"Dear Matt,
I want to thank you for removing the bats and flying squirrels from our house. I found the process very interesting and I am glad that no animals were killed. I found you guys to be very 'down to earth.' My son keeps asking me 'When is Bat Man coming back?'
Thanks again."

Kelly B., Upton MA

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Little Brown Bats or Big Brown Bats?

Massachusetts is home to two species of bats- Big Brown Bats and Little Brown Bats. A question that I am frequently asked by my clients is "What species are the bats in my attic?" It is very difficult to differentiate between the species as they are almost identical in physical appearance. On this page we will discuss the differences in the species, their habits and also how to tell them apart.

Disclaimer: I am not a biologist. However, in any given year I deal with as many bat infestations as anyone in New England. Working in the field and studying bats for many years I have accumulated a lot of knowledge about our local bat populations. I'm sure that some of what I write will conflict with information that is found in books or on academic websites and I'm fine with that.

Big Brown Bats

Big Brown BatBig Brown bats are the most common species of bats in Massachusetts. Big Browns are year-round residents in the Bay State. We typically find colonies of Big Brown Bats in attics that range anywhere from 2-200 bats. They are active from late winter through the late fall. Once the cold weather arrives in the late fall and the nighttime temperatures dip down into the 30's Big Browns will go into hibernation. With very little natural habitat left in Eastern Massachusetts these bats now hibernate in attics and rooflines of homes. Big Browns will burrow under insulation or drop down wall-voids in an effort to stay warm. Here is a picture of hibernating bats.

I have found that these bats seem to have a high tolerance for cold weather. Every year we see Big Brown Bats flying at night and even roosting on the exterior of houses in January and February.

Big Brown Bats definitely cause the most problems for homeowners in Massachusetts. These bats are not afraid to enter attics and go exploring once they're inside. They will venture deep into cathedral ceilings and other areas inside the structure. 99% of the bats that we remove from the living space of homes are Big Brown Bats.

Big Brown bats are commonly found in attics, rooflines, cathedral ceilings, chimneys, roosting in gable-vents and also behind rake-boards.

Little Brown Bats

Small Brown BatLike the tourists on the Cape, Little Brown Bats migrate to our area every spring from the mid-Atlantic states. These bats start to show up in May and are usually here in full-force by the second week of June. Little brown bats are found in VERY large colonies throughout Eastern Massachusetts (and also RI). The 5 largest colonies of bats that I have ever witnessed have all been little brown bats.

One thing that I have noticed over the years, after dealing with hundreds of colonies of little browns is that they don't seem to like insulation (or attics that are very hot). We rarely find inside attics and the attics that we do find them inside usually have little or no insulation. They also seem to prefer to roost in large open attics or barns rather than tight cluttered attics or wall-voids.

Rake BoardsThe most common problem associated with Little Brown bats is droppings. Because they are found in such large colonies they tend to produce large amounts of bat guano wherever they reside. One spot that Little Brown Bats like to roost is behind rake-boards on houses. The area under these rake-boards is usually covered with bat droppings. See pictures below:

Bat GuanoLittle Browns also present another problem - They are very difficult to control. Little Browns can squeeze through very, very small openings. These bats will take advantage of the smallest gaps, cracks or crevices to gain access to houses and buildings. It takes a very skilled technician to rid a home of a little brown bat infestation. They will also use entry-points that are very low to the ground and I have seen them enter through foundations on more than one occasion.

Big Brown vs. Little Brown - Which is Which? How do you tell them apart?

The physical differences between little brown and big brown bats are subtle. Even with one in each hand it is still difficult to tell them apart. I have studies both species for many years, I have taken thousands of photos and I have also studied them in person every chance I got. Only now am I confident enough to say that I can tell them apart.


The most obvious physical difference between Big Browns and Little Browns is the nose.

Little Brown Bat

Little Brown Bat - The nose of the Little Brown bat is short and almost entirely covered with fur. It also looks "smooshed" and is very short.

Big Brown Bat

Big Brown Bat - The nose of the Big Brown bat is generally longer than that of the little brown bat. The space from the ear-lobe to the tip of the nose is almost fur-less.


Large Big Brown BAtGenerally speaking, size is not a good way to tell one species from another. There are just too many variables that go into the size of a bat (age being the most significant). If you had one bat of each species that were both the same age the Big Brown bat would be significantly larger. The picture above is a Big Brown bat. I have found that the the wingspan of adult Big Brown bats in Massachusetts is between 11-13 inches. Adult Little Browns on the other hand have a wingspan of 6-9 inches.


HUGE Bat Guano PileThe guano (bat droppings) that each species generates is also completely unique. The droppings from little brown bats are physically smaller than that of Big Brown bats. The easiest way that I can describe the size difference is to say that little brown guano is smaller than a grain of rice and Big Brown guano is larger than a grain of rice.

The smell of the guano is also completely unique. Guano from little browns smells really rancid and almost has an ammonia-like odor to it.

Guano from Big Browns always smells like it is wet and moist and doesn't smell nearly as bad as guano from little browns (at least in my opinion). In most cases we can determine the species of bats that we're dealing with as soon as we open the door to the attic just by the odor from the guano.


Colony of Little Brown BatsAs mentioned in the preceding paragraphs Big Browns are usually the bats that enter rooflines and attics and go exploring. These bats are the ones that show up flying around your living room in the middle of the night. Little browns on the other hand seem to prefer open roosting areas such as rake-boards, gable-vents, barns and attics without insulation. We rarely find little browns inside the living space of houses. Another interesting fact about little browns is that they are much louder than big browns. Little browns are very sociable and squeak very loud, especially when in large groups. I have pulled into client's driveways and heard the bats under the rake-boards before I even got out of my truck.

Location and Distribution of Little Brown and Big Brown bats in Massachusetts

Colony of Bats

Big Brown Bats, Resident Bats

I find that Big Brown bats are pretty evenly distributed in Eastern Massachusetts and Rhode Island. I think its safe to say that every town in our area has a healthy population of big brown bats. We find large colonies of these bats in rural and suburban areas around the route 495 loop and throughout RI. We also see healthy populations of these bats in downtown Boston. The towns of Brookline, Somerville, Newton and even downtown Cambridge all have healthy populations of big brown bats. Certain areas also seem to have more than their share of big brown bats. In certain towns we have performed 7-8 exclusions for colonies of big browns inside of a 1/2 mile radius.

Little Brown Bats, Migratory Bats

The fact that Little Brown Bats migrate hundreds of miles each year makes them a bit harder to pattern. These bats seem to show up in clusters for us every spring. Some years we get dozens of calls for little browns along the coastline and won't get any from inland areas. The following year all of our calls for little browns will come from inland areas and we won't get any calls whatsoever from the coastline. As I've already mentioned I am not a biologist so I won't speculate as to why these bats are so unpredictable and difficult to pattern. Every year they keep us guessing.

*The two largest colonies of Little Brown bats I have ever seen (both over 400 members) were in Topsfield Massachusetts and West Greenwich Rhode Island*


The towns of Westport Ma, Marion Ma, Little Compton RI and Tiverton RI will get inundated with these bats some years and the following year we won't get a single call for them in this area. We also find big numbers of little browns in Worcester County and the towns in the greater Andover area. These bats seem to stick to the rural areas of Eastern Mass and generally stay away from the cities.

Rhode Island

South County Rhode Island is an area that has a very healthy population of Little Brown bats. Every year we find large infestations of Little Browns in Misquamicut, Richmond, West Greenwich and Narragansett. We don't get very many calls for Little Browns in the Northern half of the state. 99% of the bats we find in the greater Providence area are Big Browns.


I hope you enjoyed my article on the Big Brown and Little Brown Bats in Massachusetts. If you have a bat problem you can find more information on our Bat Removal.

Matt Grady
BatGuys Wildlife Service