Image of little brown bats roosting on a building

Bat Roost Monitoring

In general, bats are cryptic animals. However, two bat species in Wisconsin form large maternity colonies in summer and give a rare glimpse into the ecology of these bats. Both little brown bats and big brown bats are known to roost in bat houses, attics, barns and other buildings where it stays warm over the course of the day and through the night. Generally these maternity colonies are easily located and monitored because they are in or near human dwellings and the colonies can be quite large. Little brown bats form maternity colonies of up to thousands of individuals. Big brown bats tend to form smaller colonies, but roosts of up to 200 big brown bats are often reported. Knowing the locations and approximate sizes of these colonies has helped the WBP gather data about statewide distribution, population estimates and roost preferences before and after white-nose syndrome arrived in the state.

Bat roost monitoring is simple and can be an enjoyable experience. Monitors identify bat roosts and sit outside the roost entrance in the evening to count the bats as they emerge. Bats will start to exit the roost just after sunset, and will emerge one or two at a time making counting easy. The bats will continue to exit for about 40 minutes. How often a roost gets counted is up to the volunteer, however the program appreciates at least two counts- one in early June and one in late July. These dates are chosen based on the volancy (flight) of the young born in June and can help us determine recruitment of the colony. Check out the video below for more information, and use the roost monitoring packet to submit your surveyor, site and count information. You can also submit your counts if your site is already in our database.

Enter counts
image of a bat emerging from a bat house
Little brown bat emerging from a bat house.
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