Bats in a Detroit Attic: Why Timing Matters

Why does timing matter so much with bats?

Female bats raise their young in warm, protected spaces such as attics during early summer. The pups cannot fly for several weeks. If the adults are shut out during that window, the pups are left behind and will die in the wall or attic, so exclusion is scheduled around the colony's calendar.

Bats are not like mice. A mouse problem can be tackled any week of the year, because a trapped mouse is simply a trapped mouse. Bats in an attic in June may be a nursery, and the adults leave each night to feed and return to nurse the pups. Closing the exit in that period does not make the adults give up the colony. It makes them stay outside, circling the house, while the pups cry from inside.

That is why experienced wildlife technicians say the same thing: there is a window when bat work is a bad idea. In Michigan the usual advice is to wait until the young are flying, which is typically late summer. The situation is uncomfortable for a homeowner, because the bats are in your attic now. The honest answer is that a few weeks of waiting is far better than a mass of dead pups behind your ceiling.

Waiting does not mean doing nothing. The sections below explain what to do in the meantime, and a technician can inspect during the nursery season, identify the entry points and plan the exclusion for the right date.

What are the dates in southeast Michigan?

Commercial sources put maternity colonies in southeast Michigan from about the third week of May through late July, with exclusion avoided from roughly May to mid-August and no one-way doors in June or July. Treat these as typical timing from commercial sources, not an official calendar.

We could not find an official Michigan DNR page with a set of dates, so the timing below comes from commercial wildlife companies that work in the region. The sources agree on the broad pattern. Colonies form in late spring, pups are born, and by late July they are starting to fly. Weather matters: a cold spring can push everything later, and an unusually warm one can pull it forward.

A technician on site can usually tell more than a calendar can. Evidence of young bats, such as noise at dawn and dusk and the pattern of bats leaving at dusk, helps them decide when it is safe. We therefore treat the table as a planning guide for you and your technician rather than a rule.

How can you tell you have bats in the attic?

Common signs include scratching or squeaking at dusk and dawn, dark stains or grease marks around a small gap, droppings that crumble to a shiny powder, and bats seen leaving the roof at sunset. A technician can confirm the species and locate entries without guessing from sounds alone.

Bats leave a roost shortly after dusk and return before dawn, so watching the roof line for about thirty minutes at dusk on a warm evening is the most reliable check. You may see small shapes flying from under a soffit, a gable vent or a gap near a chimney. A gap that bats use can be small, and they leave smudged rub marks along the edges from oils in their fur.

Droppings, called guano, look like dark, small pellets. Mouse droppings are similar in size, but bat guano is made of insect parts and tends to crumble into a glittery dust when pressed. Guano piles under an entry point or on an attic floor suggest a regular roost. If you find a lot, do not sweep it dry or disturb it, because dust can be a health concern, and ask a technician about handling it.

If a bat gets into the living space, do not handle it with bare hands. Close interior doors, open an outside window if you can, and call. Bats can carry rabies, so any direct contact with a person, child or pet is a reason to call a doctor or local health department promptly. Our bat exclusion page has more on what a visit involves.

• Scratching, squeaking or chirping in the attic or walls, loudest around dusk and dawn
• Small dark stains or grease marks around a gap in the soffit, fascia or gable
• Droppings that crumble to a shiny, powdery dust
• Bats flying out of the roof line at sunset
• A strong, sharp odor in the attic during warm weather

What do you do while you wait for the season to end?

Have the house inspected, keep people and pets out of the attic, leave the roost alone, and fix any other gaps that bats are not using. Do not seal active entry points or use smoke, noise or repellents. A technician plans the exclusion for when the young can fly.

A summer wait can feel long. A few things help. Keep the attic closed and make sure no one stores items there. If you hear bats in the walls, shut doors between the wall space and living space and make sure gaps around ceiling fixtures or attic hatches are blocked. Do not put mothballs, ultrasonic devices or smoke in the attic. They do not reliably move bats and can harm the animals and you.

The most useful step is an inspection. A technician walks the exterior looking for entry points, identifies the main exit, and notes how many exits there are. That information lets the exclusion run efficiently when the time comes. It also reveals other points that can be sealed now without trapping any bats, such as unused soffit gaps far from the roost.

Neighbors and landlords are part of this too. If you live in a two-family flat or an apartment, the roost may be shared, and the exclusion has to cover the whole building. Raise it with the owner early so the work can be scheduled for late summer.

How does exclusion work when the time comes?

A technician finds every exit, seals all but the main ones, and fits one-way devices on those so bats can leave but not return. Once no more bats are leaving, the devices come off and the exits are sealed permanently. Timing keeps the nursery season out of it.

Exclusion is slow and methodical. Bats can squeeze through very small openings, so the job begins with a careful survey of the roof line, soffits, fascia, ridge vents, chimney flashing, siding gaps and the junction between additions and the main house. Once every possible entry is mapped, the technician seals the secondary ones with durable materials and installs one-way devices over the main exits.

The bats leave at night, find they cannot get back in and move to another roost. When no more bats are leaving, the technician removes the devices and seals the final entries. Because a sealed gap can be reopened by determined animals, quality of repair matters more than speed. That is why exclusion is a different job from simply putting up a screen.

Cleanup is a separate step. If there is a large amount of guano or soiled insulation, a technician or restoration specialist can advise on removal. Treat this work with care, because dust and droppings are a health consideration, and ask what protective equipment will be used.

What is different about older Detroit houses?

Older Detroit houses often have slate roofs, dormers, carriage houses, deep eaves and chimneys, which give bats many places to get in. Large homes in neighborhoods such as Indian Village and Boston-Edison have more roof line to inspect, which makes the survey longer and the sealing more detailed.

Research notes on Indian Village describe large masonry houses with slate roofs, dormers and carriage houses, built mostly between 1895 and the late 1920s. That kind of roof has many junctions, flashing and overlapping details. Those features give raccoons, squirrels and bats roof-line entry. Boston-Edison has similar large, older houses, and the same logic applies there: the more complex the roof, the more places to check.

Older masonry can also move over time. Mortar joints open, flashing lifts and wood trim shrinks. A small gap that was sealed decades ago can reopen. It is worth knowing that bats are not the only animals using these routes. If a technician finds signs of squirrels or raccoons, our squirrel and raccoon exclusion service handles that, and the repairs often overlap.

Carriage houses and detached garages deserve attention. They are often less maintained than the main house and can host a colony that moves into the attic later. Include them in any inspection.

Who is allowed to do this work, and what about the law?

Michigan licenses pesticide application businesses through MDARD, and bat work may involve rules beyond that. We do not state a legal rule or a fine here, because no official Michigan DNR page confirmed the details. Ask the company what rules apply and for its license number.

You will find websites that claim specific penalties for harming bats. One search result mentioned a dollar-per-bat fine, and we left it out, because we could not verify it. If you want to understand your obligations, ask a licensed wildlife professional and contact the Michigan Department of Natural Resources directly.

Michigan's commercial pesticide applicators are certified by MDARD, with category exams that include vertebrate pest management. A company doing bat work should be able to explain how it handles the maternity season and give you its license number if you ask. Ask for a written plan that states the date work will begin, how many exits there are and how the final seal will be done. You can reach us at (855) 974-4733 to talk through your situation.