Bat Signs In Winter

By Chris Williams on February 20, 2015.

bat in cave

John Maher:  Hi, I’m John Maher. Today I’m here with Tim Chace of Colonial Pest Control. Tim is an entomologist and pest control technician. Today, we’re talking about bat signs in winter. Welcome, Tim.

Tim Chace:  Good morning, John.

Signs of Bat Infestations in Winter

John:  So Tim, what are bat infestation signs in a house, in winter, typically?

Tim:  Typically, bats that have decided to over winter in your house will begin to hibernate as the temperature gets lower and lower. Some bats will end up in areas underneath the insulation, or places that offer them a little shelter from the coldest weather of the winter. If they should become active at any time, you might hear a very discreet scratching noise. In some cases, you might even hear a very, very light fluttering if they’re in the wall void or in the attic in the winter, as they’re flying from location to location within the structure.

You might even hear, as they try to communicate, a very, very low squeaking sound. Different than other animals, it’s a very high pitched squeaking, so that would be an indication that something is going on up in the attic in the winter.

Do bats migrate in the winter?

John:  OK. Do bats migrate in the winter, like birds, in Massachusetts and New Hampshire?

Tim:  That’s a good question. The most prevalent species of bats, the small brown bat and the large brown bat, are known to do a little bit of both. If they have a location that’s suitable for them to hibernate, they might stay in your attic over the winter, and in some cases they might fly a couple hundred miles to another location that they find more suitable, like a cave that they all congregate in for the winter.

That’s typically what happens in the New England area, so we have a combination of them migrating to a more favorable spot, and some of them will actually stay up in the attic over the winter.

Bats in Summer vs. Winter

John:  Is it less common to find bats in your house in the winter than in the summer?

Tim:  It generally is. A lot of the bats will head to a better location than your attic. In modern construction, with the way that a lot of homes are designed today, the attic gets quite cold in the winter. If you compare that to an underground cave where the temperature hovers in the 50s, that might be a better place for them to congregate over the winter. But certainly, there are locations within the house that they could deem desirable.

What do bats do during winter?

John:  OK. And what do bats do during the winter? Are they generally just not active? You said sometimes you might have them scratching in your walls or that sort of thing. Do you generally just not even know that they are there, or are they sleeping?

Tim:  Once the temperature drops sufficiently and stays cold, bats should be almost non‑active because there’s no food and they’ve got very small fat supplies. They really don’t want to do a lot in the winter, except sleep in that hibernating mode where they’re just alive, but they’re really not moving. Their metabolism gets very, very low so they’re not using up a lot of energy.

However, due to global warming and other factors, temperature‑wise, we could have — let’s say, for example, in January the temperature goes up to 55 degrees. The attic warms up too, the bats wake up, maybe they won’t even go back to sleep. So, now you’ve got a group of animals that have woken up after their suspected winter sleep, and now they’re rustling around looking for dead insects to eat. Maybe accumulations of things that they can feed on that are up in the attic, but they typically won’t fly outside unless there’s a food source.

I’ve got a large brown bat that lives at my neighbor’s house. When he first comes out in the springtime, you can actually observe him coming out of the corner of the house in the day time. It’s kind of neat, he will fly down and take a drink from my pond and go back to his little spot.

It’s really neat to see these guys when they’re active, but that’s a spring event. I’m sure he might stay up there all winter, but large brown bats do migrate to other locations, sometimes several hundred kilometers away. Depending on what the bats do and what their colony is up to, really dictates what they’re going to be doing in the winter.

John:  Does that make it harder to get rid of bats in your house in the winter, because they’re not flying in and out of the house, generally? They’re more hibernating, and so does that make it more difficult to get rid of them?

Tim:  Absolutely. What happens is bats will actually mate in the fall, and the female bat actually holds the sperm in her uterus until ovulation and fertilization take place in the springtime. Most bats will do that on their own, so the male and females get together in the fall. Once the colony disperses to their hibernation site or wherever they migrate to, when they come back the females will start having their young.

Typically for the small brown bat and the large brown bat, they’ll have one, maybe two young. It takes a few weeks for them to learn how to fly. By about August or September, the little guys are able to fend for themselves, just about where the colony is going to maybe disperse from your attic and go to their migratory location. Or if they’re going to hibernate in your attic, they’ll find a spot up in the attic and remain there over the winter.

When to Exclude Bats

John:  Is that the time when it’s easiest to get rid of them, when they are leaving your house and that’s the time when you can seal up the gaps and things like that, and just not allow them back in?

Tim:  Yes. It’s probably the best time of the year to do that, and then again in the spring, before they’ve actually had the babies. April through July is typically when the babies are being born, depending on the temperature and the regional location.

John:  If you can seal up your house before that happens, then that’s a good idea.

Tim:  Yes. Set up the one way doors at that point, get all of the adults out of there and seal it up. But you don’t want to seal up the house while the babies are in there, because they’re completely dependent on the mother’s milk for their food. So, we want to make sure that the beneficial species get back out to nature to eat all the insects that we don’t like, and they’ll be free to live.

John:  Right, right. Well, that’s really helpful information. Tim Chace, thanks very much for speaking with me.

Tim:  Thank you, John.

John:  For more information, you can visit the Colonial Pest Control website at, or call 1‑800‑525‑8084. That’s 1‑800‑525‑8084.

Photo credit: USFWS Headquarters / Foter / CC BY



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