall as animals disperse. Some structures empty entirely.

Winter. This is where the big brown bat differs from most of its relatives. It will hibernate inside buildings — in attics, in wall voids, behind chimneys and in any pocket that stays above freezing. A mild spell rouses hibernating animals, and a roused bat moving through a wall void is how a household ends up with one in a bedroom in January. That is why a mid-winter appearance in living space is a strong indicator of a resident colony rather than a stray, and it is also why an autumn or winter survey has to be interpreted carefully: an entry that shows no traffic in December may be extremely busy in June.

How the timing is handled. The constrained period is not a matter of trade opinion. The New York State Department of Health, in its fact sheet on bat habitat inspection and batproofing at children’s camps, instructs operators not to batproof buildings “during the period from late May to mid-August, to avoid trapping baby bats inside the building” — which is the clearest published statement of the window a New York building owner will find. The species question sits on top of it: the northern long-eared bat is federally endangered and therefore endangered under New York’s Endangered Species Law, and DEC’s Long Island guidance records the highest remaining density of that species in the state as being in Suffolk County. The current position gets checked before anything is scheduled rather than worked from memory. If the answer is that the work has to wait, we say so and explain what happens in the meantime — and the interim is not idle. Every secondary gap on the structure can be identified and closed, the primary exits can be established by emergence watching, interior entry points into living space can be sealed so animals stop appearing in rooms, and the full repair scope can be prepared so the exclusion proceeds the moment the window opens.

What is actually at risk from a colony in a roof?

Accumulation, the insects that live with the colony, the cleanup itself, and a public-health question that belongs to somebody other than us.

Guano accumulation. A colony roosting at a fixed spot deposits beneath it continuously, and over years that builds a pile on the attic floor, in a soffit cavity or inside a wall. Accumulated guano holds moisture, produces a strong persistent odor, stains through finished ceilings and can eventually load a ceiling assembly. It also degrades insulation it sits on.

Staining and odor at the entry. Bats leave a greasy dark rub at a crevice they use repeatedly, which stains siding, trim and masonry, and a large roost produces an odor that becomes noticeable indoors in hot weather.

Ectoparasites. Roosts support bat bugs, which are closely related to bed bugs and feed on the colony. They are of no interest while the bats are present and in place. When the colony is excluded, the remaining insects go looking for a host and move along the same wall voids into rooms below, which is why residents occasionally report bites starting shortly after a bat problem was resolved. Removing the roost material and treating the cavity is part of the job for exactly this reason, and it is the step a cheap exclusion leaves out. The insects themselves are dealt with the way any similar problem is; the bed bug profile covers what that kind of work involves.

The cleanup itself. Disturbing an accumulation dry puts material into the air, which is why guano remediation is done with protective equipment, containment and wetting rather than with a broom and a shop vacuum. This is a specified scope of work, not something to attempt on a Saturday.

The public-health routing. Bats are associated with rabies as a general public-health consideration, and that is a matter for health authorities rather than for a pest control company’s web page. The practical rules that matter: never handle a bat with bare hands; do not let children or pets near one; and if there has been any contact, or a bat has been found in a room where someone was sleeping, contact your local health department or a physician for guidance. Make that call first. We will work around whatever they tell you.

What is not a realistic risk is aggression. Bats do not attack people, they are not interested in hair, and a bat in a room is a lost animal looking for a way out.

What are the signs, and how do you run an emergence watch?

Look for the pile, the stain and the animals themselves at dusk — and the dusk watch is the single most valuable hour on the whole job.

Guano. Look for accumulations rather than scatters: a pile on an attic floor beneath a fixed roost, a line of droppings along a soffit, material collecting on a windowsill, a porch floor, a deck or a garage floor directly beneath a crevice, or dark grit in a gutter. Crushed, guano crumbles to a powder that glitters faintly with insect fragments; mouse droppings do not, which is a field test anyone can run with a stick.

Staining. A dark greasy smear at a slot, a seam or a louver edge, often with a slight sheen, produced by animals crawling in and out at the same point over a long period. On a light-colored fascia or a stucco gable end it is unmistakable once you know to look for it.

Sound. A soft high chittering and rustling from a wall or a ceiling, rising in hot weather when the roost heats up and the animals become restless. It is quite unlike the walking of a raccoon or the daytime rolling of a squirrel.

Odor. A distinctive musty ammonia note from an accumulation, usually strongest on hot days and in a closed upper room.

The emergence watch. This is how the exits are actually established. On a dry, still evening, position people so that all faces of the building are covered, and start about half an hour before sunset. Watch the roofline against the sky rather than against the building, because a bat is far easier to see silhouetted. Animals will drop out of a specific point, and they will do it repeatedly over a period of perhaps half an hour. Count them and note every location that produces an emergence. Roosts frequently have more than one exit, and finding only one is the most common reason an exclusion fails: the device goes on the obvious opening and the colony carries on using the other. On a complicated roof this is worth doing on more than one evening.

Why are trapping, spraying and sealing all the wrong answers?

Because two of them are unlawful and unnecessary, and the third makes the situation dramatically worse.

Trapping and lethal treatment are not on the table. The protection is real and specific — a federally endangered species in the group, listed as endangered in New York on that basis — and the handling itself is licensed work under DEC’s nuisance wildlife control operator credential, which is why the device on a bat job is fitted by a partner firm holding it rather than by us. There is no chemical approach to a bat colony and there will not be one, and any company that offers one is telling you something you should act on immediately. There are no legitimate repellents, ultrasonic devices or deterrents that resolve a roost either. The animals are extremely site-faithful, and a colony will tolerate a great deal of disturbance rather than abandon a good roost.

Sealing an occupied roost is the worst available option. A roof cavity is not a sealed box; it connects to soffits, to wall voids, to plumbing and electrical chases, and through top plates, recessed light housings, switch boxes and gaps behind trim to the inside of the house. Close the exit on a colony and the animals do not disappear. They move within the structure looking for another way out, and the remaining routes point inward. What follows is bats appearing in bedrooms and hallways over successive nights, which is precisely the situation nobody wants, and any that do not find a way out die in the cavity with all the consequences described above.

Sticky products and improvised devices cause suffering, are not a method, and produce animals that have to be dealt with by hand.

And doing nothing has a cost too. A colony left in place continues to accumulate, and the entry crevices continue to widen as the roof assembly ages. The problem does not stabilise.

How is a bat colony actually excluded?

Survey, seal everything but the exits, fit one-way devices, monitor, close, clean — and check the timing before any of it.

Establish the timing. Whether the work can proceed at all is the first question, and the published window described above answers most of it.

Split the job correctly. The device work is licensed nuisance wildlife control and it goes to a partner firm that holds the DEC credential. The survey, the sealing, the guano remediation and the permanent closure are Graduate’s, and on a bat job they are by far the larger scope — a roof assembly with dozens of candidate crevices is exclusion work of exactly the kind this company was built on.

Survey the whole roof assembly, twice. Once from a ladder and from inside the cavity in daylight, looking for staining, guano, roost sites and daylight; and once at dusk, watching emergence to establish which crevices are actually in use and roughly how many animals are involved.

Seal every gap that is not an active exit. On a bat job this is a large piece of work, because a roof assembly of any age has dozens of candidate crevices and the animals will simply move to the next one. Ridge, valleys, dormer cheeks, flashing edges, fascia joints, soffit seams, louver frames, chimney counter-flashing, and every trim joint that has opened. The primary exits are left open deliberately.

The partner fits the devices. For bats these are lightweight one-way fittings — a tube, or a sheet of fine netting hung above and to the sides of the exit and left open only at the bottom. Bats drop out of a roost to take flight, so they leave under the netting without difficulty; returning animals land at the crevice and find the material blocking their crawl-in, and they cannot work back up underneath it. Nothing is confined and nothing is handled.

Leave it up and watch. The devices stay in place long enough for the whole colony to have left over several successive nights, including animals that may not feed every night in cool weather. Emergence is re-watched to confirm nothing is still coming out.

Then close permanently. Devices come off and the exits are sealed to the same standard as the rest of the roofline, matched to the substrate: sealant and backing at fine joints, metal flashing and rigid closure at larger ones, correctly sized screening from behind at anything that must keep ventilating. A ridge vent, a gable louver and a soffit vent all exist to move air, and blocking them to keep bats out trades a wildlife problem for condensation on the underside of the sheathing. That principle runs through all of our structural exclusion work.

Then verify. Return after weather and after a warm spell, and re-watch at dusk. Bats test a roof hard, and a missed crevice announces itself quickly.

What happens to the guano, and what comes after the bats?

Remediation, then the insects, then the ordinary maintenance of a roof that is now closed.

Remediation. Where accumulation is significant, the material is removed rather than left, using protective equipment, containment and wetting rather than dry disturbance. Contaminated insulation comes out. The cavity is cleaned and treated, surfaces are addressed, and the affected area is re-insulated. Where guano has been sitting on a ceiling assembly for years there may be staining or damage below that needs a separate repair.

The ectoparasite step. Bat bugs left behind in an emptied roost go looking for a host, so the roost material is removed and the cavity is treated rather than simply closed over. Skipping this is how a household ends up reporting bites a fortnight after being told the problem was solved.

Odor. Persistent smell after remediation is generally a sign that material remains somewhere — inside a wall cavity, in a soffit run, or under insulation that was replaced around rather than removed.

And an alternative roost is worth offering. Bats consume very large numbers of night-flying insects, and a colony that has been excluded from a house has to go somewhere. A properly sited bat house on a pole or an outbuilding, away from the building and with the right sun exposure, is a reasonable thing to install as part of the work. It will not do the excluding for you, but it is a decent outcome for an animal that is doing your property a service.

What keeps a roof closed to bats?

Maintaining the assembly at the top of the building, because that is where every crevice is.

Deal with the roofline at re-roof time. The single best opportunity to close a roof to bats is when it is being re-covered anyway. Ridge detail, valley closures, drip edges, step flashing and counter-flashing all get done properly at that point at very little marginal cost, and the alternative is doing the same work later from a ladder for considerably more.

Screen the vents from behind, in metal. Gable louvers and ridge vents are the two that fail most often. Fabric or plastic screening degrades; correctly sized metal does not.

Keep trim joints closed. Rake boards, fascia, corner boards and soffit returns shrink and open as they age, and every opened joint is a candidate crevice. This is caulking and carpentry, not pest control, and it is the highest-value maintenance on the list.

Fit and fasten chimney caps and keep counter-flashing tight.

Survey the outbuildings. Detached garages, barns, pool houses and sheds are frequently the actual roost on a property where the house is getting the complaints.

And check after any roof or gutter work. Trades open things. A survey at dusk in the summer after a re-roof is twenty minutes well spent.

Related work

Exclusion method, one-way devices, timing constraints and the repair specification sit under wildlife management, the parent service for this profile, and the sealing discipline itself is described under structural exclusion.

Among the other profiles, the eastern gray squirrel and raccoon pages cover the two animals that use the same roof assembly through openings a bat would find enormous, and it is entirely ordinary to find all three histories on one building. The cluster fly profile is worth reading alongside this one, because cluster flies exploit the same shrunken trim joints and gable-end crevices in autumn — a roof sealed properly for bats generally solves that problem as a side effect.

If you are finding accumulations beneath an eave, a stain at a ridge, or animals emerging at dusk from a fixed point on your house, tell us what you have seen and where, and note the time of year — that determines whether the work can proceed now or has to be scheduled. The areas we cover are listed under locations.

Treatment for this pest is covered under Wildlife Management.