Species’ names:
LITTLE BROWN BAT, Myotis lucifigus
Size:
3–4 inches long
1/16–1/2 ounce
9″ wingspan
New York’s most abundant species of bat.

BIG BROWN BAT, Eptisicus fuscus
Size:
4–5 inches long
3/8–5/8 ounce
12″ wingspan

There are nine species of bats in the Northeast, but these are the only two that commonly roost in buildings.

Signs of their presence:
1. Swarming and flying around the entrance hole several times before entering the roost. At dawn or dusk, may see bats entering or leaving the building.

2. Sounds: Roosting bats may squeak or scurry when disturbed.

3. Droppings: Piles of black, dry, guano usually found under the main exit hole in the attic. Can be found scattered throughout an area, particularly in roosts where bats enter and fly about, but roost in a specific area such as a wall void or the intersection of the beams and rafters. May also see scat on the side of the house, usually below a hole or crack. Large piles of guano are usually found beneath areas that are used often, or by large numbers of bats. Individual droppings for the little brown bat are about the size of a grain of rice. The scat of the big brown are about twice that size, and are typically chunkier, due to the types of insects they eat, such as beetles. Bat droppings look like mouse droppings, but mouse scat isn’t found in large piles, and is also a little smaller. Also, bat droppings will crumble into powdery dust but mouse droppings won’t. You may see pieces of insect wings, or their reflections, in bat scat.

4. Rub marks along the edges of exit holes: Slight brown discoloration that’s a mix of body oils and dirt.

Roosts: During the summer, they’ll use different roosts during the day and night. The daytime roost is usually in an attic, barn, garage, soffit, cave, underneath shutters or roof shingles, in wall voids, or behind siding or chimneys. At night, they’ll rest in a breezeway, under an awning, or in a garage or similar areas. In the winter, both species hibernate in colonies in caves, mines, and deep rock crevices. Big brown bats are more likely to hibernate in buildings, often in the attic or in wall voids.
Common nuisance situations:

Time of year: Peaks from the third week of July through the first week of August, although situations arise any time of year.
Females may roost in colonies in buildings to raise their young. Scat and urine can damage insulation and household goods and attract other pests.

Lone bat enters the house and flies around. This usually happens in July and August, when the young are learning to fly.

During an extreme heat wave, several bats may enter the living quarters, seeking a cooler roost. This is when they’ll show up in places they normally don’t use.

Big brown bats will hibernate in buildings (little browns don’t seem to.) In the winter, a big brown bat may leave its roost in the attic and fly around in the living spaces. This usually happens when the temperature of the attic roost changes dramatically, disturbing their hibernation—during a thaw, or during the very coldest part of the winter, if the attic is much colder than the rest of the house.
Disease risks: Rabies, histoplasmosis. In New York, bats are a rabies vector species.
For more information:
http://www.nwco.net/08-AppendixBNuisanceSpecies/8-01-Bats.asp