Attic with Bat Guano
Over time, a colony of little brown bats such as I discovered roosting in our "belfry" can grow to a population of a few hundred, leaving behind an accumulation of guano that is benign if undisturbed but that, over time, generates an undesirable odor.
What we had, high up on the beams where they meet the chimney, was a maternity roost. With bat exclusion, timing is everything. Females give birth, one pup a season, any time from May through August. (The one upside: In a single day, nursing mothers can eat up to half their weight in insects.) So that no young bats are trapped in the building, exclusion must be conducted either in the spring when insects have appeared but before the pups are born or after the young bats are capable of flying, ideally early fall. By the time frost appears, little brown bats will have migrated to caves where they hibernate for the winter.