Most Ravens naturally nested around others for protection, never straying too far from their flock. This was a natural delf-defense mechanism of their colony, both for protection in numbers as well as to alert their unkindness 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.