Most types of Bats naturally stayed together for protection, huddling below cavern ceilings in hundreds or even thousands. This was a natural delf-defense product of their colony, both for protection in numbers as well as to alert their fellows of any danger. Certain types of swarms, however, were not benign assemblies of little creatures. They formed as a result of a sinister or unwholesome influence, and would not leave the swarm even when in danger.
Even powerful Druids could not dissuade or charm these swarms, which had an aggressiveness that was borderline unnatural.